[petek]

If the harness is already set up to use an ammeter then installing an ammeter will be fine. The wires are probably something on the order of 10 gauge (voltmeter harnesses can be as small as 30 gauge, although most of them seem to be around 18 gauge).

If you were to install a voltmeter into the ammeter harness, the voltmeter will work but the car won't run. Voltmeters are high resistance (20k ohms). You'd be dropping all of the battery voltage across the voltmeter.

You could use a milli-ohm shunt and measure the voltage drop across the shunt to determine amperage. I = V/R For example, let's say you had a 1 m ohm shunt (.001 ohm) and a millivolt meter. You place the voltmeter across the shunt and measure 10 mV. In other words, the shunt is dropping 10 mV. Calculating current, I=V/R, I = 10mV/1mohm = 10A. Note that the voltmeter is going to have to be capable of measuring in the millivolt region. (Ammeters are really millivolt meters with milliohm shunts.)

But what if you wanted to use a "normal" voltmeter with a range say from 0-20 volts. Ah, you could just increase the milliohm shunt's resistance. Let's say you take the shunt's resistance to 1 ohm. Now you get 1V measured for every amp flowing through the shunt. But there is also a big problem ... the shunt is dropping one volt for every amp. Draw 10 amps and you'll drop 10 volts across the shunt. That leaves only 3.8 volts for the rest of the car!

I hope this isn't too much nerdy information. Use an ammeter if your harness is set up for it and you'll be fine.