Q: My ABS light came on last week on my 2006 F-150. The auto parts store read the code and said it was the rear center ABS sensor. I bought one, replaced it, and cleared the codes, but the light still comes on as soon as I hit the brakes. I looked at all the wiring and don't see anything wrong. Thought it could be a bad new part, so I bought another one from a junkyard but it still doesn't work. It's driving me nuts!
A: Proper diagnosis is a step-by-step process. I wish you included the ABS diagnostic trouble code; one related to the rear wheel speed sensor points toward a signal failure, another an open or shorted circuit, and still another a speed calibration problem. If you have a multi-meter you can do a quick check at the speed sensor. It should measure 3.8 to 5.2 MegaOhms. Out of that range, it's a bad sensor. Next, with a voltmeter, measure between the two sensor pins on the harness side with the key on. You should have about 9 volts. If not, there's an open or short in one of the two circuits between the sensor and the ABS module, or even a bad module. This requires further testing of the circuits, with both the sensor and module disconnected, to pinpoint the exact location of the fault. If sensor resistance and module output voltage are correct, you need to wire an ammeter in series between the sensor and harness. Here we're looking for current flow jumping between 6.3 milliamps and 15.4 mA while slowly spinning the rear wheels. If the current flow is not changing, or out of range, it still could be a bad sensor, but may also be a mechanical problem with the sensor ring inside the rear axle assembly. A lot of times simply changing parts works, but not always. Get it to a qualified technician if you don't want take diagnosis any further. And be sure all tires are the same and correct size. A mismatch can cause ABS issues.
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