1977 FJ40. White Wire off Battery lead, now grounds (1 Viewer)

[Jason A]

Anyone know why all the sudden this wire would short out? It appears to go to the main wiring harness that runs along the passenger side, into the firewall.

It now grounds and the FJ will not start. If the wire touches, it arks
I just got this truck about a month ago, and don't know much about the wiring.
Thanks
JA

 

[DanS HJ-45]

You've got a short somewhere. Time to start looking under the dash for some melted wires or burn marks.

That big, solid white wire is part of a circuit that includes the alternator B nut (the big wire on the alternator) and the main power to the fuse box.

Dan

 

[spotcruiser]

Sounds like the wire from the ammeter to the fusible link, but hard to tell without a photo. You need 10 posts to be able to post photos and you need a wiring diagram.

 

[Jason A]

Here is the white wire I was talking about, comes off the positive post and into this thick harness. Basically the truck was running good all the sudden this wire just burned in half and now grounds.

 

[Fast Eddy]

This is the fusible link. You could just solder it back together and see if it keeps burning out. If so, you are drawing more current than you should for some reason, probably a short as mentioned above.

 

[spotcruiser]

Yep, the white wire runs from the battery to the ammeter. The fusible link, the short black wire with the two connectors, burned up. That's what it is supposed to do to protect the harness. Either you have a ground on that white lead [or the associated white/blue lead(s) or fuse block] or you pulled the instrument cluster while the battery was connected and allowed an ammeter stud to touch the dash. It's easy to do.

 

[Jason A]

Spotcruiser thanks! Can you walk me through step by step where to check for this ground? Any steps that I need to take under the dash? Any grounds that I need to check for and where to look? Thank you so much in advance

 

[spotcruiser]

Did you happen to pull out the instrument cluster with the battery connected?

That white wire runs from the battery to the firewall, through the main harness grommet at the firewall, then up under the dash to one of the ammeter studs. I would disconnect the battery and pull the cluster to look at the ammeter terminals. As mentioned, disconnect the battery. Pulling the cluster is a little difficult, since you have to disconnect the speedo cable and there is no easy way to do that. You have to remove the two cluster screws and tilt the cluster down as best you can (it's hooked at the bottom) to get access to the speedo cable connection. The speedo cable makes it difficult to tilt the cluster down, but you have to reach in there and release a plastic clip to pull the speedo cable out of the speedo input. Once you've done it, you will know next time, but it's difficult even when you've done it many times. When the speedo cable is disconnected, you can pull the cluster up and out a little. It will have two wires to the ammeter and a harness with an octal plug for the rest of the connections. You can pull the octal plug out of the cluster and pivot the cluster out to look at it. The ammeter has connection studs on the back of the right side of the cluster. If you don't see a problem near the ammeter, you can pull the fuse block and look for a problem there and/or look for issues with the harness between the fuse block, ammeter, and battery. If you don't have a grommet in the firewall where the white wire passes, that's certainly a probable place for chaffing and a ground. I'd check those places first and go from there.

Good luck.

 

[Jason A]

Nope. The truck has been sitting for 11 years. Just flushed the gas tank and changed plugs, points etc. it was running good and we were about to take if for a spin. Then I saw smoke. That lead wire shorted and will not reconnect without buring up. I'll do whatever you think, I can try removing the instrument cluster etc. should I look for anything on the fuses? just let me know your thoughts

 

[spotcruiser]

You'll just have to look it over thoroughly. Anything downstream of a fuse would most likely blow the associated fuse. The main part of the harness is only protected by that fusible link, so it's most likely upstream of the fuses. Follow that white wire as best you can. It could be chaffed and grounded along it's length (make sure it has a grommet at the firewall). Look at the large white/blue wire from the alternator. Look at all the fuses (just for GP) and look at the back of the fuse block and it's associated wiring. Look for chaffing and burning. Look for something scabbed-in by a previous owner for continuous power (like a radio or lights). Disconnect the battery to begin with (the fusible link is blown, but it's just a good idea anyway). You'll find the problem at some point, hopefully not a melted mess in the harness. Look for the obvious first.

If you put an ohmmeter to the harness side of that white wire, you should find that it's grounded. You'll have to find out where.

By the way, you need a wiring diagram. There have been many posted here on mud. If you can't find one, I can post one.

 

[Weber Sarge]

Check those firewall grommets very closely - it's a great place to cut through wire insulation against the sheet metal .
Sarge

 

[jcj_78FJ40]

I would begin by disconnecting all wires to the starter and then the alternator and see if the short to ground goes away.

 

[B y r o n]

Sitting that long, mice could have gotten in and chewed through the insulation under the dash. Short burns out wires and they burn down the cruiser!