Grumbling/scraping feeling from brake pedal?

abs
brakes
pontiac
torrent

#1

I have 2006 Pontiac Torrent AWD with front disc and rear drum brakes. I need help with my ABS & traction control lights come on together (sometime) when I hit brakes slowly to slow down my car around 20-30km/h with a grumbling/scraping feeling sent to my feet from brake pedal (feel like rumble strip on highway). ABS & T/C lights go away if I shut off engine and restart car. It also happens if I accelerate at that speed. This issue does not happen when I hit brake hard. Has anyone experienced this issue?


#2

The first step would be lift the car, pull all four wheels and carefully inspect the brakes for wear and damaged parts…Carefully inspect the ABS sensors. Are they positioned correctly? Are the wiring connections undamaged and positioned correctly? Check the wheel bearings…If they are loose, the rotors will wobble…


#3

Is the ABS sensor on this car part of the wheel bearing? Seems like a bad bearing triggering everything along with the noise.


#4

When the ABS activates it can make a similar sound. Maybe that’s all that is happening. That’s the way ABS works, when it determines a wheel is locking up during braking it pulses the brakes automatically to prevent the lock-up, which the driver can often feel in the brake pedal and hear.

That wouldn’t explain the acceleration part of the problem tho. With the traction control function, if a wheel starts to spin during acceleration, a little braking might be applied to that wheel using the ABS system to prevent it from spinning & losing traction. So again what’s happening may be exactly what is supposed to happen.

If this all didn’t happen until recently, most likely a problem with the brakes or a wheel speed sensor. Did you have any tire or wheel work done on this car immediately prior to noticing this?


#5

Agree with @Caddyman… make sure the brakes themselves are in good condition. Pulsing brakes can fool the ABS. So can worn wobbly bearings since this car has the ABS sensors in the bearings. Once all that checks out, you may need a mechanic with an ABS scanner to do a ride-along to see if it is going into ABS and at which corner. That may mean a flakey ABS sensor sensing noise the the ABS computer.

I have been through this on a GM truck. The ABS cycled as I slowed below about 9 mph nearly every time I stopped even on dry pavement. The solution was to adjust the left front torsion bar to bring the sagged corner back to level. Problem gone. Shows how sensitive these can be.


#6

@Mustangman writes …

The solution [to a faulty ABS system] was to adjust the left front torsion bar to bring the sagged corner back to level.

Interesting … Was the sagging corner somehow causing that wheel speed sensor to flake out? If so, that seems like an undesirable amount of sensor sensitivity to frame geometry.


#7

@GeorgeSanJose The sagging corner allowed the opposite side to carry more load and the sagging side less. The brake torque is the same side to side but since the load holding the tire onto the pavement is less, the rotational speed differed just enough to flake out the ABS. Not flake the sensor but the ABS itself since it compares wheel speed left to right and front to back. Pretty sensitive calibrations in the software. (I could dig up charts showing the percent slip as a function of tire load and brake torque if anyone is interested.)

Or at least that’s the theory I developed AFTER I saw a forum post suggesting this as a solution to the phantom ABS cycling with no explanation. Easy enough to try so I twisted the torsion bar bolt to level the truck and the problem was cured.


#8

Good explanation. It goes to show, there’s a lot of unexpected relationships that have to be considered when it comes to repairing cars.


#9

And you never noticed the corner was sagging before this brake problem came up?


#10

I experienced this problem a lot driving GM Suburbans and Chevy Van based School buses. The solution was always to replace a wheel speed sensor. In 3 years I had 4 sensors replaced on one van.
The problem was always at low speed and the van would take a long distance to stop when it was doing this because the ACS was cycling and preventing the brakes from applying.

I had someone I worked with who rear ended someone ahead of him going downhill out of a parking lot at 10 mph because his 6 month old Chevy pickup did this.

I had so many problems with ABS with vehicles I drove at work and ABS and Traction Control issues with a 98 Olds Intrigue that I owned that when I needed a new car in 2004 I bought a brand new PT Cruiser because they didn’t have ABS.

All of my ABS problems have been with GM products.


#11

“The problem was always at low speed and the van would take a long distance to stop when it was doing this”

Sounds like a design problem that justifies a safety recall, IMHO.
I’d rather not have ABS at all than chance that happening.


#12

I guess an unintended consequence due to the complexity of the new car safety features is sometimes the cars are less safe than before. Some good, some bad, it’s a compromise.