Troubleshooting Left Turn/Brake Lights Not Working on Trailer But Testing for Power at Socket  

Question:

Helpful Expert Reply:

If you are testing the left brake/turn signal lights on the trailer right at the socket of the assembly itself and getting power, but the lights themselves don't work when the bulbs are in place then my first thought is a weak ground connection on the assembly.

Since you would be grounding the circuit tester as well, this basically could be strengthening the ground enough for that circuit so that you get power when testing but not when the bulbs is back in place. Trailer lights will either use a dedicated wire for its ground or will ground through their mounting hardware. In either case you will want to make sure that its grounded to a clean, bare metal portion of the trailer frame that's free of any debris or corrosion.

If this doesn't resolve the issue then the other cause is a loss of power from the tow vehicle back to the trailer lights. It sounds like you're using a standard circuit tester and not a multi-meter that would actually show the amount of power at the testing point. Because a circuit tester won't require the same amount of power that the bulbs would, it's possible that you're getting just enough power to light up the tester but not enough for the signal light.

I recommend inspecting both the tow vehicle and trailer side connectors for any debris/corrosion built-up inside the pins, cleaning out as needed. Then try to inspect any wiring you can gain access to for the left turn/brake circuit to see if you can find any pinched or exposed areas that might be interrupting power flow.

If you still aren't able to locate and resolve the issue please let me know and we can continue to troubleshoot.

expert reply by: Chris R

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