Trailer Light Issue

I have a 2004 Toyota 4Runner, and I'm having issues with my trailer lights not working. I know the issue is with the 4runner not my boat trailer. The light work fine when I test the trailer on my on my neighbors truck. I also hooked up to his trailer and still the lights would not work. I have checked the fuse and everything is fine. I check the voltage at the plug in and I'm getting the correct voltage. Not sure why when I hook the 4runner up to trailer the trailer light will not work. Has anyone else had this problem? I'm out of ideas on how to trouble shoot.

Telling us the lights are "not working" isn't enough to go on for anyone to help diagnose your issue. What exactly is not working? What type of wiring plug do you have on the truck? Factory option or installed aftermarket? By whom?

None, of the trailers lights are working. I have a factory installed 7 pin plug, that I'm using an adapter to convert to a flat 4. I have test the voltage directly on the 7 pin plug and the voltage with the adapter plug in. Both ways I'm getting the correct voltage.

I assume the trailer is flat 4. If that's correct, then I'll venture the following guesses:

When your buddy's truck is connected, the trailer gets Ground via the hitch ball to the trailer frame. This isn't the proper way to get Ground for the trailer, but that is likely why his truck works with your trailer.
Your hitch ball doesn't have continuity to Ground so nothing works when connecting with your truck for either trailer.
You have a Ground on your truck's 7 pin connector, but the white wire on the trailer side is not connected.
That white is failing to connect the Ground from your 7 pin connector to the trailer's frame.

The white wire on the trailer side of the flat 4 needs to have a ring on it and be under a screw into the trailer frame making metal to metal contact (no paint) for the circuits to complete.

Let me know how right or wrong I am.....

Got to be a lack of ground.

Yep, my long-winded post was to help him understand how that can be and what to look for as most likely.

Yes sir, you take the time to explain it.

You need to go under the truck and take a wire brush wheel on a drill or a side grinder and clean a spot to bare metal and connect a wire from to the ground of your trailer connector. The spot does not have to be huge just big enough to use a self tapping screw to connect the wire to the vehicle. A bad ground will cause all kinds of strange trailer light issues. Make sure the white wire on the trailer harness is connected to the trailer in the same fashion. Once you do this you wont have to guess if you have a good ground.

Hitec, I think you are off-base on there being an issue on the vehicle. The OP already stated that he checked voltage of the circuits at the trailer connector on the vehicle and all was good. This says that the Ground on the connector is good, so there is most-likely no need to mess with that connection.

He has zero lights of any nature. I'm betting the trailer-side white Ground is dangling, which I've said he needs to connect if I'm correct. His trailer ball is likely isolated, hence all lights out. Of course, grounding through a trailer ball is never a good plan anyway.

If you have an ohm meter, back the truck close to the trailer and hook it up but don't let the truck or hitch touch the trailer. Now measure from truck bumper to trailer tongue. There should be zero ohms if the harness is correct. If not, disconnect the trailer wiring and locate the ground wire on the hitch connectors and measure from that wire to ground on that vehicle. Or, might just do that first and save driving the truck. You'll test both grounds this way.

Cast, you're making this more difficult that it has to be. If you read correctly, you would see that the OP already used a voltmeter to measure the circuits at the truck's trailer connector. He's got Ground at that connector or he wouldn't be reading 12V there for the various lamp circuits. There is no reason to doubt the Ground connection for the truck side - unless his check was somehow invalid, and it doesn't sound that way.

Additionally, checking Ground by measuring from the trailer frame to the truck bumper is not always a valid thing to do. Why? Because there is no guarantee (nor need for) a bumper to actually "be" Ground. Some bumpers may have grommets or a full paint coating that may prevent the bumper from measuring as Ground - when there is no need for it to be. His tests put the issue past needing to check the vehicle's Ground connection, but if he did need to, you were close - the points to check are the trailer frame to the truck frame (not the bumper, which may or may not be grounded).

Flippin-Out, your analysis was correct. I hooked back up to the boat trailer, then moved the trailer ground around to different spots around the hitch, until we found a spot that worked. Wired the ground to a bolt, now everything works. Thanks for you help.

Ohhhhhh, what a lucky guess.lol


Actually, grounds are the biggest problems almost all of the time. Anytime you have a problem, check the fuse first and then the clean the ground.

I agree with you and I also suggested that he do the same thing on the trailer. If you do this on both the trailer and the truck then you are guaranteed to not have a ground issue. I hate light problems and I do this an all my trailers and so far I have had no issues. I wont haul a trailer when the lights don't work. A pet peev.

I am glad you got it working.