Trailer wiring harness on the fritz-help!

[Thomas DeBusk]

I have a factory 7-wire trailer wiring harness on my '83 Wag. It worked for a while, but now every time I turn the headlights on and off, the parking lights stay on.

I've traced the problem to the black box behind the rear bumper. It has 4 relays in it and a 10 amp circuit breaker.

The one farthest from the circuit breaker seems to be the problem. When I unplug it, the parking lights go off. So I got a new relay. It seemed to work the first couple of times, but then it started doing the same thing and now does it all the time, too.

This factory harness has a broad, flat, 6-way plug and a separate couple of plugs on red wires. Those red wires go to the parking lights. To install this trailer harness, you unplug the vehicle's 6-way plug and plug in the harness. When I unplug the red wire that goes to the front with the 6-wire plug connected, the parking lights go on and off like they should. That is, the front ones do. The rear ones are disconnected.

I need the harness to run my trailer. Can anyone help here?

[andy d]

this is pure speculation. check and see if you have a good ground. also see if none of the wires are shorting out.my trailer uses the smaller plug on the 88 so its of no use for this diagnosis

[Thomas DeBusk]

I don't think it was grounded very well, so I put an alligator clip from the black ground wire in the box to a good ground. Parking lights still stay on.

As for the short idea, I like this one. How do I track that down? And please use very simple language. I'm an ignoramous when it comes to electrical problems.

Thomas

[Thomas DeBusk]

Doesn't anybody else want to help an electrical ignoramous? I still haven't cured this trailer wiring problem.

Here's some more info. The vehicle seems to work fine without the trailer harness plugged in. That is, the parking lights don't stay on when I turn them off. But if a short somewhere in the front of the vehicle is keeping the current going that's keeping the relay energized, shouldn't that short show up with the trailer harness disconnected?

But a service station that checked out my electrical system about a month ago said there wasn't a battery drain present more than the few milliamps that run the digital clock. Am I working on a defective theory here?

Also, I notice sometimes when I'm driving that the amp gauge quickly ticks toward the charging side about once every second. This only happens sometimes and when it is happening seems to go away when I turned on the headlights. This have any relationship to the trailer harness problem?

[woodybeone]

Do you have a multimeter to troubleshoot ?

[Thomas DeBusk]

Sure do, but that doesn't mean I know how to use it intelligently. But I follow directions very well.

[andy d]

uhhm lessee, to test for a short with a multimeter.set the meter for resistance (the omega symbol) or if you have a fancy one the tone.or diode test. anyway,kill the power by un hooking the battery. put one lead to ground, the other to the wire. if the resistance goes to zero, that is a short. if the resistance stays at one,that is an open. you can also use a this to test for continuity of a wire by putting the leads to both ends of a wire. if you keep the ckt powered, the tests can be performed with a test light.wiring faults are usually simple to fix once they are found. finding takes patience,logic and luck.

[Thomas DeBusk]

OK, more info:

I located & disconnected the power wire to the taillights, set my multimeter to 250m DCA, and put the positive lead on the wire and the negative to ground. With the light switch on, it pegs the meter. With the light switch off, the needle wobbles just a little bit, but very distinctly. That indicates current's flowing, doesn't it? And it shouldn't be, should it? And isn't that enough to keep the relay energized?

Since I posted last I've disconnected and cleaned all taillights, marker lights, and turn signals. No change. I did discover that with both rear taillights disconnected closest to the bezel, or with the upper taillight/turn signal bulb (mine's an '83 Wag) out of the socket, the lights work correctly. But plug the lights back in and the parking lights stay on when you turn the headlight switch completely off.

[ January 30, 2002: Message edited by: Thomas DeBusk ]</p>

[Thomas DeBusk]

BTTT

[andy d]

you are losing me with your procedure. the meter should be set for ohms(resistance) and the ckt should be dead. an open read on the ohms scale will go to infinity. a short will read zero. i would disco the battery to do this test. if you are set for 250 dc milli amps you have to be in series with the load to get a reading and that isnt what you want to measure.

[Veepster]

sounds to me like your problem is in the trailer wiring......from my quick glance over the thread....if everything works w/o the trailer connected and it doesn't with the trailer connected...seems to me like short or bad ground is on trailer....just a thought!

[Thomas DeBusk]

andy d:

Thanks for jumping back in here. Told you I was an electrical ignoramous. However, I think I now understand your method enough to try it as you describe it. Did you see my other post, where I said I discovered a completely fried ignition wire under the dash? Just last night. Unfortunately, that apparently wasn't the wire responsible for this problem.

I've also discovered the rear defroster circuit includes the power seats and power door locks, too. I want them back!

Veepster:

Can't be in the trailer wiring. I've never had it connected. When I say "trailer wiring harness" I'm referring to a factory-made pigtail that splices into the factory vehicle harness by unplugging the connections at the back and just plugging the pigtail in. The pigtail includes a 7-wire vehicle receptacle for the trailer to plug into, and an attached black box that apparently is designed to supply higher voltage to the trailer than just wiring into the taillights. It's a box of four relays that is supplied by the heavy wire that runs to the rear defroster. When the relays close, they supply voltage from that wire to the trailer wires. Pretty slick.

Except mine's on the fritz. Really, I've examined the pigtail carefully, and there are no bad wires in it. And I've tried 3 different relays in the same position as the one that sticks closed, and they all behave the same. It's not the relay.

Thomas

[Veepster]

sorry for the misunderstanding Thomas....I should have taken more time reading the thread.....seems like Andy is guiding you well! Good Luck!

[Thomas DeBusk]

I just tried the "Andy" test. I get roughly 20 ohms resistance on the taillight, auxiliary, ground, and brake prongs of the 7-wire harness. That's with the battery disconnected.

So obviously I have a shorted wire somewhere, right?

The resistance on the ground wire just tells me I have a good ground, right?

By the way, Andy, for the first time, I think I understand how this test works. The battery in the multimeter supplies the power to the "circuit", right? Which wouldn't exist if there weren't a ground, right?

So I guess the next thing to do is start tearing apart more wires to find the short(s). I suppose I should start yanking things apart and seeing if/when the resistance changes?

Thomas