Brake Pressure Sensor (DSC/ABS/BRAKE Trifecta)

Ive spent about 12hours pouring throught the mounds of the ABS/DSC/Brake trifetca issues for a week and havent found any good DIY on Brake Pressure Sensor testing.

If anyone can point me to a DIY thread for Brake Pressure sensor testing, please do so, however I coultnt find any specifics from bluebee or 540iman that specify a process.

I have an 02 E39 with the ABS/DSC/Brake light issue for about 6 months however speedo and cruise work fine. I have not heard of anyone having this specific issue, but all 3 lights usually pop on for me 3/4 the time when Im braking at a stoplight, (light sometimes turns off/on while brake pedal down) which leads me to think its the brake pressure sensor (vs ABS module or speed sensors.) The other 1/4 is during normal driving.

I did the "BlueBee wheel sensor diode test" which did show failure in the RR, however my LR diode test was only working when I reversed the leads. Im still questioning this test vs the gold standard which would be to actively measure the sqaure voltage output of each, however I havent the time or equipment to do this. Despite my DMM diode function working, I cant get any Ohm readings. All sensors have been pulled and cleaned, still with trifecta issue.

BRAKE PRESSURE SENSOR TESTS

There are 3 wires in the rear of the sensor. After much testing I came to this conclusion.
-blue/black (ground)
-purple/black (+5V)
-white/blue -variable voltage per psi

SENSOR TEST
To test the sensor output with DMM, ground the (-) probe, and tap the (+) probe into the blue/white wire, turn key on and meausre voltage change with brake pressure.

Despite 0 -> full brake force, I could only get a 0.2V increase from .69V to about .89V. The other wires were a constant. I have read the voltage should approach over 4V at maximum pressure.
Does anyone have specs on the resistance through this sensor?
Is there a pin from the ABS pinout that corresponds to this wire for testing?

CONCLUSION
It appears my brake sensor is the likely culprit here, however I am still not convinced I dont have a bad wheel sensor or ABS module, although all speedo and cruise controls work fine.
I have heard of erroneous diode testing even thought the wheel sensors are normal....
And I am not sure why when I do the diode tests at the actual rear sensors, they are all faulty with OL/OL. I am also perplexed why my LR diode polarity is opposite the other 3 (verified this 5 times), as this could possibly mean a faulty ABS module?
I'll probably finally load up GTI and INPA for more data, as Ive been meaning to do this, As for now its nice to do peel-outs with the DSC failing

Any thoughts appreciated!
Sorry to post yet another 1000+ thread on the trifecta of death.

-James

Originally Posted by bjay27

If anyone can point me to a DIY thread for Brake Pressure sensor testing, please do so, however I coultnt find any specifics from bluebee or 540iman that specify a process.

I'm surprised since the canonical ABS BRAKE DSC trifecta thread has lots of information on testing the brake pressure sensor (but I would agree it's scattered about within the 500 posts of that thread).

The test we came up with (initially from 540iman and Max_VQ) was based, I'm sure, partly at least on this PDF:
- DSC_COMPONENTS.PDF

I'll look separately for that brake pressure sensor test and include the information for you in a later post.

Originally Posted by bjay27

I have heard of erroneous diode testing even thought the wheel sensors are normal....

As I remember ... any wheel speed sensor that tested bad (when the other three were tested as good), 'was' bad (but I'd have to check the 500 posts to be doubly sure of that off-hand assumption).

Originally Posted by bjay27

I'll probably finally load up GTI and INPA for more data, as Ive been meaning to do this

Start here:
- How to diagnose the BMW amber ABS BRAKE DSC/ASC trifecta or bifecta (1) & what are all known options when your ABS control module is bad (1) & Quick99Si's explanation of why the 10-minute wheel speed sensor diode-action quick test doesn't always work, especially on brand new non-OEM wheel speed sensors (1) (2) & explanations by 540iman as to why the diagnostic tools very often (extremely often, like almost all the time) fail to properly diagnose the trifecta or bifecta (1) (2) (3) (4) (5) (6) & where to get the BMW diagnostic tools for free once you're aware of these limitations (1)

As promised, below I bring over what we long ago had ascertained about how to test the brake pressure sensor (BPS) which starts around post #62 in the canonical ABS BRAKE DSC trifecta thread and continues to about post #473 of that same thread.

Post #62:

Originally Posted by bluebee

This errant brake pressure sensor reading might be the reason some people replace sensors and the ABS module yet still have the three warning lights lit.

It seems there is a "pressure sensor" P/N 34511165467 $111.68 screwed into the back of the hydro unit in the engine compartment of the 2002 E39.

I wonder what an "offset" error indicates?

Any advice how to test this ABS hydraulic unit "pressure sensor"?

Post #64:

Originally Posted by bluebee

Your documents are FANTASTIC to help me understand the system!
- BMW_ABS_BRAKE_DSC_TRIFECTA_COMPONENTS.PDF

One way, if we can figure it out how to do it, to test this front-brake pressure sensor would be to do a "flying lead" while the car is running to see if a 0 to 5 volt signal is generated while using the brakes.

But, it would be easier to do a static test of this front-brake pressure sensor with the car ignition off from the ABS module connector.

Do you have any ideas how to test the "offset" of the front-brake "pressure sensor" that is screwed into the rear of the ABS hydraulic module.

Post #65:

Originally Posted by Max_VQ

Here is my thought on testing the sensor...
Since the output has a range of 0-5volts and it measures pressures from 0-250bar we can figure out the scale of the output:

250 bar = 3,626 psi
3,626 / 5 = 725.2 psi/volt or 1.3 mV per psi.

