Relay or Switch

[Jeremy Desmond]

So Im having a strange 4 pin tow light issue.

I originally thought it was a ground issue on the trailer but found with a LED 4 pin tester the problem is with the truck.

I scraped the truck frame and drilled a new ground with a self tapping bolt.

I cut the 4 pin connector off and check the voltage with a DMM.

With Headlights on and hazards on:

I get 12 v on the headlights steady (brown wire)

Left blinks 12V and the right stays at 1 volt

and other time the right will blink 12v and the left hold 1v.

With the head lights off I get 12v blinking on both yellow and green as I should.

Bottom line I get the correct voltage on the turn signals with the headlights off

and only get one side blinking with the headlights on.



All lights on the truck work in all conditions. This issue is just at my 4 pin connector (cut off)

This is checking the voltage on the cut wires about a foot up from where the connector was.



Could this be caused by a bad headlight switch or Relay?



Thanks,

Jeremy

 

[user 69094]

Sure sounds like a ground problem.

Try running a ground wire directly to the battery.

 

[Jeremy Desmond]

OK Ill give that a shot today when I get off work at 330. I run a test ground outside the truck from the neg batt term to the 4 pin connector. I went ahead and got a new switch for $25. I figured Id replace that too seeing its cheap, and while I have the dash apart Ill remove a dead horse fly from my instrument cluster.

 

[Duane Foster]

I had a similar issue with (both I think) my '78 F150 and '94 Explorer. My issue as I remember was that everything worked fine with the headlights off. But with the lights on, the brake lights, taillights, and turn signals wouldn't all work at the same time. Two of the 3 would work together, but not all 3. In both cases I was using an aftermarket direct fit pigtail that plugged into the connector for the factory tail and brake lights, turning the connection into a T. I read somewhere that the problem was due to a ground wire that was needed at the 4 pin connector. I never did fix it, just made sure I was home before dark.



You shouldn't have that problem with the factory wiring all the way to the 4 pin, but a bad ground is still the place to start.

 

[Jeremy Desmond]

Turned out to be a break in the wire down the frame rail close to the filler nozzle.

Ultimately a ground issue as Dan stated.



THanks for the help!