ABS engage at light braking input

[ahess77]

I'm having a problem in that at highway speeds, +55mph, under light to moderate braking my ABS is engaging intermittantly. I get the pedal pulsation and hear the ABS grinding away under the hood. This is dry pavement, the tires are definately not sliding.

I don't believe there's a warped rotor, the braking is straight & no steering wheel shimmy. And it's intermittant, one time no ABS activation, the next time it might happen.

Anybody had this happen before?

[magindat]

ABS sensors taking a dump? :dunno:

[Aren Jay]

What size of tires do you have?

[ckadiddle]

Check brake fluid level if you haven't already done it.

[ahess77]

Brake fluid level is just below the MAX mark.

Tires are 235/55 front and 255/50 rear. No problems in the last 35k miles with traction control or abs.

[Bluerauder]

Brake fluid level is just below the MAX mark.

Tires are 235/55 front and 255/50 rear. No problems in the last 35k miles with traction control or abs.
Are you sure of this ^^^^? Seems you have aspect ratios reversed. In any case, I don't think you are (or should be) running 235/55s on the front. There is more than 4 inches difference in circumference of the OEM tire front/rear mix. Less than 1.5 inches on the set-up you described -- maybe less with wear. Could be that the near equal diameters (and circumference) is causing the T/C to act up thinking the rears are slipping.

[ahess77]

[quote=Bluerauder;650197]Seems you have aspect ratios reversed. /quote]

Yes, you are right, actually I screwed the whole tire size. I actually have 245/45 front and 255/55 rear. I've been running this way for 8 months since I had the fronts replaced and no ABS or traction control complaining.

The intermittant failure is what makes me think it has nothing to do with the tire size or rotors or pads. I'm thinking it's sensor or module related and I'd like to know first before I start replacing things.

[BlownMerc]

Had a similar situation with a CV at work. Found the front tone rings full of debris (metallic pad content, this was a taxi). Used compressed air to clean tone rings and sensors. Also noted a couple of spots on the tone ring that weren't true. The tone rings on the car I was working on were stamped steel, so I'm guessing debris such as a stray rock could have dinged the ring, causing inconsistent airgap. Did not always trip the ABS, but sometimes, if the brake pedal is pressed right when this spot crosses the sensor, the module would see it as an "impending lock-up" and trigger ABS. Check them out, straighten the tone rings carefully and clean, might be a free repair. (Tone ring is pressed on the back of the wheel bearing assembly in the front, rotor is a slide on)

[LVMarauder]

Your corrected tire sizes seem fine. +1 to sensor malfuntion.

[LeoVampire]

And the vain ring may need to be cleaned as well.

There is a seperate ABS PC port for the car with it's own brain. Have it checked if cleaning all the the wheel senors and rings do not solve the problem.

Chances are one of the sensors are failing if the problem persists. The ABS is supose to shut down if there is a problem present but may not if it is not a full time problem.

[ahess77]

Here's a simple fix to this problem, check your air pressures.

I found that my left rear was only 28lbs, the right was 38lbs. I have been setting them to 42lbs, but the left is leaking faster than the right.

Now that they're both at 42 there's no ABS problem.