With the ignition on, measure the voltage on the pressure sensor while someone is pressing hard on the brake pedal.
My guess is that should create about 3,000 psi of force and should show around 4.13 volts. At rest it should show very close to 0 volts.

Post #66:

Originally Posted by bluebee

The hardest part, I think, will be to access the signal while the components are attached to the car.

Do you or 540iman know the pin numbers on the ABS connector for the three pins are (power, ground, signal output)?

BTW, your math seems perfect. Witness this Corvette Bosch 5.3 description whose math jives with your estimates quite nicely!

Post #67

Originally Posted by bluebee

Looking at the front brake pressure sensor at the rear of the hydro unit, I see the brake pressure sensor harness has three wires that we can tap into to test live voltage.


I did notice some "stuff" on the edge of the clip, so I cleaned that off and reconnected the clip.

... Error 81 in the posted Bosch 5.7 codes appears to be:
81 Pressure Sensor

Post #68:

Originally Posted by Max_VQ

If you did test for voltage at the pressure sensor with the ignition disconnected, then it makes sense that the module would throw a "pressure sensor" code, as it did not see the pressure sensor.

Plug the sensor back in, and reset the ABS module. I don't think the "81" will come back.

To test the output voltage, you could make up something like this: http://www.picoauto.com/breakout-lead.html

Post #69:

Originally Posted by bluebee

I agree that the diagnostic information from the OBDII port seems to change so I ran it a least a half dozen times, resetting each time.

For example, from the OBDII port, after reconnecting the brake pressure sensor connection, I did reset the original error code which, weirdly, initially came back as:
81 Einlassventil : vorne links (inlet valve, front left)
105 Brems Licht Schalter (brake light switch)

But then, after resetting the OBDII error, the ABS test consistently reported:
81 inlet valve, front left
(even after multiple error resets)

I'm guessing, from the pic below, is it that you shove the leads into the BACK of the brake pressure sensor while the sensor is connected to the hydro unit?

If so, this would be the suggested procedure to test a brake pressure sensor (but now my error is "inlet valve"???).

1. Turn off ignition
2. Remove the three-wire brake pressure sensor electrical connector (do not remove the sensor, just remove the connector!)
3. Check the brake pressure sensor (male prongs) for continuity with a DMM we don't know what values to expect???? but we may as well write them here
4. Turn ignition on (car not running)
5. Test brake pressure connector (female side) for 12v supply voltage
6. Turn ignition off
7. Reconnect brake pressure sensor
... here's where the test jig comes in ...
8. Stick three 1.5mm test probes into the BACK of the connected brake pressure connector in situ
9. Attach your DMM voltmeter to the power & signal wires (we need to determine which color is power, ground, and signal)
10. Turn ignition on (car idling)
11. Ask an assistant to press on the brake pedal as you measure the voltage change
12. You should see 0 volts to 5 volts corresponding to 0 to 250 bar pressure (0 to 3,626 psi) changes in the front brake lines as your assistant presses on the brake pedal from soft to firm.

How does this look for a test sequence for the brake pressure sensor?

The questions for confirmation are:

A) Is the suggestion to stick a probe into the BACK of the brake pressure sensor to measure voltage sweep in situ?

B) Does anyone have an idea what part the diagnostic message "inlet valve, left" is referring to?

Post #70:

Originally Posted by Max_VQ

Not quite... The cable is like a "Y" adapter:
(1) one wire (for example the top left of the Y) goes to the pressure sensor
(2) The same coloured wire as (1) goes to the connector that attaches to the sensor. (this is like the top right of the Y)
(3) the "bottom of the Y" goes to your meter. (same colour as 1&2)

This allows you to insert your meter/scope in between the sensor and plug.

YOU MUST BER VERY CAREFUL NOT TO MIX UP THE WIRES. YOU COULD SHORT SOMETHING OUT AND KILL YOUR ABS controller or DME.

You don't have to buy this, as I think it is expensive, but you could make something for much less.

Post #71:

Originally Posted by bluebee

Is it a splice. Like in the picture below?

I agree, for DIY use, the professional jig is pretty expensive (for what it is) ... so I think I will scour the local auto parts stores (hardware stores?) for a tiny SQUARE wire to fit into the female end of the brake pressure sensor connector.

Does anyone know where I can get a secure tiny square female connector I can use for the male end?

Post #72:

Originally Posted by 540iman

Bluebee, why are you making this more difficult than it is? Start the car or turn the key to position#2. You don't need an adapter or any tool!

You can strip a tiny bit of insulation away from each of the three wires if it is easier than to just back-probhe into the connector.

Attach your ground VOM wire to any good ground- use the strut tower nuts.
Take the remaining positive meter lead and test each of the three wires one at a time.
One will read 0.0, one will read approx. 5.0 VDC, and one will read somewhere less like maybe .5 VDC or somewhere close.
Then, start the car and have someone press the brake pedal while you observe the wire voltage that was less than 5.0VDC.

This will be the follower voltage. That's all there is to it..

Post 75:

Originally Posted by bluebee

It's confusing to me that two different decoding systems are needed, but, that's what it appears to be (Bosch & BMW).

It's also confusing to me that the first error (front brake pressure sensor) doesn't seem to be related to the second error (front left inlet valve, whatever that is).

As another datapoint, since I worked on it this weekend, the three yellow warning lights have taken MUCH LONGER to light up (ten or fifteen minutes sometimes); and they seem to flicker on and off while I'm driving. Before, the three orange warning lights went on within a couple of minutes and stayed on.

I'm beginning to think it might be the ABS module after all ... which might explain the inconsistencies as it "tries" to sense information from the twenty or so inputs that it has to work with.

I'll keep an eye on it and test further, given your advice.

Meanwhile I'm looking around in hardware & electronics stores for that tiny 1/2 mm square female connector that is the most important one to find. You'd think in the Silicon Valley that would be easy to find ... but not yet.

