What is Going on With Trailer Lights Not Working

Question:

Hi so I have a 91 Chevrolet Silverado or a c 1500 extended cab long bed and recently I built this trailer from scratch and ran all the wiring myself and Im trying to add side markers to the trailer. I used my grinder and took it down to Bare Metal wherever the ground points are and when I checked the lights the brake lights work the turn signals work but then when I turn the running lights on or the headlights on then the tail lights on the truck no longer have the running lights lit up and none of the trailer lights ARE lit up either but the turn signals still work and the brake lights still work on both the truck and the trailer oh and also the running lights in the front of the truck are on and working the new side markers I got for the trailer are LEDs the Stop and turn signals are not LEDs and the tail lights on the truck are also LEDs and so are the turn signals or running lights on the front of the truck are also LED.

asked by: Joseph R

Expert Reply:

The main issue here seems to be not all of the trailer lights being able to function at the same time when you utilize some functions. This is often a ground issue but since you said you have a ground to a clean, bare metal surface we can probably rule that out. Additionally an overload can be a problem but since you mentioned you have LED running lights, that is likely not an issue either. So that means there are a number of other troubleshooting steps you need to take.

The first thing to do is take a look at your owner's manual to locate any trailer fuses or relays and ensure they are all there and in working order. If they are, in order to determine the specific problem you will need to purchase a circuit tester if you do not have one - the very handy Cordless Circuit Tester # PTW2979 would be my strong recommendation due to how easy it is to use - and test the connector on your vehicle after disconnecting it from your trailer. Check the wiring going into your harness from your tail lights to see if those wires are carrying the correct signals, if not then the issue is with the vehicle wiring.

If the signals going to your harness are okay, you'll need to test the signals coming out of the harness, and if they are correct then your issue is likely going to be with the trailer side wiring. You will also want to check both your vehicle and trailer connectors for corrosion, which can be green or white in color. A build-up of corrosion can interfere with the signals getting through the connectors so if it is present you will need to either replace the plug or clean the corrosion.

Finally, check your wiring thoroughly to ensure there are none that are crimped, broken or otherwise damaged.

I have added links to a pair of help articles on trailer wiring, including a troubleshooting guide which I highly recommend taking a look at.

expert reply by: Kyle S