ABS sensor repair, replace, options? - PeachParts Mercedes-Benz Forum

stephenson · #1 01-06-2013, 09:59 AM

Posting in both Tech and Diesel since is shared system for 1991.

Started working on ASD system and may have ended up with two malfunctions - the ASD Activtation (sensing slip) dash bulb was apparently out AND the right front ABS sensor was identified by setting a code in the ASD module.

The ASD module pin is reading 8.15 VDC for the right front sensor and 4 VDC for the left front sensor - both are supposed to be 4 VDC when the wheel is spun by hand at about 1 rev per second. In spite of this, I don't get an ABS malfunction light.

My understanding is the ABS module changes the millivolt AC signal coming from the ABS sensor to the DC signal the ASD module sees - anyone know if this is accurate?

If true, the AC signal coming from the wheel sensor could be in spec and the ABS ECU could be developing an out of spec AC signal that it is sending to the ASD module. The other alternative is that the ABS sensor is actually developing a bad signal, but it doesnt matter to the ABS ECU because it reads frequency not voltage?

I have reviewed 42-720 "Electrical Testing" test program procedures, but the alternatives don't show a mechanism for ABS ECU pin to ABS speed sensor pin testing with an ohm meter ... any advice on how to do this? I hate idea of just replacing a sensor when it's unnecessary ...and expensive.

Another alternative is to find a reasonably priced used sensor ...havent been able to locate one, yet ...

97 SL320 · #2 01-06-2013, 02:47 PM

Quote:

Originally Posted by stephenson

Posting in both Tech and Diesel since is shared system for 1991.

Started working on ASD system and may have ended up with two malfunctions - the ASD Activtation (sensing slip) dash bulb was apparently out AND the right front ABS sensor was identified by setting a code in the ASD module.

The ASD module pin is reading 8.15 VDC for the right front sensor and 4 VDC for the left front sensor - both are supposed to be 4 VDC when the wheel is spun by hand at about 1 rev per second. In spite of this, I don't get an ABS malfunction light.

The sensor output should be AC. The actual sensor is a coil of wire wrapped around a magnet, the end of the magnet is near a toothed wheel. As the teeth pass over the magnet, the voltage swings from positive , through zero to negative.

Now, if the module is connected there might be a DC component if the module uses that to assess continuity. I don't know if MB uses this strategy or not.

Quote:

Originally Posted by stephenson

My understanding is the ABS module changes the millivolt AC signal coming from the ABS sensor to the DC signal the ASD module sees - anyone know if this is accurate?

Just to keep things in the same terminology, there are 1,000 mili volts in a volt but I don't think that is what you were reading.

For a generic ABS, the frequency of the AC wave is used to sense wheel speed. Trying to measure wheel speed by measuring voltage would be a real mess since sensors , toothed wheels and sensing distances would offer a variation that would have to be tuned for at each wheel.

Quote:

Originally Posted by stephenson

If true, the AC signal coming from the wheel sensor could be in spec and the ABS ECU could be developing an out of spec AC signal that it is sending to the ASD module. The other alternative is that the ABS sensor is actually developing a bad signal, but it doesnt matter to the ABS ECU because it reads frequency not voltage?

I don't know how this specific system works but there is a chance something is lost in the rebroadcast.

A lab type oscilloscope would be helpful here.

stephenson · #3 01-06-2013, 03:09 PM

Thanks ...agree speed sensor puts out AC, but since is passive device it is milliV, right? And, yeah, frequency vice V since need the stability and accuracy.

So, where does the conversion from milli VAC to VDC occur to provide signal to ASD? In the ABS ECU? Since test procedure calls for about 4 VDC from the sensor (read at ASD module printout), and I am getting over 8 VDC (left front sensor is 4), what could cause? Just a bad sensor in voltage, but stable in frequency?

I just checked all three speed sensors between ABS and ASD modules - all three passed continuity test - each was 0.2 ohms (spec is less than 1 ohm).

Even though had previously removed and tested brake light switch, went through the test procedure for the brake light ...both the operation and rest tests were fine.

Back to the RF wheel speed sensor ...does this mean, given the previous test with twice the VDC output at the ASD module that it is bad? Even with it passing the continuity test?

Any thoughts?

stephenson · #4 01-07-2013, 06:21 PM

So, drove it today and it consistently did the following...
- got ASD triangular light during start sequence (normal)
- ASD light went out after starting
- ASD light came one as accelerating then went out at about 5 mph
- I felt ABS pulsation just prior to stopping every time
- ASD light came on during pulsation a

I've never had ABS do this ...

ASD light is supposed to come on when average of front wheel speeds is compared to rear differential speed ... Only the right front wheel sensor reads out differently than test calls for ...8.15 VDC vice 4 VDC.

Arrrghhhhh ....

stephenson · #5 01-07-2013, 09:38 PM

Did more careful test drive ...all same EXCEPT ASD light did NOT come on during stop, but DID come on after releasing brakes and accelerating as passing 1200 rpm then went out.

I also noticed the ASD would occasionally kick in while cornering at low speeds (it doesnt operate at anything over about 17-18 mph).

I pulled the ASD module and got same results, except the ASD operating light (triangle) did not illuminate at all and there was no ASD actuation.

stephenson · #6 01-07-2013, 10:50 PM

The description in this thread seems consistent with what I am experiencing ...

85 300SD - Brakes - ABS noise/softish pedal?

I'll do the tone ring cleaning on RF sensor ... the sensor checked OK in resistance test ...but the 8.15 VDC expecting 4 VDC is still the only thing I can consistently point to as likely cause factor.

stephenson · #7 01-08-2013, 06:56 PM

Well, today things got better .... finally.

Today I removed the RF wheel sensor, and carefully examined it, the wiring from the sensor to the grommet at the wheel liner, the knuckle where the sensor is attached, and the tone ring. Not finding anything unusual, I carefully cleaned the sensor, sanded the outside of it lightly with very fine sandpaper, including between the two tips on the end of the sensor, sanded the base plate where the sensor attaches, sanded the inside of the sensor mounting plate, sanded the inside of the sensor receiving tunnel, carefully cleaned around the base of the tip of the sensor with a small tipped awl, inserted the awl through the access tunnel and used the tip to clean every tooth in the tone ring, then sprayed everything with brake cleaner, using compressed air to completely remove any disturbed material.

I then tested the output of the RF wheel sensor and found it, again, to be 8.15 VDC with ignition on, but when spinning the wheel at 1/s it now at 4 VDC (the spec number)! The LF wheel always tested 4 VDC with wheel spinning, but always tested at zero (0) when the wheel was not spinning.

Put it all back together and everything worked per spec – the ASD triangular light illuminated during start sequence, the ABS did no actuate during stops, the ASD triangular light illuminated when I accelerated and spun the wheels on a gravel/dirt road and then went out per spec.

A good exercise in troubleshooting!