I'm going to try the following local connector companies:
- Molex over in Milpitas (920 Hillview Ct, 408-946-4700)
- JDR in North San Jose (1723 Rogers Ave. Unit O, 408-392-0100)
- JAE in Santa Clara (5201 Great America Pkwy # 320, 408-562-6003)
- Dicar in San Jose (1285 Alma Court, (408) 295-1106)

Does anyone know the "specifications" of that particular male-female square washington-monument shaped male pin?

It looks like the pressure sensor connector male posts are little washington monuments about a half millimeter by a half millimeter in square size and about a half inch long. It seems to be the same connector that is on the ABS connector. Does anyone have a spec for this that I can fax to the companies above before I stop in?

Post 77:

Originally Posted by bluebee

... stuff deleted ...
b) Choose how to test the pressure sensor (either strip the 3 wires or build a jig; I prefer to build a jig if I can - it's more graceful)
...
d) Test the pressure sensor ... if bad ... replace (bearing in mind a $100 test jig for a $100 part isn't feasible)
...

Post 78:

Originally Posted by Max_VQ

You could try Digikey.com...

Maybe something like this:
http://search.digikey.com/scripts/Dk...name=A30305-ND

Post 79:

Originally Posted by 540iman

If the pressure sensor is at all an issue, it should be tested right at the connector for the three wires going into it.

If two or three folks compare what their readings are without any brake pressure and with a firm depression of the pedal, I would think we could tell if there was enough of a difference to be an issue.

The measurement should be taken with the car running and the trailing wire (of the three) identified. The reading would be from this wire to ground and done by using a "piercing" probe on the VOM set for VDC.

Post 80:

Originally Posted by bluebee

I already determined a 20 AWG wire would fit into those tiny square holes.

I might even buy 42 of them to make my own ABS connector breakout box so we can test the entire ABS system in situ!

I think I'll order 42 (for the ABS connector) plus 3 (for the brake pressure sensor) plus a few extras (just in case).

Post 85:

Originally Posted by bluebee

MY SAGA:
- ABS/BRAKE/DSC lights are still lit (so I have not solved anything yet)
- All four wheel sensors tested good (thanks 540iman for the definitive tests!)
- Diagnostic tools first indicated a Bosch "114 Pressure Sensor Offset"
- Now they implicate a BMW "81 Einlassventil : vorne links (inlet valve, front left)"
- I ordered the pins to test the brake pressure sensor (but I don't think it's that anymore 'cuz the diagnostic tools no longer implicate it).
- Nobody seems to know what an "inlet valve, front left" is, let alone how to test it.

MY PLANS:
- I plan on testing the brake pressure sensor and reporting the results (thanks for your comparison results!)
- I hope to locate the "inlet valve, front left" and test it (somehow)
- If both test good, I plan on sending my ABS control module out for a rebuild

Post 86:

Originally Posted by bluebee

The more I look this up, the more I think you're right that it's the ABS module masquerading as a series of red-herring errors.

For example, this BMW Bosch 5.7 error resolution says "Inlet/Outlet valve" fault codes could actually indicate a bad ABS control module (having nothing to do with the inlet or outlet valves themselves).

Note: At this point, I realized the reading for the bad brake pressure sensor 'was' a red herring - so I had the ABS control module rebuilt by ATE for about $105 (plus $25 UPS shipping to ATE) - and the trifecta went away (never to return).

Since I never actually TESTED my brake pressure sensor, I leave it to those who actually will test it to assemble the scattered information above into a simple test DIY suitable for others to just follow, step by step.

Here's what I wrote, way back in post #463 of the canonical ABS thread in August last year:

Originally Posted by bluebee

Please note post #67 of this thread which shows I originally attempted to test my brake pressure sensor (based on 'bogus readings from Carsoft 6.5).



I read what the inputs were, what the outputs were, how it was wired, where the wires went, how to make a test jig, what readings to expect under brake pedal pressure, etc.



In the end, I didn't test the sensor (too much effort for no gain whatsoever); and, I was vindicated in my approach.

Of course, had the indicated brake pressure sensor actually been the culprit, my efforts would not have culminated with rebuilding the ABS control module - and they would probably have included those tests that we came up with to test the brake pressure sensor.

It's most likely your ABS DSC module. I just went through this exact same thing on my '02 530i. At first I sent my module in to get rebuilt but the repair shop didn't do a good job so I ended up buying a new one from EACeuroparts for $489. If you go the rebuilt route, use BBA-reman or module masters. If you go the new route, you wil have to get the module coded to your DME either at the dealer or from an indy shop (about an hour labor cost).

As an aside, I took it to the dealer after I got my rebuilt module back because I still had the three lights (ABS, DSC and brake). Dealer did diagnostics and said my hydro pump inlet valve was stuck and wanted to replace the whole unit (DSC module and hydro pump) at a reasonable cost of $2800! After they revived me from falling on the floor, I said no thank you.

I still thought it was the faulty module so instead of sending my rebuilt module back to be re-repaired under warranty, I just bit the bullet and ordered a new one. However, I did get a full refund for the faulty module repair. I also, sent it back to them after the fact so they can diagnose my faulty module. I haven't got any feedback yet.

All my three lights are out now! And car drives fine. HTH with your issue.

Originally Posted by bjay27

Is there a pin from the ABS pinout that corresponds to this wire for testing?

Here's the pinout as listed in the canonical thread, starting at post #222.

Post #222:

Originally Posted by bluebee

...
I don't know why I do these things, but I unboxed my pristine electrical Bently, headed over to pages ELE-15 to ELE-18 (for DSC) and to pages ELE-21 to ELE-27 (for ASC), listing in text what the wiring diagrams showed me.
... stuff unrelated to the brake pressure sensor removed ...

