Troubleshooting Plug and Play T-Connector Trailer Wiring Harness Not Working on a 1992 Toyota Pickup


do i have the right wiring harness? I have checked the grounds on both the truck and trailer and have cleaned them up and reattached them I used one of those cheap testers and there is power at the 4 pin on the truck....however the turn signals light on the tester does not light up when the running lights and turn signals are both on. It seems to me that both of those lights on the tester should be indicating current. when you turn the turn signals off, the running light tester light signals power again. any help would be welcomed.


Expert Reply:

As long as your vehicle is a 1992 Toyota Pickup and not some other model like a T100 pickup, you have the correct wiring harness. There are two possibilities for the symptoms you are describing. A ground issue or a short between the taillight circuit and turn signal circuits.

Because there would need to be a short between both the left and right turn signal circuit and the ground I would think that would not be the issue, unless the converter box has been shorted out. Since you have already checked the grounds on both the truck and trailer and have cleaned them up, we should start by testing the converter box. I added a link to an article that goes into more detail about testing 4 and 5-Way wiring installations for you but here is how I would get started.

To test the converter box you will need a circuit tester, like part # 40376. First, disconnect the trailer from the vehicle and any adapters from the 4-Way connector on the vehicle, to eliminate the trailer wiring or adapter as a possible cause for the issue. Now, test the wires going into the converter box. You should get the right turn signal on the green wire, stop light signal on the red wire, left turn signal on the yellow wire and taillight signal on the brown wire. The white wire should be properly grounded as you stated earlier.

If you do not have all the signals going into the converter box you may find that there is a pin bent in one of the connectors that is causing all the confusing readings you have been getting. Take each connector apart and check all of the pins for correct orientation, then re-connect them making sure all the wires going into the connectors are seated in the connector and that the connectors are pressed together all the way with the locking tabs engaged.

If you get all the correct signals going in to the converter box and incorrect signals coming out, without the trailer connected, then the converter box is most likely shorted out and will need to be replaced. This may indicate a short on the trailer wiring that will need to be fixed before connecting a new harness to the trailer.

If you find that the converter is working correctly then your issue would be farther down the line somewhere. Your order notes indicate that you purchased a 6-way adapter, # 30637. Plug the 4-Way into the adapter, ground the white wire from the adapter correctly and check the pins in the adapter, without a trailer connected, for proper function (see wiring article for pin layout). If the 4-Way is working correctly and the 6-Way adapter is not then the adapter is wired incorrectly and would need to be fixed or replaced.

If these suggestions do not fix the problem, you will need to turn your attention to the trailer. Check the trailer connector for correct wiring to match the vehicle side connector. Next, check inside the back of the connector to make sure there are no wires shorted across the connection points. Check all of the trailer grounds at each light and the front of the trailer for proper connection to the trailer frame. Finally, check the trailer wiring for shorts to the trailer frame and proper connection at each light and splice in the wiring.

expert reply by:
Robert G

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