DC Ground Path

I have an older travel trailer. Three weeks ago I hooked up the rig to my tow vehicle and tested the lights. I had a problem with the left turn and brake lights. It appears the brake light is illuminated when the brakes are NOT depressed, and the left turn signal is inoperative altogether. I checked the trailer plug connections, and they were all fine. I thought it could be the tow vehicles' connections, so I tested the trailer on my other tow vehicle and got the same results. The right signal and brake lights work as they should and all running lights operate, as do the electric brakes; everything but the left turn and brake lamp on that side. Any ideas?
Chris, (Tucson, AZ)

Chris, if you are absolutely sure the trailer plug electrically matches the truck wiring (i.e. nothing has been switched around), then I would suggest you check the ground connections at the affected lamps on the trailer. Quite often, poor grounding will result in some lamps not working and others to light up when they're not supposed to. Any resistive unit (light bulb) will seek the closest/easiest path to ground. If it happens to be through another filament, it will power that lamp. Floating grounds are quite difficult to diagnose.

In the past I have used a long test cable with twin sets of leads. I connect one lead directly to the battery positive and one directly to the battery negative thereby insuring a good ground to the power source. At the other end, ground the lamp with the lead attached to the negative cable and then touch the positive lead to the wires for the bulbs. If an effected circuit is deemed groundless then a new wire can be permanently routed and connected. Use the jumpers for testing only.