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Old 01-06-2012, 02:03 PM
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bluebee bluebee is offline
Seek to understand,^Value
Location: San Jose, California
Join Date: Mar 2008
Posts: 25,236
Mein Auto: 02 BMW 525i M54 auto 130K
Originally Posted by bjay27 View Post
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 View Post
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 View Post
...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 View Post
...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 View Post
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 View Post
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 View Post
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 View Post
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 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 View Post
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 View Post
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 View Post
Hydraulic Pressure Sensor and the brake Pressure Sensor are same thing, is that correct?
Post #327:
Originally Posted by bluebee View Post
That's how most of us seem to be using the terms.

Post #325:
Originally Posted by GSned View Post
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 View Post
...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 View Post
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 View Post
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 View Post
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 View Post
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 ...
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Last edited by bluebee; 01-07-2012 at 11:26 AM.
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