Right rear locking up under heavy braking, C5Z06
06-03-2012, 09:55 AM
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Right rear locking up under heavy braking, C5Z06
I am currently preparing an '01 Z06 for hillclimbs, my first event being on June 23-24. I searched multiple times and could not find much on this subject, I am out of ideas at this point.
I autocrossed the car shortly after purchase in April, and noticed a severe right rear lockup issue. After 4 runs and sliding through the finish every time, I ended up leaving the event. I changed to Hawk HP+ pads, flushed the brakes until the fluid was perfectly clear, and the problem did not go away or get any better.
Shortly thereafter, the ABS computer started to fail. Car kept going into limp mode and throwing the No Communication code, so I sent the unit to ABS fixer, got it back nearly 3 weeks later, installed it yesterday, and the problem still exists. I thought the problem was related to the ABS computer, but apparently it isn't.
I have checked the calipers on all four corners, the pistons move freely. I am stumped. I really don't feel too comfortable racing the car as it is, and don't have the finances to throw parts at the car. Does anyone have any ideas as to what may be causing this? When the wheel locks, all others go into abs, I can feel it pulsating through the pedal, and the locked up right rear releases as soon as I lift off of the brakes. No other codes are thrown after I re-installed the computer.
I have not had the car on scales yet, but I doubt a slight difference in corner weights would cause this as severe as it is anyway. Any hard braking, instant lockup.
I autocrossed the car shortly after purchase in April, and noticed a severe right rear lockup issue. After 4 runs and sliding through the finish every time, I ended up leaving the event. I changed to Hawk HP+ pads, flushed the brakes until the fluid was perfectly clear, and the problem did not go away or get any better.
Shortly thereafter, the ABS computer started to fail. Car kept going into limp mode and throwing the No Communication code, so I sent the unit to ABS fixer, got it back nearly 3 weeks later, installed it yesterday, and the problem still exists. I thought the problem was related to the ABS computer, but apparently it isn't.
I have checked the calipers on all four corners, the pistons move freely. I am stumped. I really don't feel too comfortable racing the car as it is, and don't have the finances to throw parts at the car. Does anyone have any ideas as to what may be causing this? When the wheel locks, all others go into abs, I can feel it pulsating through the pedal, and the locked up right rear releases as soon as I lift off of the brakes. No other codes are thrown after I re-installed the computer.
I have not had the car on scales yet, but I doubt a slight difference in corner weights would cause this as severe as it is anyway. Any hard braking, instant lockup.
06-03-2012, 11:04 AM
#2
Burning Brakes
You don't think it's the wheel speed sensor on that wheel? That's an input to the ABS computer and without valid wheel speeds the computer can act flaky or shut off ABS entirely.
06-03-2012, 11:24 AM
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I have had to replace the rear calipers when unable to find the specific cause of brake not releasing/lockup. Also, I had a problem with parking brake shoes dragging and cracking the rotors. I removed the parking brake shoes. Considering ABSfixer has repaired your unit, if a good flushing of fluid and purging ABS doesn't fix it I would suggest replacing the caliper.
06-03-2012, 01:40 PM
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I have experienced the same thing. Calipers can look great and even free up shortly after releasing pedal but they hang just long enough to heat-destroy the rotor. The only relief I have found is just replace it.
06-03-2012, 02:00 PM
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I should preface this with a question. Do you mean the wheel was locking up (sliding) or do you mean the ABS was releasing and reapplying brake pressure (ABSing the wheel)?
To figure this out you need to think about how the ABS is supposed to work and determine what is going wrong. The C5 has 4 channel ABS so if one wheel starts to lock up the EBCM will command the BPMV to release brake pressure at that wheel. You will feel the pedal pulsate and the car will vibrate but that doesn't mean all 4 are in antilock operation.
Have you tried to do a hard stop so you put all 4 tires in ABS mode? That can be a violent operation since the car really does vibrate especially with R compound tires.
The 01 and up cars have dynamic brake proportioning Vs a proportioning valve like the previous C5s had. The EBCM determines how much brake to apply based on the wheel speed sensor signal.
