Hi, I have a '97 F-150

About 2-3 times a year we used to rent a trailer, the 4 prong plug in the back has always worked, so i'm fairly sure that it is wired correctly.
The problem is that we went to use the plug again the other day (finally bought a trailer, but it's used). Unfortunally when I went to plug it in the tail lights does not work. What I mean is that the left signal works, the right signal works, when I hit the brakes both lights will work. when I turn on the headlights the tail lights will NOT light up dimmly.

Using a multi-meter I found that the brown wire on the plug (the one right beside the ground) has no power when the truck is running and the headlights are on. I tried sanding the contacts in the plug and still no luck... no power.

Now, I have a few questions.

1) Is there a fuse that is dedicated to controlling the "running lights" for the trailer..... (maybe it's just a blown fuse).

2) on the brown wire, when it is working am I supposed to see 12V on this system, or is it supposed to be less (since when you hit the brakes the tail lights are a lot more bright, so i'm thinking that it might be less than 12v to make them light dimmly).

3) I found 2 plugs for the trailer on the left side of the spair tire. The gray one is connected to the 4 prong plug on the bumper of the truck. Unfortunally the colour of the wires on this plug is different than what I see on the 4 prong plug. is there a way to check this plug for power? (I have no clue what wire is what, and trying all combinations, I see no power to this plug.... maybe i just can't find the ground).

4) there is a second plug beside the gray plug (also beside the spair tire). This plug is black and much bigger. It has 2 pins and is capped off. What is this plug for?


Unfortunally, I don't have a "known working trailer" to test the the plugs with as the trailer I just purchased is used and I honstly don't trust it's electrical system competely. All I can go by is checking the plug with the multi-meter. Maybe i'm not supposed to see any power on the brown line unless a trailer is connected and I should try and fix the trailer instead?

 

isn't brown back up? it sounds like you are getting a poor ground to the trailer.

 

I have a 4 prong plug. Brown (the 3rd pin from the left) should be my marker lights. Also, i'm checking the plug with a meter so the trailer should be out of the equation for now.

I did a lot more searching on the fourm and here is what I have gathered so far.
1) The brake / signal lights are on a different circuit from the Running/marker lights. Many people on this fourm has had the trouble of the signal/brake lights going out but the running lights were working. Cause is the 20amp fuse under the hood in the tiny fuse box. My 20 amp fuse is fine (and hence why signal/brake lights works). This must mean that.....

2) The running lights are on a seperate circuit. But where is the fuse for it (it is not documented in the owners manual). One person mentioned that on the 98 and up trucks the fuse IS marked in the owners manual, would anybody be kind enough to point me to this fuse (hoping it's in the same location on my '97)

 

Sounds Like a Bad ground to me also, The voltage going to the rear bulbs is always 12 volts or more. The turn/stop filament and the running filament are different in the bulb itself. Look to see if someone put the correct bulbs in the tail light. should have 2 filaments in the bulb, 1 for stop/turn and 1 for running light. I believe the bulbs are 1157 but don't hold me to that, I have been wrong before.

 

you shold have 12 volts on the brown wire with the light swith on. make sue your meter is on a good ground on the truck. scrape off all rust or paint.--- good way to check the trailer is with a battery charger as power- touch the brown wire with hot lead and ground the other lead.--- great place for failure on a trialer is the light sockets. remove the lense and see if sockets are rusted or not grounded correctly. sorry, i dont know were the fuse is on the 98. my older truck had fuse box under the steering column bottom of dash. small panel to remove. i thiuight it said in the owners manual. if your tail lights on the truck work, then you dont have a fues problem. chekc operation of the truck tail light, turn and brake. if they work without the trailer, then trailer is the problem. (or the small 4 pin caable to it.)

 

I'm checking the lights using a meter, not hooked up to the trailer.

All the lights on the truck works just fine

The meter has a good ground (using the ground pin) cause when I move the hot lead to the left/right signal i'm showing 12v (when the signal is on).... when I check the brown wire it is dead (when the headlights are turned on).

 

the fuse for the stop turn lamps is under the dash 15 amp and the one for trailer tail lamps is under thew hood 20 amp if your fuse is good and your tail lights work then I would say you have a broken or disconnected wire try following the wire back some and take a reading if there is power there then your problem is after that point.

 

Found the problem, it was a the fuse in the main fuse box, another 20 amp fuse that was blown
(position 34, bottom left fuse).

Everything works now, thanks for the help everybody