Both Trailer Turn Signals Blink at the Same Time

Updated 05/06/2020 | Published 12/12/2012

Products Featured in This Question

T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

(201)

$35.10

Cordless Circuit Tester

(15)

$19.26

Question:

what causes both signals to blink when you only turn one on?

asked by: Jim K

Expert Reply:

This is normally related to a bad ground connection somewhere. Make sure the main trailer connector ground and the grounds for each of the lights are all connected to clean and corrosion free bare metal surfaces. Also make sure that the vehicle side and trailer side connectors are clean and free of corrosion.

If you are using a 4-Way harness then these are the most common issues. If you have a 4-Way but it has 5 wires coming out then this is a wishbone harness and it will have the running light circuit split at the trailer connector. One wire goes to the left running lights the other to the right running lights. If mis-wired it could cause some problems.

If everything is connected on the trailer correctly and the grounds all seem to be connected to clean metal then the issue could be on the vehicle side. You would want to unplug the trailer and use a circuit tester like # PTW2979 if needed to test the connector for the proper signals. If there is power on both turn signal pins at the same time then there is a problem with the vehicle wiring or tow package wiring. I would need to know the year, make, and model of the vehicle and if it has factory trailer wiring or aftermarket.

expert reply by: Michael H

Steve

5/4/2020

Just finished rewiring a tow dolly with new wires and taillights. With left turn signal I get bright left and dim right. With right signal I get bright right only. I get no running lights. It’s a flat 4 with 5 wire harness and I have the 2 running light wires run correctly. I think. One to each side. ??

Chris R.

5/6/2020

If the 5-wire harness has two running light wires (which would be correct for a wishbone 4-Way like this) then you do indeed run one on each side of the trailer. Since it's brand new wiring on the dolly, I doubt there are any exposed/damaged sections causing a short, so what I recommend doing is inspecting the 4-Way on the tow vehicle. Look for any debris or corrosion built up inside the pins that needs to be cleaned out. Let me know if this doesn't help and we'll go from there.