vivijiv

I have a 1990 129 500sl which is driving me crazy .Initially after servicing the brakes the abs/asr lights would come on after driving 20 metres.After a couple of weeks this stopped happening but when braking hard the brake pedal shudders. In this mode the speed control works and the anti skid works.
I cleaned the sensors that is the front ones as there is only a central sensor at the back. This did not help.
I bought another overvoltage protection relay and fitted it ,now the lights come on after 20 metres and the brakes are fine thus reverting to the original problem.
If I change back to the old Relay the lights stay off and the brake pedal shudders.
Please can someone help me solve this headache?
I have not changed the brake switch as it seems a major mission to remove the underdash cover to get to the switch.Is there perhaps an easy way to access the brake switch?
ANY WIRING DIAGRAMS SHOWING THIS CIRCUIT WILL BE GREATLY APPRECIATED.

vivijiv

After much research I found the problem to be a broken wire in the sheath of the RHS wheel sensor.This was after testing both sensors
So I have now replaced the brake switch and the OVP relay so I am hoping for many miles of no ABS/ASR LIGHT PROBLEMS