Q: I own a 2010 Chevy Express 2500. Last year I began having issues with the ABS on my front brakes. They would kick in at about 8 mph, down to dead stop.
I had a bad bearing, so I purchased a pair of bearings online (RockAuto). The bearings came with new ABS sensors. Installing the first bearing, I mucked up the ABS sensor, so I robbed the one off the other bearing. I ordered a replacement (ACDelco) sensor, and when it came in I replaced the other bearing.
The ABS problem continued, and I assumed it was because the two sensors were different manufacturers, so I ordered two more ACDelco sensors. Same problem.
In my research, I find passing mention of an air gap between sensor and ring. I bolted these directly to the bearing. The original had a paper-thin copper spacer that I tossed. Could this be an issue? Or could it be something else? Any and all suggestions are greatly appreciated.
-- Steve G.
A: Perhaps I should start with a quick overview of these parts before suggesting some ideas.
Your van utilizes a wheel speed sensor at each wheel so the ABS system can compare and mitigate differences in speed (wheel lock-up). A typical wheel speed sensor plugs into the hub, with its tip very close to a spinning reluctor wheel or tone ring (resembles a gear with many iron teeth). The sensor's coil of fine wire and magnet act as a tiny alternating current generator as the iron teeth pass by, outputting a positive/negative electrical waveform. The ABS system compares the frequency of the four waveforms and infers wheel speed irregularities, acting accordingly.
Correct wheel speed sensor operation requires clean reluctor teeth and a correct sensor to teeth air gap. Your reluctor is built into the sealed hub, which helps keep it clean. When a wheel bearing wears excessively and the hub wobbles, the air gap becomes irregular and the reluctor's teeth can hit the sensor, damaging it. This is likely what brought on your original symptom of low speed (4-10 mph) unintended ABS intervention.
I'm not sure how your new sensor became damaged; perhaps the cable became injured during installation. The aftermarket hub and sensor, as was the original, are a select fit, often utilizing a thin spacer beneath the sensor's mounting base as a means of setting depth/air gap. The needed gap is approximately 0.020 inch -- quite small -- and swapping parts back and forth or installing a sensor from one company into a hub made by another company gets a little scary. Too little, and the reluctor strikes the sensor or a possible odd waveform occurs. Too large, and the electrical waveform is weak/tiny at low speeds.
With your sealed hub type, it isn't possible to view or measure the air gap. I'm thinking you might remove the sensor you worked with and check to see if the tip shows signs of contact with the passing reluctor teeth. If so, it may require replacement and the addition of one or more shims to gain some clearance.
I'm hoping it's possible to gently press the sensor down into its bore and check to see if it bottoms out (a tiny gap exists under the mounting flange) or bounces as the hub is rotated. Measuring this gap with a feeler gauge, and adding 0.020 inch would provide a likely optimum tip to teeth air gap. Sensor shims appear next to impossible to purchase separately, they sometimes come with a new sensor. I'd consider making my own as needed, perhaps using thin plastic such as a cottage cheese container lid.
The best way to be certain of correct wheel speed sensor operation is to connect a multi-channel lab scope and confirm similar waveforms from each sensor. A pro-grade scan tool could also be used to verify identical and consistent MPH readings from each sensor.
Brad Bergholdt is an automotive technology instructor at Monterey Peninsula College in Monterey, Calif. Readers can contact him by email at email@example.com. Personal replies are attempted. An archive of past columns and additional consumer automotive information can be found at www.bradsautoadvice.com.
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