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78' F250 Wiring Short?

I have a 78 F250 that I have been chasing a break and blinker short in for years. I have accounted for every bare wire, bad socket, and abraded wire I can find, to no avail. I have heard of these trucks sometimes having a defective fuse block that can cause problems. Has anyone ever heard of this? Any suggestions appreciated.


Do you know if the short occurs if either the brakes or blinkers have to be turned on before the short happens. This would mean the short is after the switches. I would suspect a problem in the steering column first. If the trouble is with one the legs to the rear lights you may have to seperate them. Another common thing to go wrong is wiring to a trailer wire harness.

Yeah the short happens after some use (after replacing the fuses the brake and turn s. lights work for a little while), which tells me it’s not a dead short, but probably some intermittent grounding contact. The steering column is an interesting idea. But then that would mean that the brake issue is separate from the turn signal issue, wouldn’t it?


The brake circuit and turn signals are tied together along with the flashers also.

Maybe clean up the ground wire that connects the socket to the body or drill another hole and reconnect.

Okay, so today I removed the signal switch from the steering column, inspected it, looked fine. I re-attached the wiring harness with the switch removed and tested the signals and brake lights. I could not get the fuses to blow no matter how I messed with the switch. So I went through and shook every foot of lighting wire in the truck twice with the brakes and flashers on, to try and ground a possible hidden bare wire. No dice. After reassembly, I drove it for 20 min or so, and got the fuses to blow with the blinker on but no brakes. I think that it may be the switch, but am unsure, and it’s $95. What to do?

Later, I replaced both fuses, and drove it without using the signals, except a few brief clicks to make sure they were working. I managed to blow just the emergency/stop fuse. I guess this means there are two separate issues

I will check these more thoroughly, right now I have pretty low impedance in the circuit though.

Has it ever blown a fuse when the engine is not running?

Sounds like No from your posts.

Just a clue. I don’t know yet what it might mean.

I wonder if the brake light switch is somehow shorting to ground. The only reason that this may be a suspect is that I bought a used 1947 Pontiac years ago that didn’t have turning signals (turning signals were an option in those days). The brake lights also didn’t work. I noticed that the brake light switch had been disconnected, so I reconnected it when I installed the turning signals. The fuse blew immediately. After trying everything I could think of, I replaced the brake light switch and everything worked. The brake light switch on the 1947 Pontiac was a pressure switch mounted on the master cylinder. I don’t know if it is possible for a brake light switch that is actuated directly by the pedal could short to ground or not. The brake lights do have to operate through the turning signals, so I suppose it could be a possibility.

You stated that the short seems to not be present when the switch was removed from the column so the switch may have a problem shorting to ground somehow but I would be skepical about a bad switch. It could be though the switch is putting pressure on a connection in the column that is touching ground.

The easiest way to locate the trouble may be to use an ohmmeter on the leads from the turn signal switch to the lights. Remove the rear lights and check the resistance on the leads going to the lights. With the lights removed you normally should have an open circuit so any resistance shown isn’t good. You will have make the test points after the flasher unit. It may be easier to probe from the light socket end of the wiring.

No, and it has never blown without the truck being driven either, which is why i thought it might have to do with a bare wire bouncing around during travel.

I don’t think so, but I’m going to check this possibility out though.

The short did not occur when the switch was removed, but so far, I have only been able to blow thw fuses while driving, and I didn’t drive with the switch removed.

Thanks, I’ll give that a try, but since the fuses only blow while moving (this has got to be a key part of the puzzle), I don’t know if I’ll get any resistance.

Is there any way to know if I’m looking for a common issue or two separate issues? The break and blinker fuses seem to blow at the same time, but I can get them each to blow independently as well.

Do you have a wiring harness for a trailer? This may have a wire that is grounding out when the vehicle is in motion.

I do but I have checked it repeatedly, with no luck.

This is an old, beat-up truck, that has had so many home-job modifications that there are loose wires hanging everywhere which I have had to go through inch by inch and clean up. There must be something simple I’m overlooking.

You haven’t once mentioned using an electrical multimeter to help you chase these short circuits. Visual inspection will only take you so far. Without using any test equipment, like a multimeter, it could take you years to find where the problem is.
You need to be more precise in noting the symptoms. To wit, what fuse(s) blow? Have you driven on a rough road and repeatedly actuated the turn signals until the fuse blew? Have you driven on that road and tapped the brake pedal repeatedly to attempt to blow a fuse? If so, were there any results?
Here are the wiring diagrams which identify the wires and components of the affected circuits. Scroll down and click on an image of interest. Enlarge that quarter image. Colored pencils help to trace a circuit. The wire colors help to trace the wires in the truck. The multimeter…oh! I already mentioned that.
You can find that Gremlin!

I have used my meter to test resistance in the circuits, but I must admit that I don’t know enough about car wiring to really chase down a short in a circuit that has multiple parallel components. The bigger problem is that so far, there’s been no short unless I’m moving, which makes it hard to track down with a meter.

The Stop/emergency fuse blows, along with the Turn signal fuse. They seem to usually blow at the same time, but with the fuse block on the engine firewall, it’s hard to see when the brakelights stop working (the blinkers are more obvious). They both blow together (apparently), but the brake fuse will blow even if I don’t use the signals, and the signal fuse will also blow by itself with use if there is no fuse in the brake circuit.

I have used the signals and brakes repeatedly, and always get the fuse to blow after a bit. There is no correlation that I have noticed to the roughness of the road, but it’s an old truck with stiff suspension, so the rides always a bit rough.

I have one more thought and then I will leave you alone. You are probably on the right track that something happens while you are driving and everything in bouncing around. I had a brand new 1985 Ford Tempo and the fuse would blow that controlled the tail light circuit and the instrument lights. I would replace the fuse and all would be fine until I was driving. I figured out that the fuse would blow when I turned a corner. Since the car was on warranty, I took it back to the dealer. What had happened was that Ford offered, as optional equipment, which I didn’t have, a tail light monitor that would light a bulb on the dashboard if a tail light bulb went out. The factory had connected the tail ights through this thin wire rather than to the correct heavier wire. The insulation melted off the wire and when I would go around a corner, the wire would contact the body and cause a short to ground.

In your case, it might be that a wire to one of the turn signal bulbs is bare in a place where you can’t see it. While the tuck is in motion, it probably makes contact with the body.