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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

 
 
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Old 01-06-2012, 12:13 AM
bjay27 bjay27 is offline
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Location: Dayton
 
Join Date: Jan 2012
Posts: 7
Mein Auto: 02 530
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
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