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Mysterious dash light

I have a 1989 BMW 635csi. Usually runs great. Driving down the freeway yesterday, the car started to run rough and the yellow ABS light came on. I slowed, the car ran smoothly, but then the ABS light would come on. By accelerating or decelerating, the light would go off. But in stop & go traffic, the engine ran very roughly and I smelled unburnt or poorly burnt exhaust. Took it to the BMW place today. Runs great. They cannot duplicate the problem. Any ideas?

Well, ABS is your anti-lock brake system, so that light shouldn’t impact how your engine runs. Is it possible that “poorly burnt exhaust” was maybe burning brake pads?

Checked that at the time. Definitely not a brake smell and the brakes were not hot. Braking seemed normal as well.