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ABS warning light? It ain’t always the ABS sensor!

Old 07-08-2019, 10:22 AM
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ABS warning light? It ain’t always the ABS sensor!

My daughter’s 2007 HHR gave her a fright: the “Service Traction Control” message appeared on the LCD panel, the ABS warning light and Traction Control warning lights came on, and the car had intermittent ABS activation episodes when she made low speed left turns.

Three simultaneous warnings (Is the car about to explode!?!?) on a dash display is enough to frighten some HHR owners straight into a dealership.

I said, “Wait, lemme look at it first.”

With 123,000 miles on it, I suspected it was time to change out the front wheel bearing/hub assemblies anyways, but for some reason, I decided not to load and fire the parts cannon at this HHR, and to actually isolate the problem(s).

As it turned out, I saved myself quite a bit of work.

I borrowed a high-end trouble code reader, plugged it in and found the DTC code C0040, “Right Front Wheel Speed Sensor Circuit.”

Since this code reader was capable of displaying real-time data, I drove around the neighborhood while monitoring the AC voltage output of all four ABS sensors. Sure enough, when slowing to a stop, voltage on the right front sensor dropped to zero while the car was still rolling, and while the other three sensors were still showing declining voltage levels.

Imagine my dismay when I discovered that the sensors on these little cars are integrated into the bearing/hub assembly, and aren’t simply bolted in, readily accessible and thus easily swapped out, the way they are on many large trucks.

Still, I reasoned, the problem might not be the ABS sensor itself.

I pulled off the right front wheel, disconnected the ABS pigtail (the pins and interior surfaces of the connector were clean and corrosion-free) and noticed that the ABS wiring (which is covered with a split-loom sheath to protect the wires from the elements) leading away from the pigtail disappeared through a hole in the wheel well and ended at another connector near that hole. I removed this entire segment of connecting wire from the car, tested the resistance of each wire with an ohmmeter, and found the problem.

The orange wire in the twisted pair offered no effective resistance, while the green wire offered mega-ohms of resistance. While stripping the loom off of the entire length of this twisted pair, I found a depressed spot on the loom. Underneath this spot both wires had suffered a crush injury, seen in the photo.





About this time my two brain cells got real agitated and started talking to each other (I break out in a sweat when this happens). You see, four months ago, my daughter mentioned that the brakes weren’t as effective as they should be, and while my son-in-law and I were testing them, I found a) bad bushings on the lower control arms, b) both front struts were leaking and pretty useless, c) brake pads worn to the metal backing, with associated scored rotors, d) a sticking/frozen right rear drum brake wheel cylinder.

We repaired all of these defects, and since I have perfect 20/20 hindsight, I admit that four months ago was the right time to inspect the ABS wiring. The underperforming struts and worn/loose LCA bushings may have allowed more range of movement than normal in the suspension and steering, and helped that wire damage to occur.

I soldered in a new segment of twisted pair, shrink-wrapped the solder joints, carefully recovered the wire with split loom to protect it from the elements, and also took care to route it to avoid pinch and crush points in the steering/suspension.

A test drive caused all of the warnings to vanish.
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Old 07-08-2019, 10:38 AM
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Something was actively bending that wire, not just slight vibration or movement.

It might be helpful to others to know exactly where the pinch was.

BTW, the sensor produces an AC signal.
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Old 07-09-2019, 02:09 PM
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Good bit of detective work! Thanks for sharing!
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Old 07-09-2019, 03:36 PM
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That can definitely do it. The ABS modules actually measure current, and not voltage, like I expected. On top of that, it is expecting changes in current from the tone wheel (high-low readings). If there is not enough "contrast", it fails. If you have this situation, your high current isn't as high as it should be, and it throws a code.

I recently got educated by Continental on all of this. Bosch systems work similarly. The currents are in milli-amps, too. Touchy things... Something like a 50 milli-amp reduction in the available current will make it fail.

Thought y'all might like this. Great work by the OP finding it!
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Old 07-09-2019, 10:01 PM
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Did you mean amplitude/frequency? Since that's the variable in an AC signal.
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Old 07-10-2019, 07:38 AM
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Thankfully, Oldblue doesn’t have ABS .
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Old 07-10-2019, 08:02 AM
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Originally Posted by Oldblue View Post
Thankfully, Oldblue doesn’t have ABS .
Thanks for your pat on the back.
This guy knows stuff, he explained it very clearly.
Hope he comes back to discuss other problems with the HHR for others who have issues.
Just my 2 cents.
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Old 07-10-2019, 11:57 AM
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I’m not sure why the quote, but it’s very hot up here and Thankfully, I don’t have this issue.
Kudo’s for tracing down the pinched wire and confirming the repair fixed the issue.
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Old 07-10-2019, 06:00 PM
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Originally Posted by donbrew View Post
Did you mean amplitude/frequency? Since that's the variable in an AC signal.
The present systems for sure measure current differential. (you see how I avoided saying current systems measure current... LOL) I can't confirm an AC signal. It's a series of pulses, not a wave. Not sure how it could be AC.
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Old 07-10-2019, 07:02 PM
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Digital. Essentially sensor counts the points on the ring by producing a AC current with a varying frequency providing a digital signal to the computer because computers don't speak analog.

From Mitchell OnDemand


In addition the DTCs involved with the WSSs measure the amplitude.
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