Testing Postive and Negative Voltage Drops March, 1999

Before replacing a starter or alternator a voltage drop test should be performed to ensure good electrical paths. A voltage drop test is exactly what it infers, a test to see the drop in voltage between two points. In this case we are looking for the voltage drop between the battery and the starter or alternator in the positive and negative wiring.

To Test Positive Voltage Drop:
Using a digital voltmeter, set on D.C. in the 20 volt range

1. Connect the first lead of the voltmeter to the positive battery post.
2. Connect the second lead of the voltmeter to the power supply post on the starter or alternator.
3. Start vehicle and turn on the headlights.
4. Read the meter.

To Test Negative Voltage Drop:
Using a digital voltmeter, set on D.C. in the 20 volt range

1. Connect the first lead of the voltmeter to the negative battery post.
2. Connect the second lead of the voltmeter to the case of the starter or alternator.
3. Read the meter.

A perfect reading would be 0 (no voltage loss between points). A voltage reading over .20 volts usually indicates a bad cable or junction in the path being tested. Check owners manual for acceptable voltage drops in specific vehicle being tested. If a voltage reading over .20 exists then test each junction point in the circuit to find the cable/wire or junction causing the voltage drop. Repair cable/wire or junction as needed. It is critical to test both the supply (positive) & return (negative) circuits in all vehicles.

Effects of an existing voltage drop:

1. Low voltage at battery while vehicle is running.
2. Poor or sluggish starting and driveability.
3. Over working of the alternators causing frequent failure.
4. Starter clicking or frequent failure.