If a wheel was truly locking up (not ABSing) and there were no error codes/Service ABS messages on the DIC that means the system was seeing valid signals from the sensors but the signals were incorrect for the speed of that wheel. If you do get a code and Service ABS message that could mean there is something interrupting the sensor signals. Early C5s would have ABS issues on certain types of rough surfaces due to the way the signal changed and they would turn off the ABS. My 97 would send spurious signals to the EBCM while I was on the throttle, the EBCM would shut down and I would hear a dinging sound just as I was getting ready to hit the brakes hard. I flat spotted a very nice set of new Hoosiers due to that problem. The problem on the 97 was solved with a redesigned EBCM but 98s through 00 cars with the front mounted EBCM still had some sensitivity to bumps. A well know road racer installed a toggle switch on the I/P of his 99 kit car so power to the EBCM could be turned off and on thus resetting the EBCM in the middle of a race.
Based on what you have described it sounds like there isn't anything wrong with the EBCM since in your own words the ABS is activating. To me, it sounds like the right rear wheel is tending to lock up earlier than the left rear which is causing the EBCM to release pressure at that wheel and pulse the pedal to tell you the system is working.
Premature lock up could be due to a number of things some of which you have addressed. It could also be due to your tires. Check diameter of both rear tires. You say you have checked the calipers but you should also check the right rear caliper bracket for wear. When I had my C5Z the dealer mechanic found the bracket on one side was worn and allowing the caliper to move too much which could affect its operation.
Bill
To figure this out you need to think about how the ABS is supposed to work and determine what is going wrong. The C5 has 4 channel ABS so if one wheel starts to lock up the EBCM will command the BPMV to release brake pressure at that wheel. You will feel the pedal pulsate and the car will vibrate but that doesn't mean all 4 are in antilock operation.
Have you tried to do a hard stop so you put all 4 tires in ABS mode? That can be a violent operation since the car really does vibrate especially with R compound tires.
The 01 and up cars have dynamic brake proportioning Vs a proportioning valve like the previous C5s had. The EBCM determines how much brake to apply based on the wheel speed sensor signal.
If a wheel was truly locking up (not ABSing) and there were no error codes/Service ABS messages on the DIC that means the system was seeing valid signals from the sensors but the signals were incorrect for the speed of that wheel. If you do get a code and Service ABS message that could mean there is something interrupting the sensor signals. Early C5s would have ABS issues on certain types of rough surfaces due to the way the signal changed and they would turn off the ABS. My 97 would send spurious signals to the EBCM while I was on the throttle, the EBCM would shut down and I would hear a dinging sound just as I was getting ready to hit the brakes hard. I flat spotted a very nice set of new Hoosiers due to that problem. The problem on the 97 was solved with a redesigned EBCM but 98s through 00 cars with the front mounted EBCM still had some sensitivity to bumps. A well know road racer installed a toggle switch on the I/P of his 99 kit car so power to the EBCM could be turned off and on thus resetting the EBCM in the middle of a race.
Based on what you have described it sounds like there isn't anything wrong with the EBCM since in your own words the ABS is activating. To me, it sounds like the right rear wheel is tending to lock up earlier than the left rear which is causing the EBCM to release pressure at that wheel and pulse the pedal to tell you the system is working.
Premature lock up could be due to a number of things some of which you have addressed. It could also be due to your tires. Check diameter of both rear tires. You say you have checked the calipers but you should also check the right rear caliper bracket for wear. When I had my C5Z the dealer mechanic found the bracket on one side was worn and allowing the caliper to move too much which could affect its operation.
Bill
Last edited by Bill Dearborn; 06-03-2012 at 02:03 PM.
06-03-2012, 03:11 PM
#7
Le Mans Master
06-03-2012, 03:18 PM
#8
You have one of two things that have gone bad.
1. wheel speed sensor harness
2. wheel brg speed sensor
your problem with the wheel locking up is that the ABS is not getting the signal to the caliper.
Get someone to put a Tech II on it to check it.
1. wheel speed sensor harness
2. wheel brg speed sensor
your problem with the wheel locking up is that the ABS is not getting the signal to the caliper.
Get someone to put a Tech II on it to check it.
06-03-2012, 03:24 PM
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I should preface this with a question. Do you mean the wheel was locking up (sliding) or do you mean the ABS was releasing and reapplying brake pressure (ABSing the wheel)?
