Never Assume: Not-So Routine ABS Problem

[Joe Marconi]

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[TheTrustedMechanic]

A few weeks back a customer came in complaining that, at times, when he applies the brakes lightly the ABS would come on. He was sure that he was on firm ground, not skidding. We dispatched the job to one our diag techs. He performed his routine visual inspection and then hooked up the scanner for a road test. The tech clearly observed, thru the scanner, that the left front signal was dropping out at times. Having seen countless issues with GM wheel bearing/hub/ABS components, he advised the service writer to order a new wheel hub.

 

After the new part was installed, the tech performs a final road test only to find that the problem is still there. Is it a defective part? After reviewing the process, I instructed the tech to hook a scope right to the left wheel sensor and monitor the signal at the wheel and also monitor what the computer was seeing. Sure enough, the signal at the wheel never faulted, but the computer was seeing something else.

 

We opened up the harness, which is fastened to the suspension, and found a tiny break in the harness which was flexing when the brakes were applied. A new left side ABS harness fixed the problem.

 

The lesson: Never assume; when we see pattern failures over and over again, it can lead us down the wrong path. Always stick to the basics.

 

That is why I'm not the fastest with my diagnosis. I want to see the failure, not assume it's there. I had a similar issue with my personal car. when I bought it it had a code set for the RF wheel speed sensor. Like your tech I knew this was probably a wheel speed sensor issue so I wasn't too concerned with it when negotiating the purchase price. But when it came time to actually fix the car I did the due diligence and tested the WSS and sure enough it was open so I replaced the wheel bearing only to have the code still present. Like you a further inspection revealed the harness had a break in the wire not far from where the control arm attached to the sub-frame.

 

Even though I did test and confirm a failed component with a pattern failure history, like you wrote, "Never assume; when we see pattern failures over and over again..." Luckily it was my own car, that would not have been a happy second phone call to the customer.

 

But it still begs the question, when you find a fault, when you've tested and verified a fault, how much further should you go "just to be safe?" I only ask this because, like in the situation with my car there was more than just one failure. But if you do additional testing every time you do diagnostics you are potentially wasting time and losing money.

[bigpigdave]

A few weeks back a customer came in complaining that, at times, when he applies the brakes lightly the ABS would come on. He was sure that he was on firm ground, not skidding. We dispatched the job to one our diag techs. He performed his routine visual inspection and then hooked up the scanner for a road test. The tech clearly observed, thru the scanner, that the left front signal was dropping out at times. Having seen countless issues with GM wheel bearing/hub/ABS components, he advised the service writer to order a new wheel hub.

 

After the new part was installed, the tech performs a final road test only to find that the problem is still there. Is it a defective part? After reviewing the process, I instructed the tech to hook a scope right to the left wheel sensor and monitor the signal at the wheel and also monitor what the computer was seeing. Sure enough, the signal at the wheel never faulted, but the computer was seeing something else.

 

We opened up the harness, which is fastened to the suspension, and found a tiny break in the harness which was flexing when the brakes were applied. A new left side ABS harness fixed the problem.

 

The lesson: Never assume; when we see pattern failures over and over again, it can lead us down the wrong path. Always stick to the basics.

How did you charge the customer? Did you reinstall the original wheel bearing/ sensor assy.? Did you charge for the incorrect diagnosis time? I always have trouble with this type of situation as I don't feel ok charging the customer for what I believe is my mistaken diagnosis.

 

Dave

[Joe Marconi]

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