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Occasionally while I'm driving (usually when I go over a small bump or ramp on the road), I hear a slight clunk under my hood and my Traction Control System and Anti-lock Brake System lights turn on and don't turn off until I shut off the car. The next time I start up my car, the lights stay off until the same thing happens again. I don't feel any difference in the performance of my car (before or after the lights go on), though it may be too subtle to tell.

What could be the problem, and what should I do to fix it? I drive a 1999 Pontiac Montana.

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This is usually indicative of a sensor used by the ABS which is giving bad or intermittent signal. Basically, the ABS and traction control systems switch off because they don't trust the signals they receive from the various sensors.

Judging by the description of your problem, I am inclined to believe that the loss of signal from one of the wheel speed sensors is causing the systems to switch off.

I would do the following:

  • inspect the wiring for the wheel speed sensors to see if there are any breaks or points where a loose connection may cause temporary loss of signal from one of the wheel speed sensors

  • if possible (and this may well be a long shot) back-probe the wheel speed sensors to see if they read out voltages when their corresponding wheel is in the air and spun by hand

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