Testing an Ammeter

my gauge

I (Pati) found this amp gauge (ammeter) at a swap meet recently for one dollar! My plan was to research what vehicle it goes to, then sell it on eBay and make a few bucks. We’ll get to the profit part in a minute, but first for those of you needing to learn about electricity – what’s an amp, and what’s it got to do with a car battery?

Electricity is measured in volts, amps and ohms. Volts are the power a battery puts out, amps are how hard the battery will push that electricity through the circuit, and ohms are how much resistance to that flow is in the circuit. Most modern cars have voltage gauges with an idiot light that comes on indicating low battery charge, but older cars have an amp gauge that tells you if your battery is discharging (being used up), charging (receiving more electricity from the generator or alternator than it is using), or neither (the charging is equal to the battery use).

gauge podAmp gauge (‘BATT’) in its original instrument cluster, a 1961 Chevy pickup.

When you start a car with an ammeter, you’ll notice the needle swing to Discharge (‘D’) because of the strain the starter is putting on the battery. The engine hasn’t started yet to make its own power, so the battery is doing all the work. After the car starts, you’ll see the needle swing the other way because the generator or alternator is now working to charge the battery back to 12.6 volts, its happy place. When voltage is back to normal, the ammeter shows zero (needle straight up) to let you know your battery is good and everything is ok.

Great, so where’s the money?

This ammeter is an AC brand part, which I knew is good since AC, AC-Delco and Delco-Remy are usually associated with General Motors cars, which are everywhere. Finding an “OEM” (original equipment manufacturer, meaning the company that made the original parts for GM for installation on their cars at the factory) part is more desirable than an aftermarket (company that imitates what the OEM manufacturer did after the vehicle is released for sale) part. So far, so good. During my research, however, I found other brand-new gauges that had different ‘unplugged’ readings.

testing-an-ammeter

Six gauges had needles in the middle, including the one pictured. Only one had a needle flopped over to the right… like mine. Disappointed, I thought mine was broken. “Test it,” I was told. “How?” I asked. “Look it up,” he replied.

You mean I have to work for this maybe-not-coming profit??

There are, apparently, two ways to test an ammeter and they are both easy. The first is to check for continuity. Continuity means electricity will flow through the gauge (or wire, or circuit) you are testing. It means there are no broken wires or other breaks that prevent electricity from continuing through. To test for continuity, you need an electrical tester set to the symbol shown below:

continuity off

This arrow-plus looking symbol is in the ‘horseshoe-symbol’ group. The horseshoe (or Greek letter omega) stands for Ohms, the measurement of resistance. If we have so much ‘resistance’ our electricity can’t flow at all, we have no continuity. Hook up each wire to a post on the back of the ammeter (it doesn’t matter which goes where) and see if you get a reading. If you get any reading at all, electricity will flow and your gauge is good! The number you get, in case you are curious, is the resistance in the gauge, and of course a lower number is best.

continuity on

The second test requires no fancy equipment, just a battery and some jumper wires (with or without clip-on ends). You can use any battery, but you should NOT use a car battery. The smaller the better. I used a 9-volt but even an AAA will work. It doesn’t matter how many volts are in the battery because the gauge measures amps.

Hook up the wires (it still doesn’t matter which one goes where) and when you’re getting ready to connect the last one, just touch it briefly. The gauge should peg one way or the other and then go back to where it was. Reversing the wires will cause the gauge to max out the other way. Either way, your gauge is good.

battery off

Not touching yet….                          

battery on

Touch!

Now I can sell this gauge, confident I’m not ripping anybody off.

While many people have a fear about learning about electricity, it really is quite easy – grab an inexpensive voltmeter and start your learning!

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