Can't find electrical short (1 Viewer)

[snailwagon]

So I've been doing the frantic rush to get the pig together for a week long road trip, next week. A current problem that has been occuring is an electrical short. It doesn't help that the wiring diagrams for my vehicle do not match any color coding on the actual vehicle. This is on an early fj55. The 5amp fuse on the fuse block keeps blowing. You can idle the truck all day long, but once you drive it for 20 minutes, the fuse blows. As long as the engine is hot, it will constantly blow fuses when you replace them. I know that sounds wierd and is probably a coincidence, but that is what is happening. The circuit in question runs the 4wd indicator light, the backup lights, the fuel gauge, temp gauge, and oil pressure gauge. It may run more, but I'm not sure. My gauges are important to me on road trips.

I recently had this problem and was curious about the fuel level left on a run and replaced the dead 5A fuse with a 10A. It blew withing seconds, making me think I had a direct short. However, I'd think a direct short would blow out the fuse right away all of the time.

I've unraveled the first part of the wiring harness, it looked fine. The wiring harness to the rear also got pulled out and appears to be in good shape. I looked around under the dash, but couldn't inspect that well without dismanteling the dash. The only thing I know it isn't is the gauge cluster, since it blows without it connected.

Anyone have any suggestions of where to start looking and how to troubleshoot a problem like this? I don't have much time to wrench before our trip, so pulling the entire harness out is out of the question.

If worst comes to worst, I'll just wire in a new circuit temporarily.

 

[SteveO]

I had a similar problem in that the PO had removed the brake indicator light from the dash and then plugged the two connectors together behind the dash. It took me forever to find because it was tucked up inside the dash and it would only blow once in a while. It's just a guess but worth looking into if you're missing any lights from your dash. The two connectors look like they should plug together. Good luck. ...Steve

 

[IDave]

Sounds like when I had a bad fuse box (about the same vintage as yours). The rivet that held the clip for one end of the (in my case, headlight) fuse would intermittantly ground out and blow it. The rivet was just loose, and that was the result. I figured it out when I saw the scorch marks near the clip, but it made for some hairy driving experiences.

 

[Pin_Head]

Here is the drill for finding shorts in a circuit:
1. remove fuse from fuse block.
2. disconnect the wire for each device on the circuit at the end (eg at the sender for the gauges, etc.) and insulate the end of the wire with a piece of tape.
3. Measure the current between the two clips for the fuse in the fuse block using a high current amp meter or a test light.
If there is no current, then the wiring harness is OK and not shorted. Start connecting devices one at a time and measure the current until you find the device thqat is shorted.
If there is high current with everything disconnected, then a wire is shorted to ground. Start checking places where the wire might rub against sheet metal or other grounds. If that fails to find it you have to start looking at the fuse block itself and unwrapping the harness to find the short. Sometimes it is easier to run new wire that find a short in a harness.

 

[DSRTRDR]

Sounds like when I had a bad fuse box (about the same vintage as yours). The rivet that held the clip for one end of the (in my case, headlight) fuse would intermittantly ground out and blow it. The rivet was just loose, and that was the result. I figured it out when I saw the scorch marks near the clip, but it made for some hairy driving experiences.

yup, had the same problem once on the ignition fuse - truck wouldn't even start - I bypassed the fuse block until I had the time to re-wire the truck

 

[snailwagon]

Thanks pinhead, I've never had anyone show me around electrical diagnostics so troubleshooting is not my expertise. That drill is pretty logical and the quickest for isolating the short. I already went through about half of the harness. The fuse block looked ok, but I will inspect it further.

And no brake light indicator on this model, very limited elctronics. A short at the fuse block may make sense. Last week when I was securing my cd player in I noticed a spark when securing the radio (unplugged and ignition off) to the inner dash.

I'll get cracking at it tomorrow, thanks for the start you guys.