Problem with 02 ABS brakes

[dmince]

My ABS system is activating just as I come to a full stop. Is there something I can check and replace or does it need to see the chevy doctor?

Also, the other day the brake were grabing sometimes when I first hit the pedal. Not all the time though. Could this be a master cylinder or an ABS problem?

Thanks,
dmince

[dmince]

Nobodies had this symptom?

Seems to only happen when they/re hot. Did it more today. Is it possibly boiling the fluid. Fluid has been in for 110k miles. I haven't had the brake system open, but, could there be air in the system.

Any help would be apprieciated.

mince

[maxwedge]

No quick answe,r here the abs must be scanned with an abs function scanner to look at wheel sensor inputs, one is dropping out at low speed causing this condition.

[dmince]

Understand the low speed but it only happens after 30-40 miles when they're warm. I can't get it to chatter when they're cold. Does this still point to a speed sensor?

thanks


Added: Could the warped rotors be heating up the calibers and boiling the fluid? Seems like the heat just rolls out the front wheelwells when I park it.

[maxwedge]

Forget the caliper issue, resistances change with temps that can affect a border line sensor due to normal heat from the brakes, as I said this must be checked as I stated, you could check hot resistances on each sensor with an ohmeter, access is tough and that is not a definitive diagnosis most of the time.

[dmince]

Thanks Maxwedge. Looks like it's time for an appt.

Or, is this a bolt on replacement item not requiring any calibration?

I'll report back.

dmince

[maxwedge]

Bolt on for the sensors.

[dmince]

More help on this please.

Installed the speed sensor on the left side...no change. Still pulse/clatter near stop. Pulled right side apart and installed used left sensor on right side.

After clearing the "unknown driver" and getting it to start again, I'm getting random ABS operation. 10mph braking an random speeds not just at slow stop.

The ABS light now comes on when I start the TB. It seems to go out the first time I brake and the ABS activates.

I seemed to get alot of spark when I reconnected the battery during the right side change before I got the "unknown driver". If that means anything.

Thanks

[dmince]

I swapped out the front speed sensors and no change.

It had a rear speed sensor code stored. Ran it with the reader hooked-up and I'm getting a different speed reading on turns and about a 10 percent difference on rpm between front and back sensors.

I ordered the rear sensor. Which sensor is the ABS? (top or side) ABS & PCM?

Thanks
dmince

[maxwedge]

Have you scanned with an abs scanner and looked at wheel speed inputs> If not this will go on forever.

[j cAT]

Originally Posted by dmince

My ABS system is activating just as I come to a full stop. Is there something I can check and replace or does it need to see the chevy doctor?

Also, the other day the brake were grabing sometimes when I first hit the pedal. Not all the time though. Could this be a master cylinder or an ABS problem?

Thanks,
dmince

if oil leaks on the brakes they will grab...if one wheel had this occur it is very possible that the grabing brake on one wheel would cause the ABS to activate...

I would have the brakes checked for any problems as mentioned,,and also check that the calipers/brake hardware is not rusted and causing the brakes to drag...

[dmince]

Originally Posted by maxwedge

Have you scanned with an abs scanner and looked at wheel speed inputs> If not this will go on forever.

Yes. It had a stored code for the rear speed sensor. Then we checked the speed reading. The rear was dropping 1mph lower than the front. It showed up mostly on braking and quick right turns. On left sharp turns it picked up 1 mph over the front input. Then we check the rpm which ran about 10 percent low on the rear input.

He suggested replacing the rear sensor. Auto parts gave me the wrong one today so I'll try again tomorrow.

Am I on the right track Maxwedge??

My question at this point is which sensor is ABS and PCM on the tail piece since they are identical?? Save me one more trip under the TB please.

dmince

[j cAT]

Originally Posted by dmince

Yes. It had a stored code for the rear speed sensor. Then we checked the speed reading. The rear was dropping 1mph lower than the front. It showed up mostly on braking and quick right turns. On left sharp turns it picked up 1 mph over the front input. Then we check the rpm which ran about 10 percent low on the rear input.

He suggested replacing the rear sensor. Auto parts gave me the wrong one today so I'll try again tomorrow.

Am I on the right track Maxwedge??

My question at this point is which sensor is ABS and PCM on the tail piece since they are identical?? Save me one more trip under the TB please.

dmince

when you turn sharp right the wheels on the right rotate less...


the question/comment you made was brakes grabbing sometimes..did you check with the wheels off the ground and apply and release brakes note how the wheels properly release all the same..when the brakes where released...also any oil on the brake linings...

if the sensor was defective it would be constant...the sensors can get debris or rust lowering the signal to the pcm..this also could cause a ABS to activate..then you have wires to/from the sensor that can be chaffing into the carbody frame shorting out the low voltage produced..

[dmince]

Originally Posted by j cAT

when you turn sharp right the wheels on the right rotate less...


the question/comment you made was brakes grabbing sometimes..did you check with the wheels off the ground and apply and release brakes note how the wheels properly release all the same..when the brakes where released...also any oil on the brake linings...

if the sensor was defective it would be constant...the sensors can get debris or rust lowering the signal to the pcm..this also could cause a ABS to activate..then you have wires to/from the sensor that can be chaffing into the carbody frame shorting out the low voltage produced..

As far as the the brakes...there great now. I replaced the rotors when I was replacing the front speed sensor. The inside surfaces looked nasty. Brakes work smooth now like they used to. There were no brake fluid leaks. This part of the problem/issue is solved.

The wires and connectors looked good even where they were bent hard at the frame connector.

We probably weren't driving fast enough to get the consistant mph difference like we got with the rpm difference???

The heat does seem to be effecting the ABS though, as Maxwedge suggested. If I'm on the highway and stop there fine but in town or sit awhile (building heat under the vehicle) they activate at a stop sign.

Still looking for which rear speed sensor to replace on the tailshaft. Drivers or passenger side. There are two identical sensors on the same gear/wheel.

dmince

[maxwedge]

There is no abs sensor on the tailshaft they are in the rear wheel hubs, the sensor in the tail shaft is a speedo sensor. The abs sensor usally drops out prematurely at speeds under 10, you must look closely on the scanner as it drops out momentarily before the others causing the abs controller to assume that wheel is locked. this will not set a code.