ELE-15, ABS/Traction Control, ABS/DSC control module (A65), (3 of 4):
Pin 25 ABS/DSC control module, A65 = 0.75 WS/BL --> Pin 1 of X1654, B76 Pressure sensor

Pin 26 ABS/DSC control module, A65 = 0.75 BL/WS --> Pin 2 of X1654, B76 Pressure sensor

Pin 42 ABS/DSC control module, A65 = 0.75 SW/VI --> Pin 3 of X1654, B76 Pressure sensor
...

Reading that 500-post thread further ... and culling out brake pressure sensor test tidbits so that the OP can write a DIY ...

Post 250:

Originally Posted by jeffstri

...Fault codes are not intended to be diagnoses. Whatever scan tool you use, a fault code(s) rarely leads to a solution without more testing.
...
The software a BMW dealer uses to read fault codes (INPA, GT1/DIS) also allows measurements (e.g., brake pressure) and activation of various components (e.g., ABS solenoids, pre-charge pump). ...

Post #252:

Originally Posted by 540iman

...the diagnosis, as given by some equipment, will be wrong and may lead to replacement of parts and the problem will still be there.

You must, as you say, get more tests that will further define the problem.

Initially, I think I was of the opinion that 90% of the time, problems with the trifecta lights will ultimately be a bad module and 10% a bad sensor. I now think it is more like 70-30.

I have found more bad sensors lately, but have NEVER found one that tested bad and was really good or vice verse.

I would bet a paycheck that if Agent15 or anyone for that matter has the sensor in his hand and can get meter leads deep into the sensor such that he can test either in ohms or voltage bias forward and reverse (diode test) and the sensor tests bad, that it IS bad.

Conversely, if it tests "ok" meaning there is a definite difference when meter leads are reversed and sensor does not test open or shorted BOTH ways- it is NOT the fault.

The main point is that any diagnostic equipment that does not test by going between the sensors and the module simply can not definitively see a bad module from a bad sensor.

It is as simple as stating that a blood pressure cuff can not diagnose whether high blood pressure reading at a cuff is because of a faulty heart or a clotted artery.

You must somehow get readings I suspect (certainly not a Dr.!) between the heart valves or whatever you do to isolate. If your Indy or dealer hooks up to your OBDII port or your 20 pin and definitively tells you that you have a bad wheel speed sensor, that person is full of doo-doo.

Only thing you can diagnose correctly 100% of the time from either of these two points is a bad module due to a communication error or similar. They can not tell a bad input (sensor) from a bad module at these test points.

Post #253:

Originally Posted by Car1951

My problem (The best of my knowledge) come from Pressure Sensor, I purchased it but before to replace it I would like to ask you if I need to bleed the rear brake (my diagram says that Pressure sensor is connected on the rear brake line of the Hydro unit).

In other words; when I will replace the pressure sensor must I bleed the rear brake?

Second, are there a tool that we can use at the moment that we need to replace the Pressure Sensor without disconnect the pipe line connected to the Hydro Unit?

Post #254:

Originally Posted by 540iman

Faulty pressure sensor indication can be another one of those input sensor errors that end up really being the module.

You need to back-probe pressure sensor to see what your output is with no pedal pressure and then with pedal pushed hard.

It might be your sensor, but just like any other input to the module, unless you physically test that sensor don't be surprised if your error remains. Bad pressure sensors are very uncommon.

Post #255 (edited to make sense because the poster below didn't make any sense):

Originally Posted by Car1951

My conclusion is based on a test of brake pedal output volts at the pressure sensor which should be above 4.47 volts ... and linear volts depending on the driver pressure on the brake pedal.

Post #256:

Originally Posted by 540iman

you did not say how you tested or arrived at conclusion and many just go by carsoft error or some such crap (and I have Carsoft).

You back-probed and did your homework.

Not sure if it should be zero with no pedal, but you are dead nuts on...it should raise with full pressure toward 5VDC. May be 0-4.5VDC or .5V-5.0 point is that it varies proportionately like you said as pedal pressure changes.

Post #257:

Originally Posted by Car1951

...
I will need to replace the pressure sensor but there are a little room to do this job because line pipes and location for BMW X5 2001, I don't want to un-install a line pipe I believe that I will need to bleed the system. If I do so.

Brake Pressure Sensor

A brake pressure sensor is installed in the hydraulic unit to determine the degree of pressure build-up for Dynamic Brake Control. It is installed in the front axle brake circuit. The sensor receives its power supply from the DSC control module and produces a linear voltage input signal ranging from 0.5 to 4.5 volts depending on how hard the brakes are applied.

Brake Light Switch (BLS)

The brake switch is an input to the DSC Module to inform it that the brakes are
being applied. If the signal is received during an ASC regulation then brake regulation is interrupted.

Post #258:

Originally Posted by jeffstri

Car1951, how did you arrive at the conclusion that your pressure sensor might be bad? Did you retrieve any fault codes? If so, what software did you use and what were the codes? Or did you just start doing the tests described in this thread?

If you did get a fault code suggesting a pressure sensor problem and your X5 has DSC 5.7, I suggest you perform the pressure sensor test via the precharge pump described in the WDS:



You will need INPA or GT1/DIS to perform these tests, but you can get the software free and interface and cables for less than $50. (See my post #250 above.) That's a lot cheaper than throwing money at parts based on the voltage test described in this thread, which has never been verified.

I'm struggling with a similar problem right now. I originally got Carsoft code 86 (outlet valve front right) but now I'm getting 115 (ABS/ASC control unit internal error). (I don't find Carsoft errors to be reliable - I'm only reporting them for completeness.)

