Abs brakes modulating



I have a 1998 GMC 4WD, extended cab, automatic, 1500, short bed pickup w/ 232,819 miles I put on the vehicle since I purchased it new. It has ABS on the front disc barkes, no ABS on the rear drums. I’m a conservative driver and feel the ABS modulating the brakes during 15% +/- of my stops on dry pavement. Any clues on what may be wrong, what I may do to affect a “do it yourself” repair or should I take it along w/ my check book to a local dealer.




Are you sure that it is the ABS? What exactly do you feel? Only b/c what you might be feeling is early warping of front rotors (causes pedal pulsation). How many miles do those rotors have on them?

Brakes are not too difficult to do. I’d suggest a repair manual ($20 at a parts store). Just set aside an afternoon and make sure you have a safe way to raise & support the vehicle.

If you’d rather not bother, you don’t need a dealer. Any good, local mechanic can handle this. Just stay away from the corporate chains.


It’s a common problem on GM trucks.

One of the front wheel speed sensors is dropping it’s signal causing the system to activate. Driving it while monitoring the speed signals can isolate which speed sensor signal is dropping out.

The cure can be as easy as cleaning the the speed sensor ring or replacement of the sensor might be needed.


Thanks for the quick reply. I feel certain it’s an ABS problem because I sometimes feel it as I creep up on a parked vehicle as I pull into a parking space and want to get reasonably close to the car parked in the space opposite me. The new rotors have about 40k miles on them and I’m familar with the feel of warped rotors.


How do I monitor the individual speed sensors while driving.? Do I need some sort of high tek monitor to plug into the information port under the dash.?


Yes, you need a scanner.