Help with odd wiring harness problem!

[crazeinc]

I picked up a used trailer today and it has a 4 wire harness setup. I've verified all of the lights work properly by hotwiring the trailer harness to a battery. My '94 Explorer came with the tow package so it has the factory trailer harness. I've used it before with a Uhaul trailer and it worked fine. It doesn't work with this harness, even though they mate fine. I'm a bit stumped here. No lights or anything. Any ideas?

 

[Alec]

Try running a ground wire from the trailer to the truck.

 

[crazeinc]

I've actually gone further than that. I ran a ground wire from the trailer harness to the truck, and bypassed the harness that snaps into the wiring under the truck and ran a wire from the power lead from the truck to the power wire on the trailer harness and all that happens is the relay by the jack in the truck clicks.

 

[huskyfan23]

Our trailer was like that when we first got it. The plug went fine into the Durango plug, but nothing worked. It turned out the wiring in the trailer plug was wrong. Rewired it and everything worked fine.

 

[crazeinc]

I'm totally stumped. The relay clicks when you plug the harness in, but none of the lights come on.

 

[dogfriend]

Short in trailer harness?

 

[Crankcase]

Use this to match the trailer wires up...make sure they are the same as this....U haul wires up trailers backwards...

 

[crazeinc]

Thanks for the pic, I'll double check the wiring, but even if some of the wiring was backwards, wouldn't some of the lights still work or come on at all? I mean nothing works period and I have tried running wires to different locations without any luck.

 

[dogfriend]

That the reason that I think you might have a short somewhere: it seems like you would get something even if a couple of wires were mixed up.

I would get out a voltmeter and check for voltage at the trailer connector (vehicle side) to verify that you are getting left signal, right signal and stoplight. Then I would plug in the trailer and try to backprobe the connector to see if you still have voltage. If the voltage drops to zero, (or if you don't have voltage in the first place) then you have a short somewhere.

Also, is the relay that you are talking about part of the factory wiring or aftermarket? I looked at the schematic in the Chilton manual, and it doesn't show any relay. The schematics in both Chilton and Haynes suck.

 

[Ford_Racing_Guy]

i would also check the plugs for rust and corrosion and maybe try some grease,


also look at the wires themselves underneath your ex, the insulation may be cracked and wires maybe rusted

 

[crazeinc]

I would get out a voltmeter and check for voltage at the trailer connector (vehicle side) to verify that you are getting left signal, right signal and stoplight. Then I would plug in the trailer and try to backprobe the connector to see if you still have voltage. If the voltage drops to zero, (or if you don't have voltage in the first place) then you have a short somewhere.

OK, I have done this and the voltmeter shows power is getting to the connector on the truck. But it's just like you said, when I plug the harness in the voltmeter doesn't show any voltage getting to any of the wires. Any idea where this short would be?

Ps. There is continuity with the 2' long car harness that plugs into the connector on the truck so I know the wires aren't messed up in the harness.

 

[Alec]

So if one harness works and the other doesn't, then one is bad.

 

[dogfriend]

OK, I have done this and the voltmeter shows power is getting to the connector on the truck. But it's just like you said, when I plug the harness in the voltmeter doesn't show any voltage getting to any of the wires. Any idea where this short would be?

Ps. There is continuity with the 2' long car harness that plugs into the connector on the truck so I know the wires aren't messed up in the harness.

Some where on the trailer side of the circuit. Maybe a wire has rubbed thru the insulation and is shorted to the trailer frame? Try checking continuity from the trailer connector plug to the frame of the trailer. You should only see continuity on the ground wire.

Trailer wiring is notorious for being bad.

If it is the trailer wiring, it may be easiest just to replace it all. You can get a new set of lights with wiring for less than $30 for a utility trailer.

 

[crazeinc]

There's 3 pieces to this.

The connector on the truck says it's gets the proper voltage. When I plug in the middle piece, there's no voltage coming from any of the wires. So what I did was bypass the middle harness, and ran a wire from the ball hitch (I know it is a ground, it worked for other things) and ran a wire from the power source on the truck connector to the trailer hitch and all is it doing is making the relay click no lights come on.

This makes me believe the middle harness isn't the problem.

 

[dogfriend]

It should be very easy to eliminate the middle harness as the problem. Just test continuity for each wire from one end to the other. Then test for continuity from each pin on one end to the other pins on the same end (if you get continuity from one pin to another, then those two circuits are shorted together).

I still think that you have a short somewhere in the trailer harness.

 

[Ford_Racing_Guy]

its gotta be faulty wires

 

[crazeinc]

It should be very easy to eliminate the middle harness as the problem. Just test continuity for each wire from one end to the other. Then test for continuity from each pin on one end to the other pins on the same end (if you get continuity from one pin to another, then those two circuits are shorted together).

I still think that you have a short somewhere in the trailer harness.

I've checked continuity on the middle harness and all of the wires are fine. When you hook the trailer harness straight to a battery, the lights all work properly.