From the WEB
Loose or corroded connections on the back of the alternator can increase resistance and restrict the current flowing through the circuit. So can broken or frayed wires inside a connector or the alternator wiring harness. The connectors and wires may appear to be OK visually, but unless you actually test them, you have no way of knowing if they’re making good electrical contact and are clean, tight and undamaged.
If the wires and connectors are not checked, you may replace the alternator only to discover the new unit you just installed is “no good.” Now you get to replace it again and, on some vehicles where the alternator is buried under a lot of other stuff, that can result in a lot of lost time and labor.
You can do this test by using your voltmeter to perform “voltage drop” checks across the connections when the engine is running. A voltage drop test is done by setting the voltmeter on the 2-volt scale, then touching the positive and negative test leads on opposite sides of a connection. If there is resistance in the connection, some of the voltage will try to bypass the resistance by flowing through the voltmeter. If you see a reading of more than 0.2 volts, it means trouble. Ideally, the voltage drop across any connection should be zero, or less than 0.1 volts.
Check for voltage drops at the positive and negative battery cable connections, the alternator BAT+ power connection and the engine ground strap(s). Poor ground connections are an often-overlooked cause of low charging output and alternator failure. Voltage drops on the positive side of the charging circuit can cause undercharging. Voltage drops on the negative side can cause overcharging.
When a new alternator is installed, check the battery voltage and use a battery charger to bring the battery up to full charge before you return the vehicle to your customer. Also, start the engine and use your DVOM to check the charging output of the alternator. Don’t assume everything is working OK just because you bolted in a new alternator.