Ampmeter (not voltmeter)

Jeff65 · 02-14-2003, 03:08 PM

Ammeters require shunts. If they didn't have them, they'd soon self destruct. The factory sets up the ratio of current flow in the shunt, the main harness and the guage. Chances are you're seeing little movement because when all is normal, it doesn't move much. Here's the deal.

When you start the car, the ammeter will move negative as a lot of current flows into the starter. The battery is being drained at this time. As soon as the engine starts and the key is released from START, the current goes way down so the guage goes back towards center. As the rpm of the engine increases the guage goes right a small amount indicating it is trickle charging the battery. The amount of deflection isn't very much when you have a good battery. When you have a bad battery the deflection will be greater. This is so, because the good battery holds up to the start fairly well and the bad battery doesn't.

All you should care is:

a. ammeter needle is to the charge side when the engine is running above idle. The amount of charge indication is proportional to the deep discharge event that last happened.

b. If you see a continuing negative indication, it means you have a short or a large draw on the system the alternator can't satisfy.

c. The exact center of needle movement is not important, the direction of deflection when the engine is running is.

You can see the same indications on a voltmeter, its just the starting point is 12 volts instead of center of the scale. Voltmeters don't move much either, but its the tendency to move upscale or downscale that you have to learn to read.

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02-14-2003, 03:08 PM	#1
Jeff65 Registered Member Join Date: Jul 2002 Location: Oklahoma City Posts: 208	Ammeters require shunts. If they didn't have them, they'd soon self destruct. The factory sets up the ratio of current flow in the shunt, the main harness and the guage. Chances are you're seeing little movement because when all is normal, it doesn't move much. Here's the deal. When you start the car, the ammeter will move negative as a lot of current flows into the starter. The battery is being drained at this time. As soon as the engine starts and the key is released from START, the current goes way down so the guage goes back towards center. As the rpm of the engine increases the guage goes right a small amount indicating it is trickle charging the battery. The amount of deflection isn't very much when you have a good battery. When you have a bad battery the deflection will be greater. This is so, because the good battery holds up to the start fairly well and the bad battery doesn't. All you should care is: a. ammeter needle is to the charge side when the engine is running above idle. The amount of charge indication is proportional to the deep discharge event that last happened. b. If you see a continuing negative indication, it means you have a short or a large draw on the system the alternator can't satisfy. c. The exact center of needle movement is not important, the direction of deflection when the engine is running is. You can see the same indications on a voltmeter, its just the starting point is 12 volts instead of center of the scale. Voltmeters don't move much either, but its the tendency to move upscale or downscale that you have to learn to read.