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Old 01-15-2012, 08:36 PM
bjay27 bjay27 is offline
Registered User
Location: Dayton
Join Date: Jan 2012
Posts: 7
Mein Auto: 02 530
Brake Pressure sensor test Algorithm

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
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
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 should find .5-4.5V change

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!
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