Antilock Brakes Not Working Properly
I have found that I am able to lock the brakes on my '96 W202 (C220). Normal braking is good (I am aware ABS is failsafe so normal operation occurs even if ABS goes out). Dash light operates as it should- it comes on when car is started, but goes off shortly thereafter and is never intermittent. When I brake quickly and with a lot of force I can tell something is going on with the ABS (fast, but far too light/very mild puslating sound/feel, no real kick from the pedal as is supposed to be normal). Then it seems (one or more of) the wheels lock and the car skids. An excellent independent Mercedes mechanic retrieved two front sensor codes, tested/cleaned each sensor, and cleared the codes. The sensors give proper voltage when the wheels are spun. He then road tested the car and found the same problem. It seems that the ABS immediately kicks in, but not fully, and then it cuts out and the wheels lock. He said the REAR ABS seems to work. When he got back to his shop he re-checked for codes and found none! He set up another appointment for me for further diagnosis. Since I am feeling something different from normal braking, I'm guessing the ECU is receiving signals from the sensors, and it (ECU) is triggering the hydraulic unit, but either the solenoids/valves/pump is/are weird.
1) Is it possible that the overvoltage protection relay is not feeding adequate voltage to the ABS ECU to properly trigger the system? I have suspected possible weirdness with this (occasional small idle drop/return). The part # is the new one I think)
2) Would air in the ABS hydraulic unit cause these symptoms?
3) Are the ECU and ABS pump capable of "partial" operation (thus the mild pulsation)?
4) Why does the dash ABS light work normally, and no fault codes show up?
I have scheduled an appointment for further testing for next week, but I am really puzzled and would really appreicate any insights any of you can offer. I'm hoping I don't need a new ABS pump or ABS ECU! Thanks so much.