Voltage Gauges

A voltmeter and an ammeter show the health of the charging system, so a failing alternator or a heavy drain shows up before the battery goes flat. This section holds 52 mm gauges in black: a voltmeter reading 9–17 V and an ammeter reading from −60 to +60 A.

With the engine off, a charged battery rests a little above 12 V. With it running, a conventional alternator typically charges at about 13.5 to 14.5 V. A reading that sags under load, climbs too high or swings with engine speed points to the regulator, the belt or a poor connection.

An ammeter shows current flowing into or out of the battery. Positive means the alternator is charging it; negative means the electrical loads take more than the alternator supplies, as with fans, lights and pumps all running at idle.

Choose by what you want to see:

• Voltmeter: simple to wire, needing only a switched feed and a ground. Its 9–17 V scale spreads the useful band across the dial.
• Ammeter: shows charge and discharge on one dial. Depending on its design, the measured current passes through the gauge or through a shunt, so read its wiring diagram first.

Feed the voltmeter from a switched circuit with little other load, or it will show that circuit’s voltage drop as well. Where full charging current runs to the dash, use cable rated for it, fuse it at the source and route it clear of sharp edges. Check both gauges against a multimeter after fitting.