ABS Issues

#1

I’m trying to diagnose non-functioning ABS.

Here’s the chronology so far: ABS did not work when I bought the car. I determined that the ABS light bulb was missing. After replacing the bulb, it was lit all the time. Pulled the ABS relay and found the fusible link wire was blown. Soldered in a new wire and replaced the relay. The ABS bulb was no longer lit. (I assumed that meant the solder job on the relay was successful) Drove the car and the ABS bulb did not come on. Hit the brakes hard and the rear brake(s) locked, causing rear end to swerve to the left. The ABS light never came on.

Any suggestions on what could be wrong with the ABS or what I should test next (and how)?

Fixing the ABS should take care of the rear end swerving to the left since the brakes wouldn’t be locking, but if I can’t fix the ABS, any suggestions for equalizing the rear braking? This sent me on a wild ride in the rain at the end of the front stretch at VIR, so I’d like to avoid that in the future.

#2

I am pretty sure there is only one brake line going to the rear and it is split and goes to each rear claiper. That said the ABS system has no way of controling each rear wheel separate. If you have lock-up with one I would start by makeing sure both rear calipers are in good shape. My car has a tendency to dart Left due to the front caliper sometimes sticking when the pads get low. I now have new calipers hoping to stop this. Another thing to check would be the proportioning valve installed on the line going to the rear. They tend to go bad.

Michael

#3

ilateapex wrote:

[quote]I am pretty sure there is only one brake line going to the rear and it is split and goes to each rear claiper. That said the ABS system has no way of controling each rear wheel separate. If you have lock-up with one I would start by makeing sure both rear calipers are in good shape. My car has a tendency to dart Left due to the front caliper sometimes sticking when the pads get low. I now have new calipers hoping to stop this. Another thing to check would be the proportioning valve installed on the line going to the rear. They tend to go bad.

Michael[/quote]

Michael’s right. This isn’t an ABS problem. I bet that you have a right brake is not braking as much as it should. Too little braking is easier to imagine then too much.

Let me ask a dumb question…you mentioned the car swerving to the left. Is there more evidence that this is a rear brake problem? It can be hard to figure out which wheel is locking up, much less if it is front or rear. Because of the piston size and porportioning valve, our rear brakes don’t do much. IMO in order for a rear tire to lock up before the fronts, something assymetrical would have to occur like the rear tire hits oil.

I did a lot of brake pad testing a while back. No matter what I did, I never got the rears to lock up before the front. Not even once.

#4

To try to diagnose the ABS system, disconnect one of the wheel speed sensors and see if the ABS light will come on when the car is moving. If it does, that tends to suggest that the ABS module may be working.

The next test would be to get the car out in a wet parking lot and see if ABS will engage. You don’t need a lot of speed on the car for this test, just a slippery surface. If you get wheel lock up and no activation of the ABS, that would tend to suggest a bad ABS module or a bad ABS pump. I’d try a different ABS module before I replaced the pump.

#5

Ranger wrote:
Let me ask a dumb question…you mentioned the car swerving to the left. Is there more evidence that this is a rear brake problem?
[/quote]

The car doesn’t really “swerve” to the left, more like the rear end kicks out to the left when the brakes lock.

True, it is hard to tell which wheel locks, but the fact that the rear end wants to kick out makes me think it is the rear. Kind of like yanking the hand brake.

#6

I’m pretty sure there is only one brake line going to the rear that then splits off at the rear subframe and goes to the two rear calipers. I hadn’t thought about that. So, if the ABS senses either rear wheel locking up, the ABS brain causes it to cut pressure to both b/c there is only a single line from the ABC pump to the rear.

#7

If either right front or right rear wasn’t applying enough clamping force on the rotor, you’d have these symptoms. One way to figure it out is with an IR thermometer. After a couple hot laps there should be enough right vs. left temp differential to give a clue.

I want to say that you could tell if it was right front or right rear by how the car handled. Kind of like does the car “go left” or is it mostly “rotation”. But I’m thinking focusing on that could end up fooling a person. The thermometer idea should be pretty foolproof.