Rick's Free Auto Repair Advice

Alternator Voltage Drop Test to Find Faulty Connections

How to Perform an Alternator voltage drop test

An alternator voltage drop test is done to detect issues in the electrical connections between the alternator B+ out and the positive battery terminal and the alternator’s case grounding to the battery – terminal

You’ll need a multimeter set to DC volts to perform the test

Step 1: Test for voltage drop on the positive side of the alternator circuit

Touch one lead to the B+ terminal on the back of the alternator and the other lead to the + battery terminal. Record the value. Then move the battery probe to the positive battery post and record the value.

The voltage drop should not exceed 0.25-volts. If the result is higher than 0.25 when you touch the probe to the battery terminal but lower when you touch the probe to the battery + post, the terminal/post have corrosion and high resistance. Clean both and repeat the test. If the result is still high, check for corrosion between the cable and the terminal.

Step 2: Test for voltage drop on the ground side of the alternator circuit

Touch one lead to the alternator case and the other lead to the – battery terminal. Record the value. Then move the battery probe to the positive battery post and record the value.

The voltage drop should not exceed 0.25-volts. If the result is higher than 0.25 when you touch the probe to the battery terminal but lower when you touch the probe to the battery – post, the terminal/post have corrosion and high resistance. Clean both and repeat the test. If the result is still high, check for corrosion between the cable and the terminal.alternator voltage drop test

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Posted on by Rick Muscoplat



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