INPA consistently shows:



and DIS shows:





The brake pressure (measured with DIS) with the pedal not depressed is 0 bar, so DIS says the possible problems are a lose contact or high frequency interference:


I haven't found any loose contact and I have no idea what might be a source of high frequency interference. So I'm going through the WDS "Pressure sensor test via the precharge pump" shown above, but I'm a bit confused by the two statements "Fault currently not present" - I suspect that the second should read Fault currently present. The fault is present after a precharge pump test and the pressure with the engine running and maximum force on the brake pedal is above 80 bar (about 85-100 bar). I can activate the precharge pump with INPA - I can hear it run - but I guess it's output is too low. The only thing I haven't done yet is "ventilate DSC hydraulics," i.e., bleed the brakes with DIS, but it's looking like I'll need a new precharge pump.

In answer to your question about bleeding the brakes, you will need to but it must be done with activation of the DSC module using DIS:

Post #324:

Originally Posted by truelies

Hydraulic Pressure Sensor and the brake Pressure Sensor are same thing, is that correct?

Post #327:

Originally Posted by bluebee

That's how most of us seem to be using the terms.

Post #325:

Originally Posted by GSned

I have a friend who works for BMW who hooked up the car to the diagnostic machine which said it was a brake pressure sensor fault....I tried testing the pressure sensor by back probing and noticed I was getting a constant 4.8V from both live wires even when the brake pedal was being pushed with the engine running.

from what ive read the voltage should change depending on the pressure applied to the pedal. Does this sound like the pressure sensor or could this still be a module issue?

Post #326:

Originally Posted by bluebee

...Please post a picture of your flying-leads setup as you will be the FIRST person here to have done this so you should give back to the team and enlighten us.

Post #334:

Originally Posted by GSned

Bluebee,
You asked me to post pics of my set up for back probing the pressure sensor, unfortunately I opted for a very crude method of piercing the insulation on the 2 lives and checked the voltage that way....
Im not a mechanic or tech, I opt for problem solving using research and a common sense approach which in my case told me it wasn't the pressure sensor due to fact I had various different symptoms at different times, I guessed if it was sensor then it would be same symptoms all the time.

I put this theory to my mechanic who replied saying" sometimes you have to trust the machine" Easy when its not his cash I guess!

Anyway I stuck with my theory and got my hands on a second hand module, fitted it and problem instantly sorted.

Post #328:

Originally Posted by jeffstri

What makes you think the BMW diagnostic machines are "hooked up wrong?"

They scan for fault codes and then provide a series of tests to determine what the cause of the fault is.

It's no different in principle from what you and Bill recommend, except that the BMW diagnostic equipment allows you do more extensive testing than just measure resistances and voltages.

For example, the software that the BMW diagnostic machines use (e.g., INPA, DIS) allows the tech to activate the ABS/DSC components (e.g., DSC pump and valves, precharge pump) and measure pressures, among other things.

The pressure sensor test via the precharge pump is much more reliable than the pressure sensor voltage tests that you and Bill have suggested.

Post #460:

Originally Posted by bluebee

BMW ABS/ASC Bosch 5.7 Table of error codes:
... very many unrelated codes deleted ...
81 Pressure Sensor
114 Pressure Sensor Offset
115 Pressure Sensor Interface

Post #462:

Originally Posted by jeffstri

...
Fault codes for the ABS system are stored in the DSC module, and represent abnormal communications with the various sensors/components of the ABS system. Thus a wheel sensor code means that the DSC module sees an absent or or implausible wheel sensor signal.

The problem is not as Bill says, "the way they're hooked up," but the fact that something in the circuit that processes the wheel sensor signal within the DSC module (such as a break in the internal wiring, or a burned out resistor, etc) will appear the same to the DSC module as an abnormal signal originating in the wheel sensor or in the wiring between the sensor and the module.

The same type of thing can happen with any of 20 or so other control modules, but unfortunately internal problems in the DSC module seem to be far more common than in the other control module....

Post #473:

Originally Posted by bluebee

Quick99Si wrote up an excellent diagnostic treatise today where he successfully employed the Autologic Diagnostic Scanner, Peake, and he began using EDIABAS, INPA, GT1 diagnostic tools:
- My experience with the trifecta lights (ABS/DSC/Brake)

Reproduced below are just some of the diagnostic tests he ran!
... unrelated stuff deleted ...
TEST 4A: Brake Pressure Sensor
This sensor is located on the upper back side of the ABS distribution block. Mine looked to be in good condition despite the semi-awkward factory angle of the connector wiring, but the troubleshooting sequence asks to ensure that it detects smooth and linear pressure. I jumped into the car, started up the engine, and very very slowly applied the brakes while my buddy logged the data. It showed a linear increase in brake pressure and a max of something around 200 bar at full depression and idle engine speed. BPS is OK.
... unrelated stuff deleted ...

In post #522 of the canonical ABS trifecta thread, I've added more detailed pictures of the connectors and I've improved the pinout annotation at the ABS harness connector with respect to the diagram above.

Originally Posted by bluebee

While that may be true, it still would be nice if the OP can assemble the information above (and below) to create a simple step-by-step DIY for testing the brake pressure sensor.



Whom, may I ask, did you send yours to?
It would be good for you to post the results here:
- Summary of all known ABS control module rebuilder options & new suppliers with prices (1)

I sent mine to ASI/ATI autoecu. I chose them because they were the closest to me even though some on another forum ranked them third behind BBA-reman and module masters. I should have followed their advice. ASI/ATI does have a lifetime warranty and money back guarantee as well but I couldn't wait to send it in for another repair since my car was up for the annual safety inspection.