To figure this out you need to think about how the ABS is supposed to work and determine what is going wrong. The C5 has 4 channel ABS so if one wheel starts to lock up the EBCM will command the BPMV to release brake pressure at that wheel. You will feel the pedal pulsate and the car will vibrate but that doesn't mean all 4 are in antilock operation.
Have you tried to do a hard stop so you put all 4 tires in ABS mode? That can be a violent operation since the car really does vibrate especially with R compound tires.
The 01 and up cars have dynamic brake proportioning Vs a proportioning valve like the previous C5s had. The EBCM determines how much brake to apply based on the wheel speed sensor signal.
If a wheel was truly locking up (not ABSing) and there were no error codes/Service ABS messages on the DIC that means the system was seeing valid signals from the sensors but the signals were incorrect for the speed of that wheel. If you do get a code and Service ABS message that could mean there is something interrupting the sensor signals. Early C5s would have ABS issues on certain types of rough surfaces due to the way the signal changed and they would turn off the ABS. My 97 would send spurious signals to the EBCM while I was on the throttle, the EBCM would shut down and I would hear a dinging sound just as I was getting ready to hit the brakes hard. I flat spotted a very nice set of new Hoosiers due to that problem. The problem on the 97 was solved with a redesigned EBCM but 98s through 00 cars with the front mounted EBCM still had some sensitivity to bumps. A well know road racer installed a toggle switch on the I/P of his 99 kit car so power to the EBCM could be turned off and on thus resetting the EBCM in the middle of a race.
Based on what you have described it sounds like there isn't anything wrong with the EBCM since in your own words the ABS is activating. To me, it sounds like the right rear wheel is tending to lock up earlier than the left rear which is causing the EBCM to release pressure at that wheel and pulse the pedal to tell you the system is working.
Premature lock up could be due to a number of things some of which you have addressed. It could also be due to your tires. Check diameter of both rear tires. You say you have checked the calipers but you should also check the right rear caliper bracket for wear. When I had my C5Z the dealer mechanic found the bracket on one side was worn and allowing the caliper to move too much which could affect its operation.
Bill
To figure this out you need to think about how the ABS is supposed to work and determine what is going wrong. The C5 has 4 channel ABS so if one wheel starts to lock up the EBCM will command the BPMV to release brake pressure at that wheel. You will feel the pedal pulsate and the car will vibrate but that doesn't mean all 4 are in antilock operation.
Have you tried to do a hard stop so you put all 4 tires in ABS mode? That can be a violent operation since the car really does vibrate especially with R compound tires.
The 01 and up cars have dynamic brake proportioning Vs a proportioning valve like the previous C5s had. The EBCM determines how much brake to apply based on the wheel speed sensor signal.
If a wheel was truly locking up (not ABSing) and there were no error codes/Service ABS messages on the DIC that means the system was seeing valid signals from the sensors but the signals were incorrect for the speed of that wheel. If you do get a code and Service ABS message that could mean there is something interrupting the sensor signals. Early C5s would have ABS issues on certain types of rough surfaces due to the way the signal changed and they would turn off the ABS. My 97 would send spurious signals to the EBCM while I was on the throttle, the EBCM would shut down and I would hear a dinging sound just as I was getting ready to hit the brakes hard. I flat spotted a very nice set of new Hoosiers due to that problem. The problem on the 97 was solved with a redesigned EBCM but 98s through 00 cars with the front mounted EBCM still had some sensitivity to bumps. A well know road racer installed a toggle switch on the I/P of his 99 kit car so power to the EBCM could be turned off and on thus resetting the EBCM in the middle of a race.
Based on what you have described it sounds like there isn't anything wrong with the EBCM since in your own words the ABS is activating. To me, it sounds like the right rear wheel is tending to lock up earlier than the left rear which is causing the EBCM to release pressure at that wheel and pulse the pedal to tell you the system is working.
Premature lock up could be due to a number of things some of which you have addressed. It could also be due to your tires. Check diameter of both rear tires. You say you have checked the calipers but you should also check the right rear caliper bracket for wear. When I had my C5Z the dealer mechanic found the bracket on one side was worn and allowing the caliper to move too much which could affect its operation.
