The ammeter is reading current. Both ways. If a load draws current, it pulls the needle to minus. The alternator is wired to the ammeter so it can be read by the meter, and can get to the battery to charge the battery. As the alternator regulator adjusts alternator output to keep the battery charged, the needle will pull towards positive. Hopefully as you run down the road you will see the needle a tic on the plus side. The alternator will only put out what is needed. Probably never see it's full output used on a low accessory old truck.
As for capacity of your ammeter: The meter has two flow paths in parallel. The meter "wire" and the big shunt "wire/copper/brass buss bar". Almost all of the current runs thru the shunt, which is a bypass. Milliamps go thru the meter movement. Can't say who's ammeter will be sufficient for who's truck. It's a no brainer to go with a voltmeter. Many times you can get one the size you need and apply a face decal to match the "look". When key is off....A ammeter needle rests in the middle. A voltmeter needle rest at CCW.
1. Professionals, automotive electrical companies and Gauge manufacturing companies recommend a Voltmeter.
2. A voltmeter actually warns better and gives more info.
3. Most of the harness vendor's kits do not support ammeters.
4. The auto makers went to Voltmeters or indicators a long time ago.
5. The wiring is one 12V power source and one ground. Supplement it with an indicator.
6. No one has said an alternator will damage an ammeter.
If a persons "conversion" is to 12V. The amp draws should be 1/2. This would be less current thru the original ammeter. At the same time, if their conversion includes add-ons that draw more amps, it may cause a problem. Powerful headlights, sound systems, heaters, high watt devices. I have not researched if the milliamp meter movement is protected/limited no matter the overall amp draw and whether the shunt is robust enough to handle any draw that a person could add to their truck. This is the caveat I mentioned in my assumption paragraph in previous post. I'm guessing no problem. You could fuse it.
Last edited by bartamos; 11/19/2022 3:15 AM.
