Thanks, I'll use the 2amp fuse. This may be a stupid question, but which wire should be fused. If I connect to the lug behind the battery for one lead and the horn relay for the other, it looks like there's already a heavy wire running directly from the lug to the relay, so both ampmeter leads will always have 12v. I can't quite grasp what the fuse will protect.
What am I missing? I tried to connect a stewart warner amp gage up for a test - one term to the lug behind the battery and the other to the large terminal on the horn relay (which is essentially each end of an existing wire) and I get nothing on the ampmeter.
The factory harness (on vehicles that were ordered with ammeters) had 20 gauge fusible links at each location--one at the horn relay and one at the battery junction block. You are correct, both wire are hot at all times, even when the vehicle is off. That's why the fuse links were installed to protect these long runs of wire that always had 12 volts. If one got shorted to ground, the fusible link would burn up to protect from a fire starting.
Unfortunately the fusible links do nothing to protect the fragile ammeter movement. That's why I suggested the 2 amp fuse. Ideally (assuming you aren't building a show car), would be to intalled two fuse holders, one at each location, and each holding a 2 amp fuse.
For those that have show cars, having something un-original like a fuse holder hanging out is a big no-no, that's why you install one behind the battery where no one can see it. You don't have that luxury at the horn relay as there is no place to hide a fuse holder.
The engineers never foresaw these vehicles being around this long. What happens over the years is the main power connections get loose, or corroded (or a careless owner doesn't maintain them). Now what happens is someone tries to start the car and scores of AMPS are drawn thru this alternate route (remember the ammeter and it's wire looks like a bypass to the main 10 gauge wire running across the radiator support). The 20 guage fusible links won't blow in time to save the ammeter's internals. This is why I suggest at least one 2 amp fuse in line with this circuit. It blows to protect the meter itself.
This is the exact situation that has some many 70-72 owners scratching their heads on why their printed circuit board traces connecting the ammeter are burnt to a crisp "for no apparent reason"....