ThetaTau87

My car is an '89 GT and twice since I bought it in 2013 I've had to make a panic stop, once because a kid ran across the road and the other because a car pulled out in front of me without looking. Both times the rear tires locked up and the rear end started to slide.

I had no issues with the brake bias when I did a track day. Threshold braking performance without engaging ABS was excellent, so I don't think it's a brake bias issue. The lock up was only during a panic stop when I stomped on the pedal hard and engaged ABS. In those conditions the rear tires locked up until I let up on the brake pedal and got out of ABS engagement.

I don't have any warning lights on the dash to indicate an ABS problem. The "ANTILOCK" light comes on when I turn on the ignition and goes out immediately when the engine is started.

Is there a known fault that would cause this?

Speedtoys

If the ABS was engaging, they were not locked up..lifting turns off ABS, which extends braking distance.

Sounds like what you hears was ABS performing proper threshold braking.

FredR

If you heard the ABS unit chattering then in simple terms it was likely doing what it is intended to do.

Remember when braking hard weight transference is to the front of the car, the back goes light so not as much weight encouraging the tyres to grip at the rear.

Perhaps you can clarify exactly what was happening- no point in trying to fix a non problem if indeed that is the case here.

Speedtoys

This is an exercise we do at our NASA Teen Clinics...we put them thru ABS theory and use..most people don't know what ABS feel and sounds like, so this is common..they lift and extend braking 50-75 feet trying to 'fix' it.

AO

In the automotive world (previous life) we call ABS and other associated ESC, etc, "Slip control."

In a properly working Slip control system, the system is designed to pulse the brakes when it detects a certain amount of slip (i.e. when the wheel suddenly stops turning given a speed threshold). It sounds like you may have an issue where the rear circuit is not detecting slip, but then you should have an ABS warning, so this is contradictory.

From your description, it sounds like your panic stops were at relatively slow speeds (sub 25mph?). Depending on the calibration of the system, that might be below where it would pulse the system.

Have you done a panic stop at a slightly higher speed? say at 30+? Does it solidly lock up the rears?

If you feel the system engaging (pulsing in the pedal), then the system is engaging. There is bound to be some skidding or slip esp. with a system as old as the 928's. Modern systems do a much better job of controlling slip and maximizing deceleration (and acceleration) while providing control of the vehicle.

ThetaTau87

Both stops were from about 50 MPH. Yes, a REALLY stupid/lucky kid ran across a busy major road following his 3 friends, stumbled and fell in the middle of my lane. Luckily he was able to get up and out of the road in time. He was way too close for me to have stopped short of him where he fell.

I know what ABS engagement feels like and I also know that the rear tires were locked. I had the dual solid black stripes and tire smoke to prove it as well as the rear end sliding sideways. I don't specifically remember the ABS vibration in the brake pedal, but given the panic situation I wasn't concentrating on determining if ABS was working. I am assuming it engaged due to how hard I stood on the pedal and that there was zero front lock up and solid rear lock up for 20+ feet. If there is an ABS fault the diagnostics aren't catching it.

I guess I need to do some tests to verify if ABS does kickn in​​​​​.

ThetaTau87

I just got back from some testing and verified that my ABS is not working at all. I was able to get all 4 wheels to lock up solid above 30 mph.

Now the question is how is that possible if my "ANTILOCK" warning light is not on when driving, but does light up with all the other dash warning lights when the ignition is turned on? Also, and more importantly, how do I fix it?

GregBBRD

The entire system is controlled by an ancient logic computer.

I'd drag out my dedicated ABS system tester....and check out the entire system, sans computer.

If everything is normal. I'd plug in a different computer, after that.

If you can't find one of these testers (super rare), you might want to check for basic power and grounds at the control unit and then try a different computer.

jeff spahn

You might want to. Is it Greg or Sean. Have them test your system as Greg suggested.

AO

Brad, Puzzling for sure. If we need, we can try swapping in the computer from my GT to see if that works.