I am not very knowledgeable regarding truck electrical systems so I will describe what is happening and maybe someone can give me some clues as to where to start. When I have my work/utility trailer hooked up and plugged into the little vertical 4 pin connector on my truck bumper, all brake lights and turn signals work fine until I turn on the truck headlights. Then the left turn signal won’t blink. All other lights work ok – it’s just the left signal which stops. If I have the left signal on and then turn on the headlight switch, the left turn indicator on the dash dims and stops flashing.

I have a new F-350 so I’m thinking it is something like a short on the trailer wiring rather than on the new truck. Puzzle is that everything always worked fine, as recently as last week, when the trailer was hooked up my 89 F-150.

Thanks

 

Symptoms of a bad/corroded ground.

Ssparky

 

Oops too quick with the post button. Bad ground most likely on the left hand side of the trailer. Unless it does this with the trailer disconnected, then it could be at the front or rear of the truck. It probably does this with just the parking lights on and not the headlights. So check the light sockets and ground wires for the parking lights.

Sparky

 

[updated:LAST EDITED ON 13-Nov-01 AT 11:35 PM (EST)]Thanks sparky - It does not do it with the trailer disconnected. It does do it however with parking, low beams or high beams on. It only works properly with the truck lights off. Truck lights off = all trailer lights work as they should. Truck lights on in any setting causes the problem only with the left signal. The trailer has only one ground wire which seems to be ok. I will take the screw out tomorrow and reinsert it to see if that fixes it. Just to be absolutely clear, if the trailer plug is disconnected, truck signal works fine. Am I looking at it too simplistically to think that given this fact, the problem has to be in the trailer or in the plug on the truck bumper?

 

OK on your left trailer light assembly there is most likely a ground problem.
If it is a typical light setup, there is one 1157 bulb, which has 2 filaments. One filament is for the running lights and the other is for the brakes and turn signals. There is a tab that is inside the light socket and it is the common ground for the 2 circuits, parking and brakes/turn. This ground is mostly likely the problem it could be corroded at the socket, or where it mounts to the trailer frame. Or it could be the outside brass colored base of the bulb that is corroded.
It sounds like you are suffering from what is called a "floating" ground. The normal ground for the light is missing so the running lights are getting their ground thru the turn signal circuit causing them to stop blinking and the turn signal indicator to light up.
Hope this helps out.

Sparky
ps
The trailer lights can be grounded 2 ways. One is the tab in the light asembly is grounded to the trailer frame near the light assembly and the trailer frame is grounded to the truck frame thru the 4 pronged trailer connector. The other way is the light grounds, lh and rh, are connected together and then connected thru the 4 pronged trailer connector to the truck frame. The second is my preferred method, but the first works just as good.
Anyway, check out the grounds and let us know how you make out.

 

no!! this thread needs an answer! I have a 99 f150 xlt with this same problem but neither of the turn signals or hazard lights work on my trailer when the parking lights or headlights are on. the lights on the trailer are brand new, grounded at the tongue. somebody help (:

 

This thread was answered with the solution. If you have the same problem(lights will not work with the parking lights on, they will otherwise), take a long scrap piece of wire and hook it to the battery negative terminal, run it in the yard back to the trailer, and hook it directly to the ground for the light at the light itself, and see if that cures it on that side.

If it doesn't, then take the light apart and with the parking lights on, and the turnsignal to the light you are working on activated, jamb your temporary ground wire beside the light so it touches the brass part of the bulb. That should fix it. If it does, then you have a connection problem between the brass part of the bulb(bulb ground) to the ground wire of the light fixture. If running the temp ground wire directly to the ground wire of the fixture fixes it(the first test) then you know you have a ground problem between the fixture ground wire and the frame of the trailer, and or the trailer frame and the truck frame.

Don't rely on the ball to give you a ground, all trailer wiring plugs have a spot for a ground to pass through the plug itself.

 

+1 ---- NEVER, never - rely on the hitch ball to give you a reliable ground!

 

97 F150
I seem to have a similar problem, but it is the same even if trailer is not attached. I've checked the 4 pin plug with volt meter and also checked the pins on the wiring harness plug in the fender well. The both operate the same strange way. Checking the voltage at each pin with the ground on the 4 pin plug I get the following voltages:

..........Brown yellow green
all off.....12.......0.......12
brake.....12.......0.........0
lights.......0.......0........12
LB..........12....0/.15.....12
RB..........12......0.......12/0
light&brake..0....0.........0

Is this a ground problem too or a wiring failure somewhere else. The truck lights themselves work fine. I checked all the trailer fuses, including the one in the small 5 slot box near the firewall. They all look good. I apprecite any suggestions.

John

 

Start over. What's your problem? You rear truck lights work fine, but the trailer lights do not? What do the trailer lights do when plugged in and you go through the various stop turn motions?

 

When I connect the trailer (no truck lights on), the lights come on, right one bright and left one dim.
When I turn on lights, right one stays bright left one goes off.
When I brake, both lights are dim.
When I turn on left blinker, right is brihgt and left is dim.
When I turn oln right blinker, right light blinks and left light is dim.
With lights and brakes together, all lights off.
With lights on and left blinker, right light bright and left light off.
With lights on and right blinker, right light blinks and left light off.
These match up with voltages I read off he pins (in the last post) with the trailer disconnected, so it appears to me the problem is with the truck wiring.

Thanks for asking.

 

The first thing I would do is take a scrap piece of wire and find a good ground on the truck and hook one end to that, and then find a good ground on the trailer and hook the other to that. Just lay it on the ground, we are just experimenting right now.

 

Good idea. I Tried this with a set of jumper cables earlier today. Got good tone on a voltmeter continuity check from truck to trailer, but no difference in lights operation.

 

Look at the trailer wiring at the rear of the truck. It will be wired one of two ways;

1. Factory wired using the factory tow wiring that is built into the harness or,

2. Wired aftermarket into the trucks rear lighting. This could be a slick "T" connector you can buy at the store and insert in the factory rear wiring, or this could be a spliced in homemade type job, that could be done very nice with soldered connections, or very sloppy with stripped bare twisted wires.

Find out what you have.