Here's a new one - wiring harness switching polarity?

[Skyhog]

I've never seen this before....maybe someone else has and I'm missing something obvious....

With the headlights off, the trailer wiring works just fine. With the headlights on, they do not work at all. Bad ground, right?

Not so fast - I took my voltmeter to the harness truck side, and with the headlights off, the ground terminal is ground, and the left turn signal flashes hot on the third terminal. When the headlights are on, the third terminal becomes ground and the 1st terminal flashes hot.

Any ideas?

 

[KD4UPL]

Bad ground.
You can measure "hot" in a lot of weird places when using a meter. You can measure the voltage thru the filament of a bulb that is off. It can give you all kinds of crazy readings. You would be better off troubleshooting with a test light. This will eliminate some of the false readings a meter will give you.

 

[MTboatguy]

Sounds like the wiring in the truck needs to be checked out. How old and what make is the truck.

Also as stated, you need a test light in this instance, meters can do weird things in these types of situations.

 

[Skyhog]

Its a 2011 Ram 1500. I was using the Continuity tester mode on the meter so it was just beeping in time with the signal. I have a test light, but I think the bulb is burned out.

I have no idea where the danged wires run to on this truck - they leave the harness and immediately go into a conduit that runs up the length of the truck. Mergh.

 

[Skyhog]

OK - with a test light, it is behaving as I'd expect. First pin remains ground, 2nd pin is the constant parking light (dead when headlights are off), 3rd pin is the left turn signal, flashing (both with headlights on and off), 4th pin right turn signal flashing (both with headlights on and off).

Still possibly a ground, or am I looking at trailer side instead?

 

[briangcc]

Easy test....grab a set of jumper cables. One end to the truck hitch the other to the trailer frame. Now test the lights. Do they work? If so you have a ground issue that you need to address on the trailer.

**This is due to your wiring harness checking out on the truck.

 

[BillP]

Just a long shot but it may apply to your vehicle. I put a hitch and wiring crossover box on my wife's 4x4 Rav4. If a tail light goes out the cruise control stops and also part of the hitch lighting goes out. Toyota hand book says its designed that way and also functions the same if the 4x4 control module malfunctions. Dodge may have something weird like this. I know its a long shot but Chrysler corp has been known to do weird things...like trans calibration.

 

[MTboatguy]

Just a long shot but it may apply to your vehicle. I put a hitch and wiring crossover box on my wife's 4x4 Rav4. If a tail light goes out the cruise control stops and also part of the hitch lighting goes out. Toyota hand book says its designed that way and also functions the same if the 4x4 control module malfunctions. Dodge may have something weird like this. I know its a long shot but Chrysler corp has been known to do weird things...like trans calibration.

Yes they have over the years.

 

[Skyhog]

Easy test....grab a set of jumper cables. One end to the truck hitch the other to the trailer frame. Now test the lights. Do they work? If so you have a ground issue that you need to address on the trailer.

**This is due to your wiring harness checking out on the truck.

This did not work...

Lights Work with headlights off. Not when on

 

[MTboatguy]

Go down to Harbor Freight and get their inexpensive plug in test modules, they sell a 7 wire model and a 4 wire flat model, you just plug them into your truck and turn the lights on, they have LED's on them to tell you what is what and what the truck is doing.

https://www.harborfreight.com/four-way-trailer-light-tester-66526.html

You can get this 7 wire model with LED's from Walmart, but you can get both of them at most shops that sell boats and RV's

https://www.walmart.com/ip/Truck-Tr...MIvtPNgIfR4gIVCNlkCh3l-gmCEAQYASABEgLzufD_BwE

 

[bruceb58]

This did not work...

Lights Work with headlights off. Not when on

This is the classic bad ground behavior. With headlights off, your taillight circuit is providing the sneak path ground. With the headlights(taillights) on, the sneak path doesn't exist since the potential of the taillight circuit is equal to the brake light circuit.

When you did the test with the jumper cables, did you attach to good grounds on both trailer and tow vehicle? Bare metal with no rust?

 

[Skyhog]

This is the classic bad ground behavior. With headlights off, your taillight circuit is providing the sneak path ground. With the headlights(taillights) on, the sneak path doesn't exist since the potential of the taillight circuit is equal to the brake light circuit.

When you did the test with the jumper cables, did you attach to good grounds on both trailer and tow vehicle? Bare metal with no rust?

Yeah - that's the frustrating part - I tried multiple places on both truck and trailer. I connected it to the tailgate bolt, the frame itself, and the hitch ball. On the trailer, I connected it to the frame, the bolt that holds the jack, the bolt that holds the winch, and some metal tie downs.

I also checked the lights to make sure that the ground was attached to frame. To eliminate all possibilities, I then ran two wires direct from the harness ground to the ground on each of the lights and got the same behavior. That makes me think pretty heavily that the issue is from the truck side. I just realized though that I missed a great test, and that would be running the two separate grounds all the way from the lights to the frame of the truck to see if it works....I'll try that tonight.

 

[Skyhog]

So it's also possible that I'm an idiot...fixed it tonight.

There's 3 wires out of the light. Red, Black, and White. I assumed red and black were flasher and parking light while white was ground.

Nope. Black was ground, white was parking light and red was flasher. So with the headlights on I was grounding out the light.

Sigh.