Troubleshooting Trailer Lights Not Working

Question:

I have a newer 2018 Volvo V90 Cross Country and installed a Stealth Hitch with a dealer installed OEM factory trailer module 7-pin. I have a 2019 Nobo 10.6 with LED lights. It seems when the car does a check everything on the trailer works but when I start the car the following wont work on the trailer right side: 1. hazards 2. right turn signal 3. right brake light always stays illuminated 4. running lights on trailer stay on 100 when car is running and lights are in the off position Interesting when the car is not turned on but connected to the trailer and I use the hazards or press the brake pedal the trailer lights work just fine on the right side. The Volvo dealer cannot solve the issue and it seems to be the vehicle not the trailer since the trailer works with other vehicles. Also I have a power jack that connects through the 7-pin and it is not working when plugged in but my Prodigy RF works which uses the same pin location for power. Im stumped trying to find a solution. Dealer says they did everything correct on their end to install the trailer module and wiring, including ground connection and fuse for auxiliary power for 7-pin. Please help!

asked by: Geoff

Expert Reply:

We don't have experience with OEM parts but I can give you some troubleshooting for your lights not working, though the Volvo dealer really should be able to solve this. I say that because you mentioned the trailer lights work on your other vehicles but not with your Volvo so we know it's on the vehicle side. That points to a harness issue or something else like a fuse.

But the first thing you want to do is take a look at your grounds again - I know the dealer said they checked it but a bad ground is very often the reason for bizarre things happening with lighting so you will want to take another look. Another 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. It is possible you have blown fuses so check those.

Then, 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 you know the issue is with the vehicle wiring and you will need to have the Volvo dealer replace or repair it because tracing those wires is very difficult.

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