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Old 09-17-2013, 09:50 AM   #1
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Join Date: Apr 2013
Location: Olalla, Wa.
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Blowing Tailight fuse

I'm completely confused and hoping someone can help. I've got an odd problem. I have a 95 Jeep wrangler that has been setup for towing. It has a round 4 prong set into the front bumper for the lights and wired accordingly. All of my lights work fine until I set the toad hitch down onto the MH ball. At that point I blow the tailight fuse in the MH. The brakes and blinkers still work fine(on the jeep). My initial thought was that the tailights were shorting to ground, so I redid the ground and checked the wiring to the tailights for shorts with no help. I have diodes on the the brakes and blinks, but, not on the tailights. I think I read you didn't need them on the tailights, but, even then, aren't they only for feed back into the coach in case you flip your lights on the toad? I've tried so many things I'm now at a point of total frustration.
If anyone has seen this before, I sure would appreciate the help.

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Old 09-17-2013, 11:05 AM   #2
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First off, start from the beginning. Get a 12v test light and sit down at the rear of the coach and test all the functions of the trailer plug. Either mark the positions or make a drawing of what each pin does in that trailer plug of the motor home.

Now, get either a battery charger or, a stand along 12V battery and sit down in front of the Jeep and do the exact same thing, testing all the pins in that side of the equation. Again, mark or make a drawing of what each pin does. If all the lights function normally, that is a RT/brake, LT/brake, tail/marker/running (and to the back of the jeep to confirm each operation as you test each pin) and, finally a ground, then, hook up the pig tail to the trailer plug on the coach and do the exact same tests. This will confirm the pig tail is wired correctly, Then, compare the two (or how ever many drawings you made) drawings and see if all the pins align with their respective ones for each plug.

Now, as a separate set of tests, if you have and no how to use a Volt-ohm meter, set it to ohms, and the test each pin in the trailer plug to ground. You should not get any readings until you test the ground pin, then the meter should peg. If you get readings in any other pin besides the ground, you may have frayed wires that are touching the ground pin, on the back side of the plug.

This is the first simple set of tests for your issue. It can get a bit more complicated but, do these first. If all the pins are mating to the appropriate ones and you have not found anything really obvious as a problem, then, you need to probe deeper into either the pig tail or another section.

You say that the second you "touch" the tow bar to the Jeep that it blows the tail light fuse in the Coach? I am by far, no electrical expert but, it sure sounds to me like you've got a short to ground. And the connection or fault is not made until you touch the tow bar to the Jeep. I mean a ground is a ground and, if you find the ground pin in the trailer plug is in the appropriate position and, it transfers to the pig tail correctly then, it transfers to the Jeep correctly, than that is not your problem.

Now, I would think that, if you had short in either of the two vehicles to ground, it would manifest itself even without connecting the two together. Again, no expert here. This is interesting, please keep us informed as you continue the search for answers here. Now, as for "diodes". As you may well know, a diode is nothing but a "one-way valve" for electricity. I have installed several of them but, not for the reason you stated.

My reasons for installing diodes, in the appropriate positions of course, is to stop the signal from the coach, from traveling upstream to the "front of the toad". And the reasons are because of the turn signal lever on the steering column on the toad. Long story but, I'll tell it later. Anyway, I've never found a need to put any diodes in the line to stop the lights in the toad from lighting up the lights in the coach, never had a problem with that. Anyway, again, keep us informed of what you find.

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Old 09-24-2013, 10:22 AM   #3
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Join Date: Apr 2013
Location: Olalla, Wa.
Posts: 18
Well, I got it. I kinda feel stupid, because it was pretty easy. It ended up being the ground and the tailights being swapped in the toad connector. Scott, I had done pretty much everything you had suggested prior to your post with the exception of tracing each pin on each connector. I figured that was all fine since the lights worked fine without the hitch touching. It all makes sense now, once I touched the hitch to the motorhome the tailights would short to ground because it really wasn't the ground, it was power going across the ground. Just not sure how this got swapped. It's a new toad to me, but the PO said(whatever that's worth) that he had been towing it for quite sometime. No matter. It's all good now.

thanks for the help
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