99GMC Can

I had an accident a week ago with my truck. Normally when I'm sratching for traction the ABS does it's thing. There some whirring noises and brakes grab and release as traction permits. But, this time..

This time the brakes released!!.. and I started to coast! The pedal felt like I was stopped already and there were no skid marks.

Has anyone else experienced anything like this during evasive manuvours? There was some loose gravel on the pavement. (not a clean pullout) This makes it slippery but why would the brakes fully release? The rear wheels were still on the clean pavment when it all happened.

AZ28driver

The abs computer works by monitoring wheel speed. If the computer see's that a wheel is locked it will pulse the brake caliper on that wheel. Could it be that you were sliding on the grass/gravel and it gave you the sensation of the break coming off?

99GMC Can

There was no pulsing at all. It was a total release. The rear wheels were still on the clean pavement. I've looked through the shop manual and they refer to a feature in the ABS called dumping. This is when line preasure returns to the master cylinder and no preasure reaches the calipers. I think it is supposed to be a momentary function so that the wheels start to turn again. I made a hard left under braking then when the front hit the pullout there was a total release of the brakes. The pullout had some dirt and gravel that had blown off the road. Thing is, the wheels never did lock up or skid.
Its a strange feeling when you have to run into your friends truck to stop.

wrencher

iTrader: (2)

Dumping is release of pressure from caliper by abs hydralic control unit. It can do this dump cycle many times per second=anti lock brakes. ABS cotrol unit watches wheel speed sensors & knows when wheel lock-up will occur during braking this starts dump cycle. There are codes for an inactive abs system & codes for dump valve not resetting also . I don't think the abs system is to blame.

99GMC Can

From GM Manual

DTC C0269 or C0274 Excessive Dump/Isolation Time

Circuit Description (DTC C0269)
The EBCM microprcessor grounds the dump coil to energize and close the dump valve. This will release the excessive brake pressure which was applied by the driver to the wheels. If the EBCM microprocessor generates a dump command on one or more wheel for greater than nine seconds, the DTC will set.

Conditions for setting DTC C0269
1. Locked Rotors
2. Braking on excessive slippery surfaces for more than 9 seconds.

Circuit Description (DTC C0274)
The EBCM microprocessor grounds the isolation coil to energize and close the isolation valve. This will prevent any additional brake pressure applied by the driver from reaching the wheel. Further increases in brake pressure will be prohibited. Each isolation valve is closed independently to isolate each wheel. The EBCM microprocessor also uses software subroutine to monitor for any ABS event which would initiate an isolation command. This subroutine is performed every 1.0 second.

Conditions for setting the DTC C0274
Isolation time (pressure hold) exceeding 255 consecutive 1.0 second checks.

99GMC Can

So, the way I read this

To set the Excessive dump DTC, The EBCM has to be in dump mode for 9 sec. Next time you use your brakes, keep this in mind. You will have to have your foot on the brake for 9 sec. Try taking 9 sec before using your brake. 9 seconds! You could die three times in 9 seconds!

To set Excessive isolation DTC, the time must exceed 255 seconds, which is 4.25 minutes!! Does this mean the valve has to screw up for over 4 minutes before the dealer will acknowledge a problem?

wrencher

iTrader: (2)

There are alot more service codes in abs system. + the anything that does anthing with basic braking is a priority & they've done there homework ,if there was some kind of funky intermittant problem the legal ramifications would cost big! All auto mfg. got their **** together on their abs systems. If you got a legit problem check it out with the federal transportation safty board they have the stats on vehicle's and their perticular records.

99GMC Can

Thanks for the input Wrencher. All I realy know is that I've had ABS working for me many times. This time something failed. I underdstand fail-safe must be built in to braking systems. I suspect something is worn out. Whether GM admits to a problem (which I'm sure they don't want to) or not, I personally cannot trust my brakes anymore. If the whole release thing was a fragment of my imagination, I suppose I should see a shrink instead of typing at you. Regardless, I will be removing the assembly for my peace of mind. I would rather be in control of the skidding, than having no brakes at all. Call me old fasioned, but..
ITS ALL ABOUT CONTROL

SS-35th

iTrader: (2)

It may not be in your head. I've got a 1998 Chevy S-10 that I dirve to keep the miles off my camaro and there have been a few times (but not many) where the brakes have released on me going over extremely rough sections of road.

SRYOFCR

Actually I've had that happen to me before aswell.
about 2 weeks ago I was doing a brakestand into a burnout around a corner and I came around the corner and there was traffic so I hit my brakes but nothing happened, I had the brakes floored for a good 5 secs atleast before I seemed to have coasted into the back of a jeep.

Also a buddy of mine about 4 years ago had a jeep, and he came around a 180* corner way too fast, went to hit the brakes and they didnt engage at all, ended up hitting a curb.

99GMC Can

A little update for anyone curious:

I spoke with a GM tech who said the RCMP have been pulling the ABS fuse. Aparently when brakes are applied in midair, (good old boys jumping there new rides) the ABS does a full dump and will not recover until the brake pedal is released and reapplied. This goes agaist the idea of ABS altogether. Icey/Muddy conditions are same.