Ok Thanks everyone for all the info, I did not find the later threads discussing the brake pressure sensor test on my prior searches so thank you for pointing them out.
Bluebee thanks for the condescending ohms law refresher, but more importantly, you helped me prove the DIY for the brake pressure sensor is still incomplete.
Regarding my actual car issue, I haven’t hooked up any software/code readers so this is purely a grassroots troubleshoot, no need to get into the "fancy shmancy" software circle jerk debates.
Also, I’m not sure what you mean by "flying leads", I just use the wire pierce method with a sharp pinpoint tester/lead to test wires "in-situ".
Plan:
I will try another DMM for Ohm/Diode testing; maybe my old fluke is going out on me.
I will post again with a good DIY on brake pressure sensor testing with pictures and labels, as still no-one has discussed which exact wires to test.
I will try and correlate the 3 brake sensor wires to any corresponding pins on the ABS/DSC module pinout

Ironically, I performed my In-Situ test, just as 540iman described:
Originally Posted by 540iman
Bluebee, why are you making this more difficult than it is? Start the car or turn the key to position#2. You don't need an adapter or any tool!
You can strip a tiny bit of insulation away from each of the three wires if it is easier than to just back-probe into the connector.
Attach your ground VOM wire to any good ground- use the strut tower nuts.
Take the remaining positive meter lead and test each of the three wires one at a time.
One will read 0.0, one will read approx. 5.0 VDC, and one will read somewhere less like maybe .5 VDC or somewhere close.
Then, start the car and have someone press the brake pedal while you observe the wire voltage that was less than 5.0VDC.
This will be the follower voltage. That's all there is to it..

I did exactly this, and I’m telling the E39 folks all you have to do is turn you car or key on, ground your DMM, then pierce the wire exiting the rear of the brake pressure sensor with white on it and measure the voltage change with brake pressure. Prior posts suggest it should fluctuate between .50-4.5 volts. Mine does not, I’m going to risk replacing it.

I’m almost certain the sensor should function regardless of the ABS/DSC unit,(as long as it has input voltage) as its function is to relay to the ABS/DSC unit an increased output voltage proportional to the hydraulic pressure exerted, and the ABS/DSC unit processes this signal.
To highlight an important point Bluebee and others keeps making, if you are reading code errors which point to the brake pressure sensor, you are only testing the ABS/DSC unit's 'interpretation' of the sensors signal (as is the case with any of the sensors) and so a fault code could mean a bad ABS unit or sensor or even both. Which is why this DIY is somewhat important, as it bypasses the ABS/DSC unit.

Originally Posted by bjay27

I’m almost certain the sensor should function regardless of the ABS/DSC unit,(as long as it has input voltage) as its function is to relay to the ABS/DSC unit an increased output voltage proportional to the hydraulic pressure exerted, and the ABS/DSC unit processes this signal.
To highlight an important point Bluebee and others keeps making, if you are reading code errors which point to the brake pressure sensor, you are only testing the ABS/DSC unit's 'interpretation' of the sensors signal (as is the case with any of the sensors) and so a fault code could mean a bad ABS unit or sensor or even both. Which is why this DIY is somewhat important, as it bypasses the ABS/DSC unit.

So if you want to bypass the ASC/DSC unit because you say the sensor should function regardless of the ABS/DSC unit why don't you pull out the module and run the test again on the brake pressure? This will eliminate the module as a unknown variable. If the test still does not work then it might be the module?

The modules on these cars are a common failure due to heat issues. Why doesn't the OP send in his module to have it tested by BBA remanufacturing? I think they only charge $55. Then we will know for sure whether or not the module is the issue. You can still drive the car without the module in place. Good luck.

no-one has discussed which exact wires to test

To help answer that question, I annotated this picture (copied verbatim from post #522 of the canonical ABS trifecta thread).

Let me know if I made any mistakes or omissions.

I just referred someone from the thread below to this thread above to test his hydraulic pressure:
- E39 (1997 - 2003) > Braking Problem/Issue HELP!

And, I was wondering if the OP has an update for us yet?

I have read the existing info and have some updates on test procedure and different values than those anticipated. Sorry couldnt get pics to loadup so have links.
1.Brake Pressure Sensor
http://www.mediafire.com/i/?559fkika49tlu99
Voltage in/out to sensor is supplied by the ABS/DSC module (thus disconecting the ABS pinouts = cannot test voltage in/outof sensor)
- Reported power scheme via Bluebee:
Pin 1 (white/blue) variable 0.5-4.5V DC @ 0 to 150bar
Pin 2 (blue/black) ground 0V DC
Pin 3 (violet/black) power 5V DC
My results:
Differ from those above, I obtained a variable voltage from sensor from Pin2 (blue/black) not Pin 1 White/Blue, which was a ground for me.

1.Brake Pressure Sensor Testing Algorithm
http://www.mediafire.com/i/?cn71xxzlrrancar
Tools: DMM + Peircing probe, method to press down brake pedal
notes: would test sensor with car running, vs key on, as voltage output seemed more proportionate with car running.
1.With key on locate brake pressure sensor wire harness, and disconect
2.Ground DMM and ensure pin3 (violet/black has 5V, can also test ground pin)
3.Ensure you have access to pin 1 (white/blue), pin 2 (blue black) wires
-either via peircing probe, or by removing tiny portion of wire insulation
4.Start vehicle (can also turn key on, but more consistent results found with key on)
5.Ground DMM and use other (+) probe to test pin 1 (white/blue), pin 2 (blue black) wires
6.Repeat procedue while slowly applying brake pressure
7.you should find .5-4.5V change
http://www.mediafire.com/i/?683q898fw0gsiq4

Testing pins (male) to test Brake pressure sensor
I was unable to get any readings from the male pins 25,26,42, on ABS/DSC unit corresponding to the brake pressure sensor. I tried this with the key on/off and car running. It appears there may be a failsafe that shuts down the pins when the harness is disconnected. Maybe turning key on with harness connected, then disconnecting may fix this issue? That said, I do not feel placing flying leads from 25,26,42 for testing would work, as atleast in my situation, the ABS/DSC pins dont function unless the harness is plugged in.(Metaphoricaly similar to the heisenberg uncertainty principle). Also from the time wasted, I could have already tested the wires at the sensor.

