A voltmeter is a gauge that measures electrical current. The Mercruiser 4.3 L operates on a 12-volt battery, so the voltmeter should register around 12 volts. Sometimes the reading spikes or drops depending on electrical usage, but unexplained or extreme fluctuations could indicate a problem with the alternator, the battery, the electrical system or the voltmeter itself. You can double-check the actual voltage using a multimeter.
Sometimes voltmeters exhibit unusual readings when the revolutions per minute increase or decrease. Extreme fluctuations are unusual. Test individual wires running to the voltmeter, including the light wire, sender wire, ground wire and 12-volt wire. Unplug each, one at a time, while maintaining constant RPMs and observe what happens in order to diagnose problems.
Voltmeter problems may stem from bad connections. Check all the wiring connections leading to and away from the voltmeter. You can unscrew the face of the voltmeter or access the back through the gauge panel. You should clean all the connections while going over them individually. Dirt and debris could interfere with the readings.
In rare cases, the voltmeter may simply be producing inaccurate readings. In such a case, you will have to replace the voltmeter itself, but problems are more likely due to faulty connections or a short circuit.
