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.
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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?
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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?
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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.