"Thanks for your prompt reply. Just for clarification. The "grabbing" isn't occurring during any kind of braking. It's occurring while driving. It's intermittent and can be mild to severe. Or not at all! Worse with warmer weather, and typically worse with more engine strain (but. Not always). Can occur at highway speeds or just running around town. Like the ABS/brake light issue (where the lights would come on only after a certain drive time, temp or going over a bump), it would typically take 5-10 minutes before it would start doing it, but now it's just getting worse. So it's a mystery, seems like an electrical issue and that's what the folks at my repair shop (who've gone over this vehicle on about 5 other occasions) believe, that the ABS controller is sending faulty messages to the brakes and trying to stop the car, and not during any braking. Very weird. The ABS and brake lights have been on for some time, but braking has never been an issue, even now. Have had to hard brake on a number of occasions and they've been good. Replaced the front brakes in February, they were a little worn. Did replace the rear wheel speed sensor, too, just to see if that would resolve, but it didn't. Wasn't having a speedometer issue.
Other concern was it might be the tranny going; it's got lots of miles and does have some issues (doesn't like to get out of the lower gears that easy). But. Guys at the shop are not pinning it on anything mechanical at this point! Recently had spark plugs replaced; the #5 plug had some crazy buildup on it, the others were clean. That didn't fix the issue. Will be getting to the other regular tune-up stuff in the next week or so (rotor, distributor cap, PC valve, etc.).
Any ideas you may have would be appreciated! Don't want to put much into the ol' beast, it's been a decent vehicle and great for hauling, but I'm looking for something newer and more gas-friendly in about 6-8 months. Just need it to last that long at least!
Thanks for your help!"
First of all, the ABS should not do anything until voltage is received from the brake light switch, so yes, I suppose the computer could be erroneously activating a solenoid in the hydraulic controller which sends pressurized brake fluid to a wheel. A valve could be leaking too in the hydraulic controller. If you pull the fuses and the problem goes away, that will be a clue. A leaking valve could cause a brake to grab but it will not pulsate like they normally do when the ABS kicks in. A shorted driver transistor in the computer will also apply a solenoid without pulsing it. If you hear or feel the brake pulsate when it grabs, the computer and the hydraulic controller are doing what they're supposed to do but in response to a wrong input. That COULD also be caused by a problem in the part of the computer that reads the wheel speeds and brake switch input.
Regardless, this is not a common problem which can make it hard to diagnose. If the same problem occurs after the ABS fuses are removed, you have a regular brake problem. I have a lot of things to look at then.
Friday, June 11th, 2010 AT 7:05 PM