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  Trailer tail light fuse blows when I put vehicle in reverse.

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Author Topic:   Trailer tail light fuse blows when I put vehicle in reverse.
TetonBow posted 10-06-2009 03:39 PM ET (US)   Profile for TetonBow   Send Email to TetonBow  
I have a truck with a 7-way plug and use an adapter for a 6-pole plug from the trailer. I have checked all grounds, replaced wires and everything works fine until I put the truck in reverse and the tail light fuse blows. I still have brake and turn signals. Any help with this one?
jimh posted 10-06-2009 10:25 PM ET (US)     Profile for jimh  Send Email to jimh     
Is the circuit for your vehicle back-up lamps connected to the trailer wiring in any way?
fishgutz posted 10-07-2009 09:37 AM ET (US)     Profile for fishgutz  Send Email to fishgutz     
I think something may be wrong with the "adapter". Some trailer wiring (7 pin) has a circuit connected to the back up lights to stop the surge brakes from working when you put the vehicle in reverse. Whether or not you have surge brakes, this can very well still be the problem.

Here is an exerpt from an article on backing up a trailer with surge brakes.
"The reversing solenoid is activated when the backup lights on the tow vehicle come on. At this time, all hydraulic pressure in the brake system is released as any hydraulic out put from the master cylinder is dumped back to the master cylinder reservoir. When the back up light signal is removed, the solenoid is deactivated and the surge brake system reverts back to normal operation."

So, in your 7 pin connector there is some pin that is used for this and for some reason it seems to be shorting out and blowing the fuse.

jimh posted 10-07-2009 03:24 PM ET (US)     Profile for jimh  Send Email to jimh     
The theory of a short circuit existing in the 7-pin connector wiring of the vehicle can be easily tested by disconnecting the trailer plug and putting the vehicle into REVERSE.

fishgutz posted 10-07-2009 04:28 PM ET (US)     Profile for fishgutz  Send Email to fishgutz     
I did not say the short was in the 7 pin connector. I said it might be in the "adapter". Or it could be in the trailer. In any case he will have to trace that particular wire from the 7 pin connector back to the trailer.
jimh posted 10-07-2009 08:49 PM ET (US)     Profile for jimh  Send Email to jimh     
I am awaiting the answer to my initial question.
TetonBow posted 10-13-2009 09:22 PM ET (US)     Profile for TetonBow  Send Email to TetonBow     
jimh, thanx for your question. I've traced my problem to the vehicle side. The trailer back-up lights and tail lights run through the same fuse in the relay box under the hood. As long as I don't put it in reverse everything is fine. I've traced the wiring from the plug back, but can't see any bad connections or open wire.???
TetonBow posted 10-13-2009 09:26 PM ET (US)     Profile for TetonBow  Send Email to TetonBow     
No, the trailer lights are not connected to the vehicle light. They run through separate circuits.
jimh posted 10-13-2009 11:22 PM ET (US)     Profile for jimh  Send Email to jimh     
If the vehicle back-up lamp circuit is not connected in any way to the trailer, does the fuse still blow when the trailer is disconnected from the tow vehicle?
fishgutz posted 10-14-2009 09:46 AM ET (US)     Profile for fishgutz  Send Email to fishgutz     
This guy's giving us way too little info. First, what kind of vehicle? Next,if the trailer lights are on a separate circuit there has to be a separate fuse. Exactly which fuse is blowing, the taillight fuse for the vehicle or the taillight fuse for the trailer connection? Does the trailer have back up light? Does the taillight fuse blow without the trailer connected at all?

I still think it has something to do with the "adapter" he says he's using. It is crossing from the surge brake reverse solinoid circuit to the tail light circuit somehow.

TetonBow posted 10-15-2009 11:37 AM ET (US)     Profile for TetonBow  Send Email to TetonBow     
The truck is a '96 Ford F-150. The fuse does blow when the trailer is completely disconnected. Trailer doesn't have backup lights. It's the fuse for the trailer backup light that blows. Trailer backup lights and tail lights run through the same fuse. I'm thinking the plug itself must have a short somewhere. I live in Northern Utah where we have extreme changes between hot and cold. Cracked plug, or moisture causing short?? Replacement is $128.
jimh posted 10-15-2009 11:06 PM ET (US)     Profile for jimh  Send Email to jimh     
If your vehicle lighting system blows fuses when the trailer is not connected, there is a very good chance the problem is in the vehicle. The wiring in the vehicle may have become chafed or worn and may have a short to ground. It can be very tedious to locate a short in a vehicle wiring harness.
btb posted 10-17-2009 10:20 AM ET (US)     Profile for btb  Send Email to btb     
I would start poking around under the rear bumper, where the trailer light wiring will be accessible.

