Battery amp meter.

[ray_mcavoy]

You're welcome Gregski! Sorry, it wasn't a rare customization made for AC/DC

You're right, just looking at the printed circuit it should be pretty obvious that the meter doesn't handle the full charging system current. It's basically an external shunt ammeter but instead of using an actual resistor for the shunt, they simply use the inherent resistance in the charging system wiring.

GM started using this external shunt ammeter design in trucks back in the early 60's. Some years did have issues with melted/burnt wiring simply because the ammeter circuit was left un-fused. The fuses became standard issue in 1967 & newer trucks with factory gauge clusters and they protect the wiring, printed circuit, and meter from two things: (1) A short to ground on either leg of the circuit, and (2) an open circuit in the charging system wire that serves as the shunt. An open circuit in the shunt wire would basically put the meter in a full-flow configuration but the fuses will blow before a damaging amount of current can flow through the circuit.

So the factory ammeters are safe as long as the 4 amp fuses are present in the original locations.

On a somewhat related note, the 73-75 ammeter clusters can be dangerous if one is accidentally plugged into a 76+ truck that is wired for a voltmeter. That's because the ammeter (which has a very low internal resistance) basically ends up shorting together the +12V and ground terminals intended for the voltmeter.


In my earlier reply, I didn't go into very much detail on troubleshooting the 73-75 ammeter other than mentioning to check the fuses. The procedure is similar to what I posted for the 76+ voltmeter except for the following differences:

(1) When you remove the meter test the terminals in the cluster, both ammeter terminals should show full system voltage.

(2) The ammeter just has 2 studs on the back



and unlike the voltmeter, the ceramic block is just an insulator ... not a resistor.
The ammeter's internal coil should measure very low resistance ... only about 0.1 to 0.2Ω.