01-07-2012, 12:06 AM
Join Date: Jan 2012
Mein Auto: 02 530
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 havenít 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, Iím 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".
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 Iím 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, Iím going to risk replacing it.
Iím 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.