My Findings
Pin 1 (white/blue) ground
Pin 2 (blue/black) variable 0.6 - 2.0V with brake pressure
Pin 3 (violet/black) power 5V DC

My Conclusion: Either the diagram submitted by bluebee is incorrect, or somehow my model (02 530i) swapped pin1 vs 2 on the brake pressure sensor, or my ABS/DCS unit is somehow bad. While my variable wire only increases to 2V, and not the theoretical 4.5V, I am hesitant to say it is faulty, as the increase is linear.
Given my Brake light trifecta symptoms and all the input from everyone here, it appears my ABS/DSC truly may be faulty. I am still waiting to purchase a new DMM to repeat sensor diode testing, and setting up a laptop to run INPA for more info.
Thanks again to everyone for all the help and info!

Originally Posted by bjay27

Either the diagram submitted by bluebee is incorrect, or somehow my model (02 530i) swapped pin1 vs 2 on the brake pressure sensor, or my ABS/DCS unit is somehow bad.

Do you have any more information about which of the two above is the case?

BTW ... I'm not sure why this thread is hard to find (since it's in the bestlinks under "/brake pressure sensor") but maybe mjbennett9 (see below) can clear up the confusion about the diagrams and procedures we listed above.

Originally Posted by mjbennett9

I've been searching for a multimeter test procedure for the pressure sensor. It's been an illusive search/find so far, but will keep looking.

It looks like our friend will be replacing his brake pressure sensor based on this post today ...
> E39 (1997 - 2003) > how to change e39 brake fluid

Originally Posted by mjbennett9

Bavarian Technic says it's code 81 (Pressure Sensor Lead). I tried the DMM but since I live alone, it's too much. I am assuming the sensor is in fact bad. However, searching for a repair/replacement DIY for the pressure sensor seems to be as elusive. I've been googling and searching the forums, but have yet to find a DIY on pressure sensor replacement. Anyone replace a brake pressure sensor on a 2002 e39 525i before? Not sure if I need to just unplug 3 wire connector, unscrew, screw new one in (torque?), plug connector in, and off I go?

Sensor will be here Wednesday. I'll hopefully replace it after work, and I leave for Boston Thursday night. I'm hoping this gives me my ABS back for the wintery drive East. FYI, I've already replaced the ABS/DSC module with an ATE rebuilt module :-)

So, I asked him to post a quick DIY on the BPR R&R here to help others.

For the crosslinked record, there is an interesting discussion where users debate replacing the brake pressure sensor (perhaps errantly) ... when they try to diagnose the ABS trifecta ...

More details here:
- E39 (1997 - 2003) > ship off my abs module today

Seems like there are various versions of the same theme. I have a 2006 X5, and have just been hit by the Trifecta.
I sent it to BMW, and they have indicated that it is a problem with the Transfer box. I have difficulty believing this (probably 'cause I dont want to).
First error message was "4x4 DSC disabled", now the msg is "check brake linings"
Could the problem really be the Trf Box?

But have had misdiagnosis in the past few months. My air con was slow to start working, so I sent it into an upmarket area dealer who told me the compressor was faulty and needed to be replaced. Not convinced I got a second dealer who told me it was the electric radiator fan. Two "reputable" diagnosis, both $1500 each to repair, had I followed the first I would have doubled my cost!

Hence my lack of faith in the diagnoistics of the local dealers.

Originally Posted by GX5

2006 X5

I know nothing about the X5 so take anything I ever say with a grain of salt.

Given BMW shares components, and assuming when you say 'trifecta' you mean the brake/abs/dsc lights (or X5 equivalents), then the recommendations for you would be the same as for anyone else with an E38 or E39 or E46, etc.

1. Get out a good DMM and a thin wire and test the wheel speed sensors for consistency. If any are off from the rest, then test them closer to the wheel. If all test good, move to the next step.

2. Get out a good sharp utility knife and toothpick and slice open the ABS control module. Probe the 7th silvered wire (plenty of pictures on this forum) to see if it lifted off its gold bondpad.

3. If it lifted, either reaffix it yourself (not easy to do) or tape it all up and ship it to the top three rebuilders for them to reaffix it. If they can't reaffix the wire, they'll give you options.

4. If all else fails (this is only a small percentage of the time), then buy a new ABS control module for about $400 and change (see the thread on where to get 'em).

Total cost for diagnostics: Roughly about fifty to a hundred dollars for a good DMM (if you don't own one already).
Total time for diagnostics: Roughly about an hour or three, in toto, from start to finish (the initial test takes only a few minutes).

Thanks, will do a check as you have advised.
Does the reference to the Transfer box make any sense to you

Yet another (probably bogus) GT1 implication of the brake pressure sensor was posted today over here:

Originally Posted by David330gr

Hello everybody and a big thanks for the helpful informations..!!!
bluebee, I am reading the last 3 days your posts about the ABS BRAKE DSC trifecta and i cannot found my problem, mabye i am not a good reader and for this reason i am sorry to ask you about my problem, mabye you have post solution and i have miss it.
My problem is: ABS BRAKE DSC trifecta ,I have Gt1 and i have error message on the ABS pressure sensor,When i measure the pressure sensor with Gt1 i can read 290bar,after disconecting the pressure sensor i have the same measurement. I am measuring separately the sensor as your instructions and the sensor works.Can you help me please?

