Are you using one of the 7 place RV style plugs (with the rectangular looking contacts) or one of the 6 way ones (with round contacts)? I presume since it has a break away battery that the trailer has electric brakes. Does the truck have a brake controller?

If it's one of the 6 place ones, not all older trailers were wired to the same standard as far as what position provided what function, and trucks were often set up to match however the trailer was wired. If that's the case, it might be a matter of using a volt meter to figure out what's what, and if something's not standard (like trailer battery charging and trailer brake position reversed) it would just be a matter of swapping wires around on the socket.

If that's not it, I would presume there's no positive power at the trailer plug. Does the truck have the factory tow package, or is aftermarket?

In either case, there should be a fuse for trailer power somewhere. I'd begin by checking for power with a volt meter on both sides of the fuse. I think Fords of that era also have a relay somewhere to provide power to the trailer only when the truck is running. I'd check that, too.

Lastly, is there a battery isolator installed in the truck? If so, check that too...

Also, I presume the battery for the break-away system is also not getting charged?

 

First off, I must say you guys are all very helpful and full of knowledge. Thank you for your concern with my trailer. Now I'll answer some of your questions...

It IS a 7 place RV plug.
The trailer HAS electric brakes and we DO have a brake controller in the truck.
Factory tow package.....I went to the dealership and got a wiring diagram. The colors on the diagram were different then those on the truck.
I don't know what a batter isolator is.

So, I bought a new 7 pin plug for the truck because the old one was...well old and rusty...I pulled the old plug apart and there was 7 wires going in to it. I then replaced all seven into the new plug. I figured with all seven wires going into a brand new plug that it would work fine....Wrong....every single thing works (brakes, running lights, turn signles etc.) except for the interior lights......My friends truck will power the interior lights.... Where the crap is a fuse that could fix this problem???? I'm still optimistic. Thanks in advance for your opinions.

 

Thanks for the extra info.

Did you try taking voltage readings on all the pins on the truck, with the key on? If any of them are hot, perhaps the hot one is in the wrong location?

I don't think I have a manual that coves your particular truck. I think I'd go over to the diesel stop and run some searches or post the question.

The other possible suggestion would be to follow the wiring back from the plug, to see where it goes and where it's fused, and take voltage readings anywhere there's a junction.

A battery isolator is a device sometimes installed on rigs that tow or use a camper. It directs charging voltage from the alternator to the truck's starting batteries first, and once they are up to fully charged it will then direct power to the trailer plug to charge the batteries in the camper or trailer (which would also run the lights).

They are ofen a small metal box with heat sink fins and several large wires hooked up... Did you buy the truck new, or could a previous owner have installed an isolator?

 

Ok, I'm going to check the voltage on all the pins as soon as I get out of school today. Hopefully one of them is hot... The previous owner of the truck was the owner of a big construction company (Whitaker Const.) He never carried a camper, but towed very large trailers amost daily. (I'm now over 300,000 with orig. everything!!) He did have a C.B. Radio and a company radio installed....maybe they stole power from there. I'm also wondering if a diesel is different because it has two batteries. I really hate electrical. Thanks for the quick response...I'll keep you updated.

 

*Update* I checked all 7 plugs and no power. I know for sure now that I need constant power on the 1'oclock pin. All wires coming out of the plug are kind of thin except a blue one (trailer brakes) and orange/coral (1'oclock). For some stupid reason the orange/coral plug stops/dissapears just past the spair tire. (the blue continues)

I am going to run brand new wire and attach it to the ending point of the orange/coral wire. This should now once and for all solve the problem.

One question: where should I start the new wire?? I want it to have constant power.

 

That's really strange. I wonder why that wire would stop?

Anyhow, if you run a new wire, be sure to use a big fuse or resettable circuit breaker, and a heavy wire. I suppose you could run it straight to the battery, or to whatever lug the battery cable bolts to (to avoid corrosion).

Diesel pickups are often set up with two batteries, but they are just two normal 12 volt starting batteries in parallel (positive wired to positive and negative wired to negative).

The only drawback to having power to the trailer plug all the time is that if you park with a camp trailer, using the trailer's electric system would draw down the truck's batteries. Starting batteries don't last long if deep cycled, plus if you ran the batteries down too far...

My Dodge is wired that way factory (power hot all the time), so I just unplug the 5th wheel once I'm parked at camp. Some people add an isolator, or add a relay so the plug is only hot with key on, but for me it's just as simple to pull the plug when I don't want the trailer drawing from the truck's electrical system.

 

*Update* After all this fuss I must say that I am a retard. When it was suggested that I look for blown fuses I looked under the steering wheel at that fuse box. All was well. However, I remembered seeing a fuse box under the hood. I found a freaking huge 30Amp fuse blown in there. Replaced it for $3 and viola it works...only when the key is in the ON position. I then pulled the crap out of my wires and found that the wire does indeed continue and ends up in a relay box under the hood. Problem Solved. Thanks Everyone.

 

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