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Old 01-06-2012, 01:19 PM
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bluebee bluebee is offline
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Location: San Jose, California
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Mein Auto: 02 BMW 525i M54 auto 130K
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 View Post
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 View Post
Your documents are FANTASTIC to help me understand the system!

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 View Post
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 View Post
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 View Post
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 View Post
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:
Post #69:
Originally Posted by bluebee View Post
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 View Post
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 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 View Post
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 View Post
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 View Post
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 View Post
... 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 View Post
You could try

Maybe something like this:
Post 79:
Originally Posted by 540iman View Post
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 View Post
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 View Post
- 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.

- 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 View Post
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 View Post
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.

Last edited by bluebee; 01-07-2012 at 11:22 AM. Reason: Added the relevant posts on how to test the brake pressure sensor one by one ...
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