Seeking help from ABS expert - wheel locking and other symptoms
#1
01-13-2013, 05:22 AM
Seeking help from ABS expert - wheel locking and other symptoms
1999 AWD Grand Caravan. 85k miles.
The braking system exhibits the following symptoms:
During panic stop, left front wheel locks up
ABS seems to work correctly on other three wheels, which do not lock
Brakes remain activated until vehicle comes to a complete stop even if foot removed from pedal
Pedal does not come back up until vehicle comes to a complete stop
After multiple ABS activations, such as multiple test stops in a gravel parking lot, aftermarket wheel temperature sensor system indicates left front wheel is running about 20-25% higher temp than right front.
The most problematic aspect of these symptoms is the brakes continuing to activate even after pedal released. On slippery roads, even at low speeds, the ABS system is easily triggered, and there is no way to regain control of braking until the vehicle comes to a complete stop.
The ABS light and brake warning lights are not lit, and the system in not throwing ABS codes. If the ABS fuse is pulled, the brakes work well even in hard stops, and the vehicle doesn’t pull to one side or the other during hard stops.
The calipers, pads, rotors and flexible lines have been replaced on all wheels, front calipers actually replaced twice, most recently with Bendix remans. The master cylinder has been replaced with a new Bendix. Proportioning value replaced. The brakes have been pressure bled multiple times. While putting in the new parts has made normal braking performance quite good, it has not addressed the ABS problems. I even installed a second HCU/Cab unit, and took it into the shop for the pressure bleed/shop activation HCU bleed procedure—still no change. At the shop, the tech indicated that the ABS system is not throwing codes. The ABS light does not remain lit after start up.
Any ideas what could cause these symptoms or what I can do to better diagnose the issue?
The braking system exhibits the following symptoms:
During panic stop, left front wheel locks up
ABS seems to work correctly on other three wheels, which do not lock
Brakes remain activated until vehicle comes to a complete stop even if foot removed from pedal
Pedal does not come back up until vehicle comes to a complete stop
After multiple ABS activations, such as multiple test stops in a gravel parking lot, aftermarket wheel temperature sensor system indicates left front wheel is running about 20-25% higher temp than right front.
The most problematic aspect of these symptoms is the brakes continuing to activate even after pedal released. On slippery roads, even at low speeds, the ABS system is easily triggered, and there is no way to regain control of braking until the vehicle comes to a complete stop.
The ABS light and brake warning lights are not lit, and the system in not throwing ABS codes. If the ABS fuse is pulled, the brakes work well even in hard stops, and the vehicle doesn’t pull to one side or the other during hard stops.
The calipers, pads, rotors and flexible lines have been replaced on all wheels, front calipers actually replaced twice, most recently with Bendix remans. The master cylinder has been replaced with a new Bendix. Proportioning value replaced. The brakes have been pressure bled multiple times. While putting in the new parts has made normal braking performance quite good, it has not addressed the ABS problems. I even installed a second HCU/Cab unit, and took it into the shop for the pressure bleed/shop activation HCU bleed procedure—still no change. At the shop, the tech indicated that the ABS system is not throwing codes. The ABS light does not remain lit after start up.
Any ideas what could cause these symptoms or what I can do to better diagnose the issue?
#2
01-13-2013, 08:51 AM
#3
01-13-2013, 10:20 AM
#4
01-13-2013, 07:10 PM
Try check the Brake switch above the pedal. Maybe your ABS is recieving a false signal that the brake pedal is still being applied.
Just look at your brake light in the rear, does it ever stay Lit up even if you release the brake pedal?
regarding the wheel lock up, maybe you have a bad brake hose, since you alrdy replaced the HCU and CAB unit.
Just look at your brake light in the rear, does it ever stay Lit up even if you release the brake pedal?
regarding the wheel lock up, maybe you have a bad brake hose, since you alrdy replaced the HCU and CAB unit.
Last edited by steak59; 01-13-2013 at 07:13 PM.
#5
01-16-2013, 12:23 AM
Montreal 300, I am wondering if a speed sensor can be good enough not to trigger an error code, but defective in some way that triggers a wheel to lock up during a panic stop.
Cruzer, I've had three different calipers on the wheel that locks up, and they have all behaved the same, so the odds seem low it is the caliper itself. The pads have also been changed. The only constant is the knuckle itself. I've heard that grooves worn into the pad mounting surface can cause this, but on this particular knuckle the grooving seems pretty slight. Also, the locking seems most strongly associated with ABS activation.
Steak59, Your comment on the brake switch opened up some interesting possibilities. I looked more carefully at this aspect of things. The switch seems to be working right, but three new data points. First, I noticed the pedal return is slow, about 2X slower than my Volvo which has no braking issues. The pedal does start to return immediately after removing my foot from pedal, but it is noticeably sluggish. Secondly, when pushing the pedal down by hand, I can hear a sound that is like air being compressed by a pump, a light "whooshing" sound. Third, on the rubber bellows of the power assist unit above the brake pedal, I found some drops of brake fluid. So now I am looking again at the booster as the potential cause of the brakes not letting go upon pedal release in a panic stop. The booster clearly performs its main function correctly, in that the pedal drops on engine starting and braking does not require excessive effort. However, I've been reading that there are valves inside the booster that are connected with releasing the pedal, and that brake fluid contamination can cause them to malfunction. Does it make sense that a defective booster could cause the ABS system to keep braking by somehow holding the pedal down? It does feel like the pedal is almost actively being pulled down during a panic stop. This doesn't happen when the ABS pump fuse is pulled. Can ABS activation somehow interact with a defect in the booster to cause the pedal not to return?
Overall, I'm starting to think that this has been so hard to diagnose because there are two separate problems, and both of them don't show up under all conditions. Looks like I may have to swap out the booster to find out if that is some or all of the problem.
Cruzer, I've had three different calipers on the wheel that locks up, and they have all behaved the same, so the odds seem low it is the caliper itself. The pads have also been changed. The only constant is the knuckle itself. I've heard that grooves worn into the pad mounting surface can cause this, but on this particular knuckle the grooving seems pretty slight. Also, the locking seems most strongly associated with ABS activation.
Steak59, Your comment on the brake switch opened up some interesting possibilities. I looked more carefully at this aspect of things. The switch seems to be working right, but three new data points. First, I noticed the pedal return is slow, about 2X slower than my Volvo which has no braking issues. The pedal does start to return immediately after removing my foot from pedal, but it is noticeably sluggish. Secondly, when pushing the pedal down by hand, I can hear a sound that is like air being compressed by a pump, a light "whooshing" sound. Third, on the rubber bellows of the power assist unit above the brake pedal, I found some drops of brake fluid. So now I am looking again at the booster as the potential cause of the brakes not letting go upon pedal release in a panic stop. The booster clearly performs its main function correctly, in that the pedal drops on engine starting and braking does not require excessive effort. However, I've been reading that there are valves inside the booster that are connected with releasing the pedal, and that brake fluid contamination can cause them to malfunction. Does it make sense that a defective booster could cause the ABS system to keep braking by somehow holding the pedal down? It does feel like the pedal is almost actively being pulled down during a panic stop. This doesn't happen when the ABS pump fuse is pulled. Can ABS activation somehow interact with a defect in the booster to cause the pedal not to return?
Overall, I'm starting to think that this has been so hard to diagnose because there are two separate problems, and both of them don't show up under all conditions. Looks like I may have to swap out the booster to find out if that is some or all of the problem.