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Troubleshooting Left Taillight Malfunction When Right Taillight is Working a 4-pole Wiring Harness


Right tail light functions but Left does not.


Expert Reply:

There are several possibilities that an contribute to why you have one tail light working but not the other. The first diagnostic approach is to isolate the cause of the problem, it could be the vehicle or the trailer. Then once that is isolated you can proceed accordingly.

The first step that I recommend is to check the connector on the vehicle. If the connector appears to be in good working order and is free of corrosion or road grime, then I would use a circuit tester like part # PTW2993 and test the output on the trailer connector. To do this you would need to test the connector while the function (running lights, turn signals, and brake) is activated, and make sure that the signal for that function is present at the vehicle connector.

If your existing 4-pole wiring harness includes a modulite you would want to test power into the modulite and out of the modulite to make sure there is no interruption of signal through the modulite. In testing or checking this modulite you will also want to check any fuses that are related to your wiring harness and make sure there is not a blown fuse that may need to be replaced.

Also on the vehicle side make sure that the 4-pole is securely and properly grounded to a metal surface. Make sure that there is no damage to the ground or evidence of corrosion.

If you have checked the all the vehicle related components and do not see any issues related to the trailing wiring, and are still experiencing the issue then the next step is to proceed to checking the trailer. Again I would suggest checking the trailer connection and make sure that there is no corrosion or debris that is preventing a secure connection between the the vehicle connector and the trailer.

Since you said the left tail light was not working, I would suggest checking the connection to that light first. Make sure that the wires that connect to the light have a secure connection, and do not show any signs of damage. Also check the bulb (if a bulb and not LED) to make sure that it is functional. Some trailer light connections ground through the light connection point to the trailer, check to see if this is the case for your trailer lights, and make sure there is a secure and proper ground at that point.

Another frequent problem is an improper ground on the trailer. Check the main ground connection for any paint, corrosion, or build up, and make sure that it is secured to a proper metal surface. Also check the ground at the light assembly. If there is not a proper ground at the light assembly you should run a separate ground wire directly to the main trailer ground. This is especially crucial if you have a tilting, or folding tongue trailer, since the additional movement can disconnect or interrupt the ground signal. An interrupted or incomplete ground signal is also common on aluminum trailer due to the limited conductivity of aluminum.

Additionally if you recently installed a plastic license plate holder that might be preventing a complete ground signal and you will need to install a license plate holder like the # LP15SB, that is steel.

If you have checked all these options and are still experiencing a problem the best thing to do is to trace the wires and make sure they are in good condition. Any breaks, damage to the insulation, any signs of exposed wires, or corrosion could be causing the issues. Since your problem is specifically the left signal check that side wiring first and pay special attention to areas near any moving parts like the suspension and make sure there is no wiring damage.

If you do find a damaged wire, then you will need to remove and replace the damaged portion of wire.

I have provided links to the parts listed above, two helpful articles that explains troubleshooting trailer wiring issues, as well as a demonstration video for troubleshooting a modulite for your reference.

expert reply by:
Rachael H

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