Hashish

iTrader: (1)

Doing a search did not yield the results I had hoped for, so here goes.

I have a 2001 Trans Am that I autocross regularly. My brakes are kind of old but they work for street use and reluctantly take the abuse of the monthly autocross. The issue is very apparent during hard braking. When I am braking hard, but not even close to locking the fronts my ABS will kick on and force my foot back, thus ruining my braking. While this is happening the low trac light on the instrument cluster comes on. I thought it was just me being stupid and thinking it was something I was doing wrong, but I had a more experienced friend and autocrosser who has been racing a 4th gen of his own for many years take a run in mine and he agreed that something is definitely wrong. Has anyone ever had a similar problem? Again this is only on moderate to hard braking. Could my ABS module be going bad, or maybe a wheel speed sensor? The rear is a 4 channel if that helps or makes a difference. Any help would be awesome.

Xsta Z 28

iTrader: (2)

I would best guess at a wheel sensor going bad . . .

V68Sweep

Sounds like you and I are having the exact same problem. I posted a thread also looking for advice, and most people mentioned the 2 things you did: ABS or wheel sensor. I haven't yet had the time to try to figure it out, but on the street I almost rear ended a car when traffic came to an unexpected stop and it was cloooose

TurbopigB4C

iTrader: (5)

Unplug your abs module and if its fixed then you know its a sensor or the module. It is likely a wheel speed sensor though. Also check to make sure your wiring looks fine and if you have an ohmmeter then see if any of the sensors are open by unplugging them and checking resistance between the two terminals. Find someone with a tech 2 and see if any of the sensors are giving any erratic readings. Different diameter tires from front to back can also cause this problem.

LS1CMR

iTrader: (12)

I have the same problem, feels like i only have rear brakes when it happens, i have 3-channel ABS. My problem is a bad unit so i am deleting ABS completely.

Hashish

iTrader: (1)

What kind of resistance should I be looking for when metering the wheel speed sensors? Should I also meter with the wheel moving?

Ironhead

Sounds like a really smart guy. I'll bet he is ruggedly handsome and a helluva driver too.

If you're going to keep the car for a while I strongly suggest purchasing THE FOLLOWING.

Yes I know those are a bit pricey but you can find them cheaper. Years ago I purchased a full set for less than half that.

Those things have proven to be invaluable many, many times.

Sam Strano

iTrader: (41)

First of all, if you had a bad sensor the ABS would NOT work at all, not work at strange times.

Second of all, the setup matters. One of the first reasons I went to a big front swaybar was because these cars can carry an inside rear wheel under trail-braking which makes teh ABS go into what's called ice-mode (it thinks you are on ice as one wheel is locked) and that causes the car to stop @ about .3 g vs. the about 1g it can normally, because..... it's thinks you are on ice.

Third, it can be a shock issue. A shock with a compression damping issue will allow the tire to bounce, locking it and freaking the ABS out. Are the shocks old?

Ironhead

From what I could see / feel, the shocks are in good shape Sammy. They're newish Konis, bought from you last year. He's on your springs so the car isn't slammed.

The car doesn't just do it trailbraking into a corner, it also does it braking in a straight line.

The only thing that gives me pause (because it's something I have no experience driving) is his staggered rubber set up. He has 315 street tires on the back and 275s up front.

I have no idea how the LS1 - Bosch era ABS / brake proportioning reacts to that.

TurbopigB4C

iTrader: (5)

With different overall diameter tires front to rear the ABS will not work properly. The module will see two different wheel speeds because of the difference. If you insist on using different size tires from front to rear just unplug the module and the car should stop fine and just not use ABS.

TurbopigB4C

iTrader: (5)

I am not positive on the spec but all should be close to each other. The main thing you are looking for is a sensor that will not give you a reading when checked. If it does not give you a reading that means its open. If the sensor is open it usually will throw the ABS light and just disable the system entirely but I have seen a few sensors that open intermittently and will not throw the light but its not very common. If you have one sensor that reads way different than the other see if a dealer will give you the resistance spec off SI.

Ironhead

A 275/40-17 tire has the same diameter as a 315/35-17 tire.

LS1CMR

iTrader: (12)

If you unplug the module you can potentially lose control of the vehicle due to the fact that there is brake bias at that point and the rear will lock up even under normal braking. I know this from experience.

Hashish

iTrader: (1)

Ok, a tad more info on the setup I guess. Strano springs and koni 4/3s bought and installed Feb. 2010 and 275/40-17 front and 315/35-17 rear, both Nitto NT555. I let Korry (Ironhead) drive the car so he can speak first hand on what the symptoms are as well. It does happen in a straight line as well as in a corner. So if not a wheel speed sensor, then what could this be? My rear brakes need a service pretty badly and vibrate the rear of the car when slowing repeatedly from interstate speeds, but I never feel the vibration while racing (when the problem comes up). Could the two be related or am I just being dumb?

Sam Strano

iTrader: (41)

Still could be ice mode... And frankly that's my almost 100% guess. I can happen in a straight line, especially if the car isn't on very good tires. Add bumps and it's even more likely. I obviously can't see what is happening.

And if the tires are significantly different diameters that will mess things up a lot.

One more point. If you go directly from wheelspin to ABS the car again assumes it's on ice, as it can't fathom that you could be torching the rear tires one second and locking the very next if you are normal.

Hashish

iTrader: (1)

Ice mode sounds right as you described it Sam. I have driven the car on ice and the best way to describe the way the pedal feels as it pushes back is like you are braking on ice. Thanks a bunch, my first step will be to service the rear brakes as I feel like they could be the root of the problem. I think my tires are fine as far as grip (they are no purple crack but they are fine for the street), and they are the same diameter front to rear. I will update as progress is made, June 26th will tell if the rear brake service was the solution.

V68Sweep

What would this mean? That the wheel would always lock up under hard braking? Because that's not happening to me. I'm not trying to hijack the thread, but I am having the same issue as the OP. My shocks are absolute junk (I'm waiting for new shocks to arrive in the mail), but even years ago, when they weren't great, but not terrible, I had the same problem, whether in a corner or straight line, under hard braking i'd get (and do get) a crazy knotchy feeling in the pedal, not only a lifting feeling...it's scary!

MrEddie

That is good info concerning the ice mode - did not know this. We sometimes autocross at a location that has some elevation changes. The "ski jump" as the CP guys call it, has turned on my ABS and traction control lights and kept them on until shutting down and restarting.

thunderstruck507

iTrader: (18)

I brought my Snap-On MT2500 from home this weekend. IIRC it can scan the ABS system, if you want we can take a look.

Hashish

iTrader: (1)

Hells yeah! We will take a look at it when the weather clears up