YACT: 'Low Trac' light when braking

[Aquila76]

96 Pontiac Grand Am Sedan w/160K mi, 2.4L I4 Auto, FWD, ABS (Disc front, Drum rear)

For the last week or so, when I come to a full stop (like a parking space or red light), sometimes the 'Low Trac' warning lights up on the dash. Normally, this comes on when one of the wheels is spinning faster than the other.
I've checked the tire pressure and it's where it should be. The tires (Yokohama Avid H4S) have plenty of tread and show no signs of defect.
I'm not stopping in puddles, oil, loose dirt, etc. so I know I'm not sliding which can trigger the Low Trac or ABS. The ABS is not kicking in, which you can really tell on this car when it does.
This car is prone to wheel speed sensor problems (from various review sites), so that was my first guess. However, OBDII is not triggering wheel speed sensor (or any other) problems.

Any ideas/suggestions/tests?

 

[Zenmervolt]

Aside from replacing the wheel speed sensors and seeing if it goes away, not really. A problem such as you describe would not trigger the OBD II code for the wheel speed sensors because the system still thinks the sensors are sending good data. If the sensors were sending bad data and the system recognised it as bad data, it would disable the traction control system.

ZV

 

[Heisenberg]

I'd start with the wheel speed sensors since that's the obvious thing. A decent shop should be able to plug a scantool in to monitor the speed sensors during a drive to see if one or more sensors reports the wrong speed.

 

[Shame]

My 1995 Pontiac did this for a while. Then a couple of months later, the ABS light came and stayed on.

 

[Aquila76]

Dammit! Anyway of diagnosing which sensor(s) this is(are) myself? The part alone is ~$90 for each front WSS and ~$180 for each rear one. The rear one has the wheel bearing & hub/lug assembly built onto it. I really can't afford the repair shop costs right now.

 

[Trikat]

Ah yea. My 2002 Grand Am also has the loud ABS IS ON YOU ARE GOING TO DIE sound.

 

[Aquila76]

Originally posted by: Trikat
Ah yea. My 2002 Grand Am also has the loud ABS IS ON YOU ARE GOING TO DIE sound.

The first time my ABS kicked in, I literally thought the master cylinder or something related was going to explode. It's fairly smooth stop-wise, but that sound scares the hell outta my passengers. It's so loud that pedestrians stop and run for cover. I think I'm going to stick w/German ABS systems from my next car on.

Click-rattle, Click-rattle, Click-rattle. Now that I think of it, it sounds a LOT like an IBM Deathstar in it's final minuteses.

 

[Aquila76]

OK, I keep seeing this as the method for diagnosing the wheel sensor. Seems fairly simple; but is it accurate?

Basically plug a DVM (set to Resistance @ 20K Ohm) into the wheel speed sensor. If you get minor reading, sensor is good. If you get an open circuit, sensor is bad.

Should I also give the wheel a spin and see if I get a pulse (DC 12V, I'd assume)?

 

[Pepsi90919]

the low trac light shouldn't come on when stopping.

when you make your first acceleration, you should hear the trac self-test too.

 

[Aquila76]

Originally posted by: Pepsi90919
the low trac light shouldn't come on when stopping.
Yeah, I've read that it's usually a sign of a wheel speed sensor failure. The ETS thinks one of the wheels is moving while the other is stopped and triggers a false 'low trac' warning. In theory.

when you make your first acceleration, you should hear the trac self-test too.
What would that sound like?

 

[kranky]

Is it even necessary to repair it? Doesn't seem to be causing you any problems.

 

[Cattlegod]

Another vote for wheel speed sensor


edit - if the car is running fine, i wouldn't bother doing anything to it. your only problem will be the ABS kicking in when it shouldn't be (bad because it stops you from stopping ) . but you don't even have to fix that, just pull the ABS fuse. you will just be without an ABS.

 

[Aquila76]

Yeah, I'm planning on replacing the car in the fall or next spring, so I don't wanna sink a lot of money into repairing it. It's not even kicking the ABS on, which is weird, just lighting the Low Trac indicator. I haven't tested the ABS yet, that might be a good idea and I can do it in my workplace parking lot. If I have some time this weekend, I'll Ohm it out and check junkyards. They're easy enough to get on/off - well, the front ones anyway. If it's the rears, fsck that! Disassemble the whole wheel assembly my ass.

Thanks guys. I think I'll fix it with electrical tape over the indicator for now.

 

[sao123]

Originally posted by: Aquila76
Dammit! Anyway of diagnosing which sensor(s) this is(are) myself? The part alone is ~$90 for each front WSS and ~$180 for each rear one. The rear one has the wheel bearing & hub/lug assembly built onto it. I really can't afford the repair shop costs right now.

option #2... remove ABS fuse... problem solved for free.

 

[AdamSnow]

Check your brake fluid... when low, it will make an alert show up on the dash. On my car it's the e-brake light, but on different brands this will change.

 

[boomerang]

I don't think it's a wheel speed sensor at all. Low Trac should come on when accelerating or cruising not when braking. If it was a WSS, your ABS light would be on.

There's a decelerometer in the vehicle somewhere. They are usually somewhat centrally located. I'm not saying that's what it is because I don't know, but I do believe it's an electronic problem.

