The brake pedal on my 1999 Jeep Cherokee pulses violently when braking on snow covered roads and the brakes may be locking up. They work fine on dry roads no matter how hard I brake, but performance on snow or ice is scary once the ABS kicks in. I have had this vehicle for 10 years, so I know that the behavior is abnormal. The ABS light does not report a failure. Any help would be appreciated.

 

Well, it's obviously the ABS if you're pedal is pulsing. So it could be an ABS switch or there could just be dirt and debris in the "control module" for lack of a better word. This was happening to someone I knew in a RAV4. We popped off the wheel and there was a whole bunch of crap in the little slots for the ABS. We cleaned it out real good with some carb clean, and it worked fine. Hopefully you understand what I'm trying to explain. It's just too early right now, haha.

 

they are called Tone Rings. The sensor picks up the speed of the wheel by the magnetic pulses the tone ring generates in the sensor. If they are broken or rusted too badly they will generate a very week signal messing up the performance of the ABS. It needs to know how fast each wheel is going to control the braking.
If they are in good shape maybe the amount or depth of the snow your driving in is messing up the signal.

 

Well it's nice to know the real name for them now. Sounds logical about the depth of snow you're driving in. I'd give em a good cleaning and see what happens.

 

Thanks for the replies. I noticed the problem in one inch of snow and on black ice. The big snowfall is due to begin tonight. Can I assume that the place to start is by inspecting and cleaning all four Tone sensors?

 

That surely could not hurt Hull.

 

Yea, just clean em real good with some carb clean and see if it helps before you take any other course of action. The ABS was probably kicking in because of the black ice. If it becomes a real problem and constantly locks up, you could temporarily pull the relay.

 

Thanks for the feedback. I hope to get the vehicle indoors early next week, after the weekend storm, and I'll report back with the news. Meanwhile, if there are any secondary things to check, please keep the ideas coming.

By the way, I would have expected the ABS light to go on if the tone rings are gummed up. I assume the next thing to check after cleaning the tone rings would be the sensors and cabling. Would the computer report defective sensors?

 

My friends Exploder had a defective ABS sensor and he got no warning light about it. It would just shake violently around turns. He changed it out yesterday and it was fine. Same with the RAV4 that had the pulsating brakes, there was no indication light.

 

I think it will only show a light if the computer senses an open circuit. If there is continuity it's looking for an electrical pulse generated by the tone ring.
A short will still show continuity as the circuit is still "Live" just not picking up the pulse, in effect telling the computer that the wheel is stopped or not moving.
If the computer sees 3 wheels moving and one already stopped it will release the pressure to that wheel. Remember the reason for ABS. A locked wheel is a wheel out of control. you lose control of a vehicle when all the wheels are locked up.

 

OK. I understand why the light may not come on. However, if cleaning the rings doesn't correct the problem, and if the sensor cables are not damaged, how might I determine which sensor is the likely culprit?

 

You'll need a friend for this.
He/she needs to watch for which wheel trys to lock up when you are stopping on ice or snow. It's the one the computer already thinks is stopped and will keep releasing and reapplying the pressure to.

 

They say there is no such thing as a stupid question... but I know this one is gonna be real close!

I have never had ABS till now. While under the Jeep, I noticed what looked like over sized axle splines and a sensor on the inward side of my drums. Is this what you are calling the Tone ring? I ask because my XJ had the same symptom on the first snow day but never has reoccured after that...

 

Easiest way to check is to lift up the back seat as if you were to fold down the back or get at the jack. On the drivers side will be a big black box. Lots of wires going into it. Thats your ABS control module.
Another way is to look under the hood. You will not have the big vacuum brake booster but a lot of brake lines going into something that looks like a pump.

 

Clever idea, Jaime... except for the "friend" part, unless I give him binoculars. Just kidding. Thanks for the suggestion.

One more question: If I decide to temporarily disable the ABS system, is it best done by pulling the ABS relay? If so, where do I find the relay on a 1999 Cherokee?