Many people use a trailer once or twice, and may go to a place like Uhaul to rent one. Those Uhaul people use all sorts of really crappy trailer-wiring hook ups, they could have got it wrong, or could have left exposed wires.

To me the backup solenoid theory is worth exploring very carefully - a mis-wired connector or plug/socket (on the truck) seems like the most promising idea so far. Do a search on Trailer Wiring Diagram and find your truck, then check out the way your trailer socket is wired.

Chuck Tribolet posted 10-17-2009 08:43 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Moisture isn't going to cause enough leakage current to
blow a fuse, even salt water.

It blows fuses when the trailer isn't connected.

Does it blow fuses when trailer isn't connected,
but the adapter is installed?

Does it blow fuses when the trailer isn't connected
and the adapter isn't installed?


Chuck

TetonBow posted 10-21-2009 10:51 AM ET (US)     Profile for TetonBow  Send Email to TetonBow     
The Fuse blows both when the adapter is connected and not connected. Haven't had time yet to trace wires more carefully, but will look closer at plug. Haven't thought about back-up solenoid, but worth a check. Thanks!
jimh posted 10-21-2009 08:34 PM ET (US)     Profile for jimh  Send Email to jimh     
Finding short circuits in vehicle wiring can be very tedious.
Bulldog posted 10-24-2009 09:05 AM ET (US)     Profile for Bulldog  Send Email to Bulldog     
Don't worry about the reverse trailer solenoid, a 96 Ford or any truck won't have it, as it applies to trailers with DISC brakes. The solenoid is added to trailer circuit to allow backing with disc brakes, and is a plumbing part in the hydraulic line of trailer. Your problem is in the wiring of the truck, have at it. You can take out the reverse bulbs in the truck one at a time to see if that narrows it down for you. I've worked on a friend's F-150 of that vintage, trailer blowing fuses and dim bulbs, traced to chafed wires about halfway along frame....Jack
jimh posted 10-24-2009 10:42 AM ET (US)     Profile for jimh  Send Email to jimh     
Finding short circuits in vehicle tail light wiring can be tedious. Here is an anecdote about vehicle tail light wiring:

One evening my wife was stopped literally as she pulled into our drive way by the local police, who issued her a ticket for having a burned out tail light. The ticket had a 7-day reprieve, and if a working tail light was demonstrated to the police department, the offense would be expunged and no fine would be levied.

I immediately investigated the problem. The fuse for the circuit powering the tail lamp was blown. Inserting a new fuse resulted in another blown fuse. I experimented with a fuse with slightly higher current rating. It worked briefly, but it also blew after a minute or two. I opened the trunk and removed the tail lamp assemblies. All looked normal. I removed the bulbs. I tried another fuse, but had the same result: fuse blew. I removed some more of the trunk interior panels, looking at the wiring, but did not find anything suspicious. That ended my investigation.

Due to the timing of the ticket's 7-day window and our work schedules, we really did not have much opportunity to investigate further, so we decided to take the car to the dealer for repair. The service write-up technician asked, "Has this vehicle been involved in a rear-end collision recently?" In fact, it had. The car was dropped off at 7:15 p.m. The dealership called at 7:45 p.m. to say the car was fixed and we could pick it up. We drove right back and had the car before 8 p.m. Evidently, the dealer service personnel knew precisely where to look for the chafed or shorted wires from experience with this particular model of vehicle. The cost of the repair was about the same as the fine for the ticket, so we did not feel too bad about having to pay for the repair. It was somewhat embarrassing for me, personally, that I couldn't find the defect myself. I think the pinched wire was related to the bumper movement during the rear-end collision. It was hidden somewhere under another removable panel.

clark2334 posted 10-14-2010 02:25 AM ET (US)     Profile for clark2334  Send Email to clark2334     
Pull off both tail light assemblies and see if any wires were pinched when the were bolted in. If you see nothing, try a new fuse and see if it blows. There could also be a wrong bulb in one of the sockets causing this.
jimh posted 10-14-2010 08:29 AM ET (US)     Profile for jimh  Send Email to jimh     
Thanks for the advice, but since this discussion has been dormant for a year, I think we can assume that this problem has been resolved.

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