Trailer hitch wiring mystery: 3 terminals, but 4 functions


Old 11-24-09, 05:11 PM
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Location: Wisconsin
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Trailer hitch wiring mystery: 3 terminals, but 4 functions

I was working at this rental today and this old guy was interested in buying the tenants 4 x 8 trailer. He got me involved, when he found out I was 'handy', because he did not know if his pickup truck's jack connector was the right one for the trailer jack. Plus he said he cannot hardly see. It was. It was in sorry shape with crud in some of the female terminals and green corrosion. I cleaned it all up for the guy.

Then I volt meter tested. I was only able to get 12 volts from one jack terminal to the green wire jack terminal, no matter what we did in the way of............

...well.......this is the mystery: The jack has 4 total terminals of which 3 are female, and one is male. One of the female's is the green and carries the ground. So answer me this: How can the trailer have 1. Lights on - 2. stop lights - 3. Left directional, - and 4. Right directional....when there are only 3 terminals and a green ground wire terminal? ???????

The old man tired of my testing and questioning finally said, "Let's just pull up to the trailer, hook my jack to the trailer, and see what happens." Well, right off the bat, the lights were on, but really dim. You could only see the faint glow when looking into their lenses head on. Then he tried one of the directional signals and they worked, but were dim. The brake lights did not cause the lights to get brighter, even though they did on the pickup itself.

The pickup's jack connector, and about 18 inches of that flat wire containing the 4 wires, was spliced into the individual wires near the rear of the truck. And one would presume that whoever did it (the old man bought the junk pickup truck that way) got it right, or else would have redone the job till it was right. But for some reason, I had either no power or like 4 volts of power to say a couple of those terminals. (Yes, I had thre polarity correct for DC testing, but I even switched my probes just to make sure) If it was MY truck, I would have guessed the connectors got corroded inside and needed to be redone, even though by looking at these specail square plastic coated connectors, I saw no evidence of salt/ corrosion The old man said he'd take care of it.

But what is still vexxing to me, as I am not familiar with trailer wiring jacks....... even if 3 of the terminals of the jack had 12 volts for each proper light function, along with the 4th ground on earth do you get the 4 functions that I listed with only 3 wires + ground? Yet, that was a typical standard jack. You always need lights at night + brakes + left signal + right signal. ?????
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Old 11-24-09, 05:25 PM
Join Date: Jun 2008
Location: Eastern Georgia
Posts: 486

color code for a 4-wire connectors as follows:
Yellow- left turn
green- right turn
brown-tail lights
white- ground

the brake light work thru the same wires the turn signals do.
hope this clears it up.
Old 11-24-09, 05:35 PM
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Join Date: May 2007
Location: Wisconsin
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White is ground? Why not green? What are they trying to do, confuse people?

So a directional filament is the same filament used for brakes? Is that the case also for the rear lights on a vehicle, usually?

Old 11-24-09, 07:37 PM
Join Date: Jan 2007
Location: North Central Indiana
Posts: 916
Trailer lights

You are working from house wiring when you think of green as ground. On a vehicle this rule does not apply. I fell in to this trap one time on a travel trailer.
To expand on the turn/brake lights, when you apply the brakes current from the brake switch goes up thru the turn light switch. If the turn lights are off the current continues to BOTH brake lights. If a turn light is on the switch interrupts the brake current to that light and takes it over as a turn light. This applys to older vehicles, on the new ones with multiple lights its a different ball game

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