Q: My abs and brake lights periodically come on. Sometimes my speedometer fluctuates. But only when the truck bed is empty.

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Recently got this truck, the abs light and brake light will come on at random times. My speedometer will fluctuate at random times as well. If I have a heavy load in the back everything runs fine. I have tried replacing the speed sensor but it doesn't help. Someone told me it could be a ground wire but where?

My car has 173600 miles.
My car has an automatic transmission.

The problem sounds like you have a wire or connection problem with the rear axle speed sensor. Verify the connector and wiring at connector is not broken and then check the harness up to frame to make sure the wiring cover is not out of position. The sensor puts out very low voltage signals and is susceptible to interference between axle and antilock controller. make sure harness is not misrouted back to controller and near other high voltage wires in system. Also check connections at ABS module they are clean and dry.

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