The first thing to look for is a shorted trailer wiring harness. On minivans, those often are wrapped around the trailer hitch, then they drag on the ground when transitioning up a grade, such as a steep driveway. On all vehicles, we used to see aftermarket trailer harnesses installed by U-Haul that had three red leds in the connector to make circuit identification easier. Those leds would short two or more circuits together when they failed.
A potential clue to a shorted trailer harness is the tail lights and dash lights turn on when the brakes are applied, but those tail and dash lights only turn on when one turn signal is turned on, but not the other signal. The other clue is both rear lights act the same way.
If you turn on the tail lights and both sides light up the same, then one side gets brighter and the other side goes out when the brakes are applied, that is due to a bad ground circuit. The way to identify that is to remove the bulbs from one side, then observe the other side also no longer works. With the bad ground, current can't find it intended path to ground, but it doubles back through the other filament in that bulb, then over to the other side of the truck and through that bulb, then to ground. If you look real close at the brake / tail light bulbs on both sides, you'll see one filament in the bulb on one side is glowing, but not full brightness, and both filaments in the bulb on the other side are glowing, but also not full brightness.
You might find more ideas in this article:
https://www.2carpros.com/articles/no-tail-lights-exterior-running-lights-tail-lights-out
Step 15 gets into the symptoms of a bad ground.
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Monday, March 1st, 2021 AT 12:30 PM