Bill
Yes, I have done multiple "panic" stops. The first time I noticed the issue was at an autocross shortly after purchasing the car. The right rear LOCKS up, and stays locked, until I release the brakes. I was literally unable to stop at the finish 4 runs in a row. It isn't just premature, the tire stays locked and the car will not stop. I can feel the pedal pulsating, but the 200 ft of skid mark tells me the right rear never released. I have also repeatedly tested it on a secluded road after my attempted "fixes". When I release the pedal the wheel starts turning again, with a slight delay, maybe a 1/4 of a second.
It was the ECBM, which I had repaired by absfixer. Still have the problem after repair, don't think this particular problem was related.
06-03-2012, 04:08 PM
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Yes, I have done multiple "panic" stops. The first time I noticed the issue was at an autocross shortly after purchasing the car. The right rear LOCKS up, and stays locked, until I release the brakes. I was literally unable to stop at the finish 4 runs in a row. It isn't just premature, the tire stays locked and the car will not stop. I can feel the pedal pulsating, but the 200 ft of skid mark tells me the right rear never released. I have also repeatedly tested it on a secluded road after my attempted "fixes". When I release the pedal the wheel starts turning again, with a slight delay, maybe a 1/4 of a second.
Replacing the calipers is one thing to try but as I suggested before take a look at the bracket before you pop for the calipers. You could also rebuild the calipers. If there is nothing wrong with the caliper sliding mechanism (other than bolts) the GM rebuild kit is cheap and it isn't hard to rebuild them if you have an air compressor to force the pistons out.
I am attaching the ABS Theory of Operation as well. If you look at the BPMV diagram you can see the right rear outlet is located at number 10 on the diagram. If one of those valves feeding the right rear wheel isn't sealing or moving correctly that could be the problem and maybe the ABS diagnostics can't detect it.
Bill
06-03-2012, 05:44 PM
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I had the same problem on my 2001 coupe. Check the flexible hose carefully for visible kinking or other damage. Mine was separating internally and would allow application of the brake but would not allow the fluid to flow in the reverse direction. It would gradually leak down and finally allow the caliper to release. Took me quite a while to figure out what the problem was. Replaced hose with SS line and all was OK.
06-03-2012, 09:54 PM
#13
If OP can't throw parts at it how about trying to swap LR hub to RR hub and see if problem travels? How about a less grippy rear pad? No put down intended are you sure you are not a brake pedal stabber? High speeds makes pedal stabbing worse. Connections of the connector to the brake hubs are not that great. I had a bad connection hitting some bumps and flew off the track last month. ABS locked up 4 wheels and flatspotted them with 1 going to the metal cords. Ruined a good set of A6 hoosiers. Check those connections to the hubs.
06-04-2012, 12:18 AM
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From what you are saying it sounds like the ABS may be attempting to release brake pressure to the right rear wheel. Two things may be happening. 1) The BPMV has a stuck valve the ABS diagnostics isn't finding (doubtful but a possibility); 2) The caliper is hanging up in such a way that only a complete pressure release by removing your foot from the pedal works. I am not sure but I don't think the ABS system completely releases pressure to a brake when the valve opens. It reduces it but can't drop it to zero.
Replacing the calipers is one thing to try but as I suggested before take a look at the bracket before you pop for the calipers. You could also rebuild the calipers. If there is nothing wrong with the caliper sliding mechanism (other than bolts) the GM rebuild kit is cheap and it isn't hard to rebuild them if you have an air compressor to force the pistons out.
I am attaching the ABS Theory of Operation as well. If you look at the BPMV diagram you can see the right rear outlet is located at number 10 on the diagram. If one of those valves feeding the right rear wheel isn't sealing or moving correctly that could be the problem and maybe the ABS diagnostics can't detect it.
Bill
Replacing the calipers is one thing to try but as I suggested before take a look at the bracket before you pop for the calipers. You could also rebuild the calipers. If there is nothing wrong with the caliper sliding mechanism (other than bolts) the GM rebuild kit is cheap and it isn't hard to rebuild them if you have an air compressor to force the pistons out.
I am attaching the ABS Theory of Operation as well. If you look at the BPMV diagram you can see the right rear outlet is located at number 10 on the diagram. If one of those valves feeding the right rear wheel isn't sealing or moving correctly that could be the problem and maybe the ABS diagnostics can't detect it.
