Backup Fuse Blows Immediately

[Cliff]

77 Cherokee, 401/QT, 100k miles

Like most of our beloved FSJ's mine has experienced some wiring hacks from PO's. Most were easy to fix but one problem persists that I cannot make sense of.

The fuse for the backup lights blows when the ignition is turned on. Even before I can turn on the head/tail lights or place it in reverse. I have examined the harness at the rear and it all appears to be untouched and clean, all connections intact. Bulbs and sockets are clean and rust free. I just can't figure how ignition voltage can blow a fuse when the circuit isn't even yet energized?

Any ideas?

[wiley-moeracing]

sounds like the switch is shorted ground or the wiring to the switch is shorted to ground.

[nograin]

We will have to look at a '77 diagram to confirm, but the reverse lights are probably fed from the 'accessory' terminal on the ignition switch.
The feed to the fuse box is probably fine.
From the fuse box, the circuit with the reverse lights goes to the reverse switch. Since it happens in neutral, its probably a short in that wire.
If it was grounded after the switch, then there is no connection in neutral.

[Cliff]

When you say "reverse switch" are you referring to the neutral safety switch, or is there another reverse switch?

[nograin]

Off the top of my head I don't know if its seperate on your truck.
With the later torqueflite automatic transmissions, the reverse light switch and the neutral safety are combined. The backup light switch gets two wires (one always hot with key on) and the NSS has its own wire.

[rang-a-stang]

QT equipped trucks had the reverse switch as part of the neutral safety switch. It's on top of the steering column under the dash. I would start there.