Antilock brakes, traction-control system linked

Q I have a 1996 Buick Park Avenue Ultra, which has antilock brakes and a traction-control system. About a month ago, after about an hour on the road, the dashboard warning lights for these two related systems came on simultaneously.

As days went by, the time between starting the vehicle and the lights coming on decreased to where now it's consistently about two minutes. Interestingly, during those two minutes, both systems operate just fine; I was easily able to test both with the recent snow.

So, it would seem the computer that controls the system is sensing a fault, from a sensor, perhaps? Or maybe the computer itself is a problem. I assume it's separate from the engine control module?

I've also heard of a procedure to clean the rotating hubs that the sensors on the front read, but I don't remember the details. Have you heard of that, and does it sound like a possibility?

A You are correct in recognizing that the antilock brake and traction-control systems are related. The antilock brake system (ABS) controls rotational speeds of the wheels under braking to prevent wheel lockup and loss of steering control, and the traction-control system controls rotational speeds of the drive wheels under acceleration to prevent wheelspin and loss of vehicle control. Both systems utilize the same wheel-speed sensors that monitor rotational speed of the wheels.

The difference between the systems is simple. When the wheel-speed sensors identify one or more wheels slowing more rapidly than the others under braking, the EBTCM (electronic brake/traction control module) commands the brake pressure modulator valve to isolate and reduce hydraulic pressure to the brakes on the locking wheels, preventing lockup. If the wheel-speed sensors identify one or more of the drive wheels accelerating more rapidly than the other wheels, the EBTCM commands the brake pressure modulator valve to isolate and apply brake pressure to the brakes on the wheels that are beginning to lose traction and spin.

At the same time, the EBTCM commands the powertrain control module to reduce torque applied to the drive wheel or wheels losing traction by retarding ignition timing and shutting off injectors.

In the case of your Buick, because both ABS and traction-control warning lights come on at the same time, the common components of both systems are suspect. These include wheel-speed sensors and their connections, the EBTCM, the brake pressure modulator valve and its pump motor. The good news is that both systems have sophisticated onboard diagnostics that can be read with a scan tool -- which is at least as smart as the ABS and traction control.

I don't think the problem is related to wheel-speed sensor connectivity, because both systems pass their initial self-test when the key is first turned to the "run" position -- the warning lights initially illuminate, then go off after 4 to 6 seconds. It certainly is possible that the signal from one or more of the wheel-speed sensors is intermittent because of damage, dirt or debris in the area of the reluctor and magnetic sensor located on knuckle and wheel hub. Once the EBTCM recognizes the loss of signal from a wheel-speed sensor, both systems will be disabled for the remainder of that operational cycle.

But if I had to guess based on your description, I'd have to say that the brake pressure modulator valve and its pump are the leading suspects.

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