Every time we hook up a trailer to my F350 and turn on the parking lights, they light up but then in a few seconds it pops the small fuse under the hood.

I even clipped the wire from the main harness and mooched one off a rear taillight.... still did it. Does it hooked to the airboat, travel trailer, lowboy etc.... and drives me nuts.

Where to look?

Thanks, jeff

 

Sounds like the harness along the frame, or down the firewall. I had the harness get too close to the muffler and melted it. It was behind the muffler and in the frame and I couldn't see it. I've also seen the wires get close to steering shafts or brake/clutch pedals under the dash.

 

What size is this fuse? Someone may have installed one that is too small. It should be around 15 amps.

 

IIRC it was what the book called for at 7.5, but someone put a 10 in there and we've stuck 10s in there ever since... Its so weird, it blows when a trailer is hooked up but not without a trailer.... I'm just wondering what does that mean. And I'd thought it might be in the main harness from up front so thats why I cut that wire and mooched one off a taillight..

Jeff

 

If you have a short to ground with the wiring on the truck, the fuse should blow regardless if the trailer is hooked up or not.

The two scenarios I can think of would be your trailer plug is hooked up wrong, and everytime you plug a trailer in, the running light circuit goes to the trailer ground instead of the lights. The other senario is the fuse is too small, so with the extra lights from the trailer, the fuse can't handle the load.

 

Fuse wont' blow until the trailer is on.. As to going to ground, I've had it apart, I KNOW its right, has to be a bare wire somewhere insulation worn....

IIRC we tried fuses up to 20 which was stupid when the book called for 7.5, still no go. Blows on both my airboats, my travel trailer, and my lowboy.... The F'd up thing here, it only blows on the round 7 pin plug, the flat 4 pin connector works just fine with trailers that have that connector.....????????????

We need an electrical specialist

Thanks, Jeff

 

It's obvious there is nothing wrong with the truck wiring. It doesn't blow unless the trailer is plugged in. What I would do is get a meter and put it on ohms, and put it on the brown wire that feeds the round plug. Put the other lead of the meter on ground. Note the reading. Then plug the trailer in.(do all this testing with the lights off). If it's blowing the fuse, expect the reading you had to go down close to zero. This would confirm why the fuse is blowing.

I would then take the trailer plug apart, trace down the running lights wiring on the trailer, and take a ohm reading. If you get a high reading(no short) then you know the trailer wiring is good(I would expect this since multiple trailers do the same thing).

If all this checks out, then you know there is something wrong with the plug.

 

if it goes out with any of your other hook ups !then it is what franklin 2 said those 7 and 4 flat connet. from corrosion common problem/and flying around +damage "shorted" block> it's time to change it >101=no go elec. problems

 

I didnt see what year your truck is. If you still cant find a problem try unplugging the connector behind the 7 pin trailer connector. This is the large hardshell connector with the safety lock that joins the vehicle harness to the 7 pin connector. Ive seen some of these full of corrosion and cause very high resistance. The harness is also very tight at that location and some rub through may be occurring at the hitch that isnt present until the weight of the trailer harness is attached.

 

If you are using a 6 or 7 wire plug, there are different wirings. 6 wire is actually very commonly switched around, so much so that Hopkins (hoppy and drawtite) have a quick way to switch their adapter around. Until they started coming standard on trucks, most people wired the 7 wires as they saw fit. The most common thing I saw in fixing trailers is the center pin being a ground. If your trailers are wire wrong, this can cause your headache.

On a four wire, I don't know, it's hard to screw those up.