The 1991 350 SDL (anti-slip differential) has a problem. The dash light does not go out when engine is startes (it should). When on ice the small light within the speedometer does not light when wheels spin (it should). Also, the system does not work.

In attaching an electronic tester, it indicated that the brake switch was bad, we replaced it and get the same report. Next we replaced the relay with the two red fuses, still get same report and system malfunctions. We did all prescribed voltage and continuity tests and all components seem to be OK. We suspect the about $500 computer for the system but do not want to replace it and find it is not the problem because it is not returnable. Any help out there?

Bob

This system is somewhat rare here in the US, so most techs haven't had much experience w/diagnosis. Make sure that the brake lamp signals are getting to the ECU.
The traction warning lamp in the speedo won't come on when the MIL light is on.
Is or does the ABS lamp come on??

Doc - Thanks for response.
The ABS system is all working correctly, both lights and its function.

The brake switch was replaced and it tests correctly, as did the old one, in doing specified function tests. At this point I suspect that the computer check that shows the brake switch as being defective is actually picking up a ECU malfunction in the part that receives the signal from the switch.

In talking to one company I was told that malfunctions in the ECU are only 300 per million so if this is true it is not a frequent problem. Know of any place that can test the ECU? Had thought of trying to find a used one but that may or may not be good.

Bob

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Bob1