Bill
I had the same problem on my 2001 coupe. Check the flexible hose carefully for visible kinking or other damage. Mine was separating internally and would allow application of the brake but would not allow the fluid to flow in the reverse direction. It would gradually leak down and finally allow the caliper to release. Took me quite a while to figure out what the problem was. Replaced hose with SS line and all was OK.
If OP can't throw parts at it how about trying to swap LR hub to RR hub and see if problem travels? How about a less grippy rear pad? No put down intended are you sure you are not a brake pedal stabber? High speeds makes pedal stabbing worse. Connections of the connector to the brake hubs are not that great. I had a bad connection hitting some bumps and flew off the track last month. ABS locked up 4 wheels and flatspotted them with 1 going to the metal cords. Ruined a good set of A6 hoosiers. Check those connections to the hubs.
, most of my experience was in non-ABS Mustangs with a ton of negative camber (where the front brakes were prone to locking), so I don't consider myself much of a brake stabber. Testing the car out though I am just nailing the brakes trying to get the rear to lock, and it does, everytime (ONLY the right rear). At the event no matter how softly I got on the brakes it was locking. I have tried two different compound pads, no change. I have disconnected and re-connected the hub multiple times. I may try swapping hubs, it needs two new fronts anyway.
06-04-2012, 03:14 AM
#16
Racer
I was having the exact same issue. Right rear locks up while all others work fine in ABS mode. No codes, no service ABS light, no other issues.
I unfortunately did a shotgun approach to the fix, so I'm not sure what exactly fixed it. The rear pads were just about done, especially the left inside pad. I discovered a very frozen slide pin on that caliper. The right side caliper looked good with evenly worn pads. While i was at it, I disconnected the wheel speed sensors to verify if I could throw the codes, they did. Button it up and took it for spin to bed the pads and voila, it was fixed.
FYI, Im running Cobalt XR-1s, C6 Z51 calipers and Hoosier A6s.
I unfortunately did a shotgun approach to the fix, so I'm not sure what exactly fixed it. The rear pads were just about done, especially the left inside pad. I discovered a very frozen slide pin on that caliper. The right side caliper looked good with evenly worn pads. While i was at it, I disconnected the wheel speed sensors to verify if I could throw the codes, they did. Button it up and took it for spin to bed the pads and voila, it was fixed.
FYI, Im running Cobalt XR-1s, C6 Z51 calipers and Hoosier A6s.
06-04-2012, 05:39 PM
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I hope! 
Thank you. I am going to replace the hose, then the caliper, then rotors, then do the same to the other side, then set the car on fire...
I was having the exact same issue. Right rear locks up while all others work fine in ABS mode. No codes, no service ABS light, no other issues.
I unfortunately did a shotgun approach to the fix, so I'm not sure what exactly fixed it. The rear pads were just about done, especially the left inside pad. I discovered a very frozen slide pin on that caliper. The right side caliper looked good with evenly worn pads. While i was at it, I disconnected the wheel speed sensors to verify if I could throw the codes, they did. Button it up and took it for spin to bed the pads and voila, it was fixed.
FYI, Im running Cobalt XR-1s, C6 Z51 calipers and Hoosier A6s.
I unfortunately did a shotgun approach to the fix, so I'm not sure what exactly fixed it. The rear pads were just about done, especially the left inside pad. I discovered a very frozen slide pin on that caliper. The right side caliper looked good with evenly worn pads. While i was at it, I disconnected the wheel speed sensors to verify if I could throw the codes, they did. Button it up and took it for spin to bed the pads and voila, it was fixed.
FYI, Im running Cobalt XR-1s, C6 Z51 calipers and Hoosier A6s.
06-06-2012, 10:19 PM
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Brake Hose
I had the same problem on my 2001 coupe. Check the flexible hose carefully for visible kinking or other damage. Mine was separating internally and would allow application of the brake but would not allow the fluid to flow in the reverse direction. It would gradually leak down and finally allow the caliper to release. Took me quite a while to figure out what the problem was. Replaced hose with SS line and all was OK.
06-07-2012, 10:27 AM
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When you got the ABS unit back you said you still have the same problem. I assume this is the lockup.
In your description of the problem you state that you had No Comm errors.
After reinstalling the unit is this still part of your comment "same problem" or did this go away?
In your description of the problem you state that you had No Comm errors.
After reinstalling the unit is this still part of your comment "same problem" or did this go away?