CobraEatr

I've had my '93 Z28 for almost 12 years and I've had this problem since the day I bought the car. I have some spare time on my hands and I'm ready to fix this issue. The problem below happens approximately 75% of the time I drive the car. Here we go......

Every time I start the car I disengage the ebrake (I have an M6) and I let the car warm up for about 30 seconds. I then put the car in reverse and begin to back up. While I'm in the process of moving backwards, I ALWAYS hear a faint grinding noise that lasts for just 1 second or so. The exact same time that I hear the noise I see on my dash that the "Low Trac" light flickers ON just for a fraction of a second. Once I finish backing out of my driveway, I put the car in 1st and slowly begin to move forward. After about 2 seconds of moving forward, the "Low Trac" light will flicker on again and then both the "ABS Inop" and "Brake" lights will come on and stay on for the duration of my drive. This happens about 75% of the time I drive my car (it's my daily driver).

I seem to remember about 10 years ago asking this question and I never really found out what it was. I think someone mentioned to me that the '93 M6s perform some sort of "Low Trac" check on startup but I can't really remember. I've tried searching but can't find anything that matches my exact situation. The car has always braked flawlessly and I've never had any issues other than the annoying lights on the dash and I've also always wondered why it makes that *grinding* noise as I begin to back up. I do all my own routine brake work (changing rotors/pads/fluid etc) but I have no idea where the "sensors" are or how to check them. Any help is greatly appreciated.

shoebox

Sensors are in the rear of each front wheel hub and in the top center of the rear end housing. I would check what you can of the wiring and connections. A scan for ABS codes might help (you will need a scanner capable of reading them). The ABS system is highly complicated and takes up hundreds of pages in the service manual. I've really had no particular dealings with it.

If you want a particular person's attention to a thread, send them an email with a link. Putting a name in the title only makes other people avoid the thread and lessens your opportunities for getting more help (MHO, of course).

Injuneer

Couple points:

Please don't "page" individuals on the tech forums. If you want feeback from a specific person, send a PM or an e-mail.

Please post on the correct forum. None of the above relate to the "engine". These topics get discussed regularly on "Suspension, Chassis and Brakes".

Thanks.

Moving.

pgerst

I am a little confused how the car can brake flawlessly with ABS Inop and Brake warning lights on? Does the abs actually work when these lights are both on or can you lock up the brakes, i.e, you do not have functioning abs? I suspect you do not have abs when the warning lights are on.

"Easiest" way to determine what is wrong without replacing everything is to get a GM scanner which has the brake software in it; the tech 1s and 1As can be found on ebay for about $200-$300. Get the error codes and go through the factory manual diagnostics which requires manually cycling the abs unit with the scanner.

I have a 95 which is a little different than a 93 but I recently had the ABS Inop and Brake lamps (though both on at all times) and while the brakes worked in the sense they stopped the car, they did lock up, so no abs; turned out to be a bad abs motor pack.

CobraEatr

I'm not sure if the ABS is working or not. One would assume that if the light is on then it isn't working, however I've never locked them up before and I've been making some pretty hard stops over the past 10 years. I'd rather not throw down a couple of hundred bucks for the scanner if I didn't have to.

Does anyone else with a M6 hear a slight grinding noise as they reverse and their "Low Trac" light flicker? I remember reading years ago that this was normal either for '93 M6 owners or all M6 owners. I can't remember exactly but I briefly had a conversation with another member back then about it.

Injuneer

Have you ever pulled the sensor on top of the rear axle housing, to check for damage indicating contact with the reluctor, or heavy accumulation of metal particles?

My car was an M6 for the first 7 years of its life, and I never noticed a grinding sound in reverse, and never had a problem with the ABS.

mediclafemme

had a similiar incident here, was the sensor Shoebox mentioned earlier