Today I went to borrow my friends car hauler to rescue my broken down commuter car and only the brake lights and turn signals would work on the trailer.  I tried my second 7 pin connector for the 5th wheel in the box and it was the same thing, so I figured the problem is upstream of the 7pin connectors.  Did a bit of reading and found the big square 40a fuse in the fuse box in the engine bay and sure enough she was toast.  Stole the 40a from the fuel heater until the parts store opens in the morning and everything was working again.

 

I'm looking for some suggestions why this could have happened?  I My guess is that it was one of the following things:

1) 19 year old truck and maybe just time for it to go?

2) My 3 year old son likes playing with water guns and could have squirted a bunch of water in the 7 pin connector and shorted something.

3) I have LED reverse lights on my dump trailer and maybe I was in reverse long enough for it to cook the fuse

4) maybe somewhere there is a corroded wire that is creating a lot of resistance

 

If the fuse pops again, where should I look first?

 

After replacing the fuse I drove around with my friends trailer and it was fine.  When I got home I hooked up to my dump trailer since I will be working tomorrow and everything worked fine with the new fuse, so I don't think there is a short somewhere since the fuse didn't pop.  The only relevant information I can think of is I recently replaced the battery on the dump trailer with a larger capacity deep cycle battery, but I don't see how that would matter.  I've had the LED reverse lights on the trailer since I rewired it 1.5 years ago.  I don't remember the numbers on them but they aren't a high power light.  Maybe if I blow a fuse again I can run a relay with the main power source coming from the dump trailer battery instead of powering straight from the trailer wiring?  Any advice would be greatly appreciated!  Thanks!

 

 

Edited April 22, 20186 yr by Timburrr
grammar

You probably have an intermittent dead short to ground on that circuit for a fuse, especially 40 amp, to blow.  Was the trailer connected to the truck when you replaced the battery on the trailer,  If it was and the positive cable touched the frame of the trailer that would have done it.

40 Amp fuse covers the power lead and the running lights. I will admit who ever trailer they better check it out. Like on my BigTex when the 40 Amp fuse blew I was hauling home my brand new zero turn lawn mower. The runs went out so stop in a passing town picked up a fuse it worked for a little while blew again. Needless to say when I check the wiring in the rear. The center marker plugged itself then shorted to ground. That 40 Amp fuse was strong enough to melt over 6 feet of wiring. I ended up rewiring the entire rear of my BigTex trailer because of the damage. 

2 hours ago, Mopar1973Man said:

40 Amp fuse covers the power lead and the running lights. I will admit who ever trailer they better check it out. Like on my BigTex when the 40 Amp fuse blew I was hauling home my brand new zero turn lawn mower. The runs went out so stop in a passing town picked up a fuse it worked for a little while blew again. Needless to say when I check the wiring in the rear. The center marker plugged itself then shorted to ground. That 40 Amp fuse was strong enough to melt over 6 feet of wiring. I ended up rewiring the entire rear of my BigTex trailer because of the damage. 

 

I'll check my trailer out and see if I can find any shorts.  Hopefully it won't be on my truck side because that will mean dropping my fuel tank again and that will make me pretty angry.  Haha.

 

2 hours ago, IBMobile said:

You probably have an intermittent dead short to ground on that circuit for a fuse, especially 40 amp, to blow.  Was the trailer connected to the truck when you replaced the battery on the trailer,  If it was and the positive cable touched the frame of the trailer that would have done it.

 

Truck was not connected to trailer, and it was about a month or so ago that I put the battery on.  I have driven the trailer frequently since then, it was just one of the only recent changes I could think of.  So on Thursday I used my trailer, everything was fine, I backed the trailer into my driveway and disconnected with everything being fine.  Last night I hooked up to my friend's trailer and I had my headlights on when I plugged into his trailer and noticed the running lights didn't come on, so there wasn't a flicker and the fuse popped or anything like that.  I'm wondering if it happened when a trailer wasn't hooked up.  I replaced the fuse and his trailer worked just fine for me.  Got home and hooked up to mine and everything works just fine.  I'll be driving it around in a couple hours and see how things go.

 

Could it have been my son possibly messing around with my 7pin plug and shorting something out?  Could it be the reverse lights draw too much and blow the breaker when I have it in reverse?  This is my biggest worry because sometimes it takes me a while to back my trailer up my driveway and position it where I want it.  I'm trying to remember if the reverse was working when the fuse was blown.  I know the 12v power, brake controller and running lights were all out.  All I had was signal and brake lights that I remember.

Drive around as much as you can with no trailer on and check the fuse regularly to see if it popped.  I know it's a PIA but intermittent electrical problems are just that, a major PIA.

 

You'll need to check the fuse before and after hooking up the trailer. 

Will do. So far no problems. I was also thinking about hooking up to my trailer, putting the transfer case in neutral and put the truck in reverse for a bit and check the fuse with s laser thermometer periodically to see if it gets hot.  Does this sound reasonable? My thermometer is a cheapo so I don't know how well it will work but I was thinking it would give me an idea if the backup lights are too much of a load. 

22 minutes ago, Timburrr said:

I was thinking it would give me an idea if the backup lights are too much of a load. 

I don't know .  I've never tried that.  It would be great if you could an amp meter in line with it and test it that way.  It would have to be a meter rated for that big of a load.

 

What is the amp/watt rating of those backup lights?   

5 minutes ago, IBMobile said:

I don't know .  I've never tried that.  It would be great if you could an amp meter in line with it and test it that way.  It would have to be a meter rated for that big of a load.

 

What is the amp/watt rating of those backup lights?   

I wish I could remember. I'm going to try to find the receipt or something to see because I don't think it's stamped on the lights. I'll check when I get home. Thanks again.

I've done lots of driving around with and without the trailer, so far everything is good.  I tried using the laser thermometer on the fuse with the truck in reverse and there wasn't a noticeable increase in the temperature so I'm HOPING it was a one off issue like my kid jamming something in the plug or filling it up with water, or possibly when I plugged into my friend's trailer there was an intermittant short that popped it right away.  I'm going to keep my eye on it and post if there are any updates.  Thanks for the help!