is this what this circuit is actually for? pic attached

[JesseA422]

truck is a 77.
from what i understand this is a constant hot.
when i got this truck the orange wire for the interior lights was connected here. as there was not any interior lights properly wired up at the time i disconnected it, and use the top male as a fused constant 12v source for a radio. it worked fine for a year and now the fuse popped while just cruising down the road.
radio is now disconnected from connection.
when i replace the fuse, it pops.
when i disconnect the battery and replace the fuse, as soon as i reconnect the battery, the fuse pops. this is without anything connected to the top male connection.

any idea where the problem could be coming from? what do i need to trace to find the problem from here?

[MikeB]

Probably just a spare terminal for accessories or add-ons. Many OE fuse panels have these for BAT (12 volts constant) and ACC (12 volts present when key is in ON or ACC position). Sometimes they are even labeled.

[Axle]

A clock is a common factory accessory for that slot. Same pigtail used in passenger cars, back when you could find old cars in the junkyard and salvage the pigtail to repurpose. I'm running my aftermarket radio memory off the secondary 'bat' slot.

[JesseA422]

which is the "second batt" terminal u are using?

why would this fuse pop out of no where, and now it pops if i replace it, without anything connected to the terminal?

where is the interior lights constant hot (orange wire) supposed to be wired to?

[MikeB]

Originally Posted by JesseA422

it worked fine for a year and now the fuse popped while just cruising down the road.
radio is now disconnected from connection.
when i replace the fuse, it pops.
when i disconnect the battery and replace the fuse, as soon as i reconnect the battery, the fuse pops. this is without anything connected to the top male connection.

Found the picture below of a 77 fuse panel. I see where the connector is labeled ACCY, which usually means it's tied directly to the Accessory bus on the back of the panel, which gets power when the ignition switch is in the IGN or ACC position. So this is probably an un-fused source where you would connect an accessory that is externally fused, like a radio with an in-line fuse.

It's possible the ACCY connector is being fed by the fuse below which is is only labeled "FUSE", but I'd be surprised. You'd need a voltmeter to find out for sure.

Either way, f the fuse below the ACCY connector is blowing, you have to determine what it's providing power to. Also, since you don't even have to touch the ignition switch to get it to blow, it's either powered by the BAT bus (not the ACCY bus) or you have a short somewhere.

What happens if you don't install the fuse? Does the engine start and run? Do the lights, heater fan, turn signals, horn, etc. work?

[JesseA422]

no, it is a constant batt + on connection, fused by the fuse right below it like i showed in the picture in the first post. like i said, i had my radio constant power connected there for months, then the fused popped and radio died. i disconnected the radio completely, but the fuse still pops even when i insert one without anything connected to the terminal. no other problems with any other system when fuse is not there.

what do i need to trace to try and find what is causing this to pop, without anything connected to the terminal?

[MikeB]

Originally Posted by JesseA422

no, it is a constant batt + on connection, fused by the fuse right below it.

What do i need to trace to try and find what is causing this to pop, without anything connected to the terminal?

OK. Interesting that the connection isn't labeled BAT. By ACCY, they mean it will power an accessory directly from the battery through the fuse below it. That's a first for me.

To trouble shoot, start by disconnecting the battery and measure the resistance from the the single ACCY terminal to ground. That's the "ohms" position on the multi-meter. If it's zero or close to zero, there is a short to ground somewhere. But since the radio power wire is no longer connected, it would have to be in or behind the fuse panel.

Also, measure the DC voltage on each of the ACCY terminals and both fuse terminals with the battery connected. I assume you will see 12 volts on the lower fuse terminal.


FYI, here's a fuse panel from an 81 truck. Notice how direct-to-battery terminals are labeled BAT, and terminals powered by the ignition switch are labeled ACCY -- and none of them are fused.