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Troubleshooting 4-Way Installation With Turn Signal and Brake Lights Not Working 2010 Toyota Tacoma


Hi, just installed a 4-flat trailer wiring harness on the truck. Removed taillights and plugged the harness into the connectors before zap-strapping it securely. I have running lights but no signal lights. Any ideas before I remove the whole thing and start again? Thanks for any help!


Expert Reply:

With a plug-in harness on a 2010 Toyota Tacoma there are only a handful of possibilities and with that I can briefly go over them. I have also attached a FAQ article that gives troubleshooting tips when installing a 4-flat.

Before starting the testing process, disconnect the trailer from your truck. The testing procedure requires a circuit tester like part # PTW2993, to verify you have voltage running through the circuits.

In the picture attached, I have the test locations (spots) noted. I also have a brief demonstration video on how to test with a circuit tester.

Have a friend turn on the left signal and check the Yellow wire before the converter box for voltage, (test spot # 1 in picture) then turn on the right signal and do the same with the Green wire. If you do not have signal, then chance are that you have either a blown fuse or relay. Check your owners manual to find which relay/fuse corresponds to your tail lights and change if blown. If you are receiving signal then move to the other side of the converter box and repeat the method.

The next step is test spot # 2 on the other side of the converter in the picture and repeat the process. If you are receiving signal then move to test spot # 3, however, if you do not have signal then you have an issue with your converter box and you will need to contact the retailer where you purchased the harness.

At test spot # 3, if you are receiving signal then the issue is with your trailer and if you are not receiving signal then you probably have a small short somewhere between the converter box and where you tested the wire in step # 2.

If all of these test spots check out positively, then you do not have an issue with your truck or your harness, but, rather an issue on your trailer.

If the issue is with your trailer then chances are you either have corroded pins in the plug, a bad ground, or a small short in the wires.

If you determine the issue is on the trailer side, first check the plug coming off the trailer. Inspect the pins to make sure they are not bent in any form or fashion or have any dirt or corrosion built up. That in itself will disable functions.

If your plug is good then make sure your ground is connected to a clean, dirt and grit free, bare metal surface. Grounding issues are weird and the littlest thing can make everything not work. Other ground causes are grounding to an aluminum frame, plastic or aluminum license plate or a painted metal surface. If your ground is to any of these locations then adding a simple grounding jumper will alleviate the problem. The final ground cause could be if you have either a folding tongue or tilt bed. Both of these types of trailers can disconnect a ground because of their movement.

Finally, if you deem both of those facets operational, then, it is time to look for a small short in your wire. Inspect the wire and look for bare copper. If you find that then use electrical tape to seal the area.

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