Thanks...!!!

I have a 2003 e39 530i. The three lights come on 5 - 10 minutes. Of driving and stay on until the engine is stopped and restarted. I had an auto-logic scan that repeatly sourced the error to the brake pressure sensor. The Trifecta is neither triggered by bumps or use of the brakes, I suspect that the error count reaches a threshold and throws the fault.

I was skeptical because of all the reading about the Trifecta being a wheel sensor or the control module. Note speedometer and cruise control works. ABS brakes work as well.

I back probed the three wires going to the sensor. Violet/black measures 4.36 against Blue/Black and 5 volts against the strut tower nut. The Violet/Black measures 0.64 volts against the strut tower nut. (Interestingly the net voltage of the two wire is 5 volts.)

I measure the White/blue with no brake pressure and the reading was .64 volts. With some brake pressure the reading was 1.25 volts. And with a lot of pressure the reading was 2.5 volts.

So do I spend the $100+ bums for a new sensor or is there a furred trouble shoot on the control module. I think I remember a post maybe by Blue Bee pointing to two suspect so feet joints and others to the fine wires being lifted.

Originally Posted by Microflight

I back probed the three wires going to the sensor. Violet/black measures 4.36 against Blue/Black and 5 volts against the strut tower nut. The Violet/Black measures 0.64 volts against the strut tower nut. (Interestingly the net voltage of the two wire is 5 volts.)

I measure the White/blue with no brake pressure and the reading was .64 volts. With some brake pressure the reading was 1.25 volts. And with a lot of pressure the reading was 2.5 volts.

This is great information!

To my knowledge, unfortunately NOBODY yet has posted a PICTURE of the actual testing procedure.

Nor, to my knowledge, has anyone actually 'needed' to replace the brake pressure sensor when they had the trifecta.

So, whatever YOU learn, please let US know as you're on ground rarely traveled!

Originally Posted by Microflight

is there a furred trouble shoot on the control module

The only test of the control module that I know of that is even close to reliable requires you to Exacto knife open the control unit to lift off the cover.

Once the cover is off, then toothpick the 7th aluminum wire to see if it has lifted off its gold bondpad. If it lifts up, then you KNOW it's the control module.

Gory details with dozens of pictures are found by typing /abs F3 in the bestlinks:
- How to test and repair an E39 Bosch 5.7 ABS control module (1)


Note: if you decide to send out the ABS control module to the rebuilders, this will help:
- What are all known options when your ABS control module is bad (1)

For the cross linked record, and to keep the BPS tests together, I bring this over from another thread which was opened today, asking the same questions:

Originally Posted by chuckjim

OK so just a bit of background. I have a 2000 528i with 140k and trifecta abs lights on and off speradically. Replaced faulty right front wheel sensor and still having issues. I have an OBDII tester that I think is giving me a 15 fault code which could be ABS pressure sensor? What is easy way to test sensor? I have multi meter. Do I test sensor or plug going into sensor? If plug in then do I use paper clip to stab into connector? I know Im testing voltage and it should be 0 volts at rest with ignition to on but not running and 0-5v with firm pressure applied to brake. Anything else?

To test the brake pressure sensor, I originally used a needle soldered to a wire that I could attach to a meter. I then found 18-22 gauge snap on connectors at Radio Shack - 3 connectors for $2.00.

I have the sensor "flying wire" soldered to the white/blue wire. I was going to peal back the wrap around the three wires and snap in the connectors but I dont think I need any more information. The blue/black is a relative ground and I measured this to the strut tower with a measured value of 0.64 volts. The Violet/Black wire measured 5 volts against the strut tower ground. Measured against each other the value was 4.36 volts. This may be ok as it sets up a range of values from the sensor wire. In fact the no pressure value was .64 volts. Some pressie on the brake was 1.25 volts and substantial pressure was 2.5 volts.

If I do any more, I will bring the sensor wire into the car and put it to a meter with the meter grounded to something in the car. Then go out and test various brake pressure up to and including the ABS pulsing to see what values there are.

Having said all of this, I still suspect the module. For now, I will just live with the sporadic trifecta bug. I have established that it will not prevent the car from passing SMOG.


BTW, Blue Bee or anyone in the Bay Area, Do you know of a good Indy. I have a chance to buy an 2004 E46 6 speed with a performance package and would like to get it looked at before I go there from Goleta.

JKM

Originally Posted by Microflight

To test the brake pressure sensor

Thanks for providing the values as this is not something that is often tested on an E39.

Originally Posted by Microflight

I have established that it will not prevent the car from passing SMOG.

I have no idea other than I had a P0500 when I removed my ABS control module for repair, and, I, unfortunately, brought the bimmer for smog a day or two after putting it back, and it failed for that reason alone (see details below):

Originally Posted by Microflight

Do you know of a good Indy

The Indy that I hear most out is is Rays BMW, over on Lafayette Street:
http://www.raysultimate.com/

I was only there with a friend once, and I was very disappointed in his prices. His service was fine as he found the problem right away. But his prices were way up there, so, caveat emptor.

Or maybe I'm just not used to service pricing since I haven't been in for service since my bimmer was under the original warranty.

Blue bee thanks for the Indy rec. Fortunately or unfortunately the guy sold the 2004 E46 with the performance package and 6 speed transmission for what he claims as $11K - KBB excellent was $9100. Difference was a bridge to far.

In our area the hourly rates vary from $85 to $139. Some of the Indys have better equipment for diagnosing and for solving more complicated problems. If you have a reasonable idea of what you need, the additional cost can be reasonable.

On the Trifecta how does the code P0500 translate to English.

Thanks