Step by step through the trouble shooting diagram in the factory service manual will be the best way to fix this without throwing parts at it. You already know they're expensive.

As a side note, the best way to keep your ABS system "healthy" is a regular flush of your brake fluid. Every two years is good, but once a year would not be excessive if you have the means to do so. This is of course won't help with electronic glitches.

 

[Aquila76]

Originally posted by: boomerang
I don't think it's a wheel speed sensor at all. Low Trac should come on when accelerating or cruising not when braking. If it was a WSS, your ABS light would be on.

There's a decelerometer in the vehicle somewhere. They are usually somewhat centrally located. I'm not saying that's what it is because I don't know, but I do believe it's an electronic problem.

...

That makes sense. I almost forgot about that sensor (Vehicle Speed Sensor or VSS). Wouldn't that trip an OBDII code, tho? My codes were clear the couple times I tested it after the Low Trac indicator lit (a week ago and last night).

 

[boomerang]

Uh, I'm not sure what you mean. You're talking VSS and I was referring to the decelerometer. I shortened wheel speed sensor to WSS, which is an acronym that I'm not sure GM even uses. So I'm not certain what you're referring to.

I quit working on cars around the ODBII days. My scanner would have required a new cartridge (to the tune of $800) and I could not justify the cost for horsing around in my garage. The prior cartridges were $125 which was still too high, but I paid the money when needed.

The deal is with these things is that you can justify in your mind a lot of different scenarios. When you don't work with this stuff all day, or even on a regular basis, you don't have a knowledge base to go from.

There are a number of replies in this thread that seem feasible. But which one is correct, if any?

That's why I suggest the factory service manual. They are available from Helm Inc. But they too used to be reasonably priced. But no more. Another reason I quit working on cars at the level I was. Try ebay. You should be able to get one for a reasonable price.

Working through the troubleshooting diagram for your specific problem will lead you directly to the culprit. A few basic tools and a DVOM are about all you'll need. If it's not setting codes, you should be able to troubleshoot it with simple tools.

A caveat though. On older cars, corroded connectors (even slightly, by that I mean it will take a close inspection) can be a difficult thing to troubleshoot.

You might even have some worn insulation on a wire that is shifting enough during braking to turn on the light.

I don't mean to discourage you. I'm just trying to point out a few things that can drive you batty.

 

[boomerang]

Originally posted by: Aquila76
OK, I keep seeing this as the method for diagnosing the wheel sensor. Seems fairly simple; but is it accurate?

Basically plug a DVM (set to Resistance @ 20K Ohm) into the wheel speed sensor. If you get minor reading, sensor is good. If you get an open circuit, sensor is bad.

Should I also give the wheel a spin and see if I get a pulse (DC 12V, I'd assume)?

In case you want to try this, spinning would be good, but even spinning slowy the pulses are very close together. A DVOM may not be able to respond fast enough to garner any real information from it.

 

[Aquila76]

Originally posted by: boomerang
Uh, I'm not sure what you mean. You're talking VSS and I was referring to the decelerometer. I shortened wheel speed sensor to WSS, which is an acronym that I'm not sure GM even uses. So I'm not certain what you're referring to.

I couldn't find a decelerometer sensor, but the Vehicle Speed Sensor (VSS) measures the acceleration/deceleration and reports it to the Engine and what passes for Traction Control sections of ECM. The WSS (still called that by GM) measures the individual wheel speeds and reports them to the ABS part of the ECM.
Believe me, I know how batty these things can get. My last car the dash lights would intermittently shut off. Ended up being a verrry tiny piece of electrical wire had some antifreeze on it that ate just enough through the wire to short it out intermittently. I think I figured that out about 3 months before I sold it.

Originally posted by: boomerang

Originally posted by: Aquila76
...
Should I also give the wheel a spin and see if I get a pulse (DC 12V, I'd assume)?

In case you want to try this, spinning would be good, but even spinning slowy the pulses are very close together. A DVOM may not be able to respond fast enough to garner any real information from it.

That's a good point. My Dad has a scope that I can use in place of the DVM. And it records over time, which is great for analyzing flaky signals.

 

[spidey07]

Originally posted by: Zenmervolt
Aside from replacing the wheel speed sensors and seeing if it goes away, not really. A problem such as you describe would not trigger the OBD II code for the wheel speed sensors because the system still thinks the sensors are sending good data. If the sensors were sending bad data and the system recognised it as bad data, it would disable the traction control system.

ZV

ooohhhhh...you're good.

:thumbsup:

 

[Aquila76]

Quick question, this is what you're talkng about by the Helm manuals?
Ebay Link
Ebay Link 2

 

[boomerang]

Originally posted by: Aquila76
Quick question, this is what you're talkng about by the Helm manuals?
Ebay Link
Ebay Link 2

Yes, those are the same manuals they use at the dealerships.
Here's what Helm wants. These manuals were typically $40 to $50 and they suddenly jumped up over $100.

 

[Aquila76]

Originally posted by: boomerang

Originally posted by: Aquila76
Quick question, this is what you're talkng about by the Helm manuals?
Ebay Link
Ebay Link 2

Yes, those are the same manuals they use at the dealerships.
Here's what Helm wants. These manuals were typically $40 to $50 and they suddenly jumped up over $100.

Yeah that was ridiculous considering my car is 11 years old now. eBay it is!