Voltage drop tests a powerful diagnostic for electrical woes

Voltage drop as a theory and voltage drop testing are often ignored because the test and its usefulness are often misunderstood. Let's clear up that misunderstanding.


First, check the insulated (positive) side of the circuit. Since the alternator is the source of voltage when the car is running, think of it that way when connecting the leads. This means the red lead goes on the alternator's output terminal and the black lead goes on the positive battery terminal. Start the engine and apply an electrical load by turning on the lights and accessories or by loading the battery with a load tester.

To check voltage drop on the uninsulated (negative) side of the charging circuit, connect the voltmeter's red lead to the battery's negative terminal and the black lead to the alternator's case and repeat the test. Remember: voltage always returns to its source, so the negative terminal of the battery has more positive potential than the alternator's case.

Starting circuit tests

To test the voltage drop in the starting circuit, first disable the engine to prevent it from starting. Connect the leads to the insulated side first. The red lead goes on the positive battery terminal and the black lead goes on the starter motor's feed terminal. Crank the engine only long enough to get a reading.

Now check the uninsulated side. Connect the red lead to the starter housing and the black lead to the negative battery terminal. Crank the engine once more to read the voltage drop on the ground side of the starting circuit.

Readings rationale

These examples only cover a test voltage drop on the starting and charging circuits, but these tests aren't limited only to those circuits. You can use voltage drop tests anywhere, but high-current circuits will have higher amounts of acceptable voltage drop than low-current circuits.

Here are some suggested maximum voltage drop values for the starting circuit:
• Connections: 0.0 volt
• Solenoids: 0.2 volt
• Starter relays (magnetic Ford type): 0.3 volt
• Cables up to 3 feet: 0.1 volt
• Cables over 3 feet: 0.2 volt

Make voltage drop testing an integral part of your diagnostic plan of attack. Partnered with a multimeter, voltage drop testing is an added powerful method for finding electrical system problems.

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