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The Top 10 Classic Electrical Snafus

The How-To Avoid Electrical Mal-

Photography by , Mark Hamilton

If your GM car is now equipped with a fusible link, you can purchase an inexpensive replacement from MAD in 12-, 14-, 16-, and 18-gauge sizes depending upon the circuit they are protecting. It's also a good idea to protect individual circuits with their own fusible links. Fusible links are sized to protect a circuit four wire sizes larger. Also remember that larger gauge numbers indicate a smaller wire. So a 14-gauge fusible link would protect a circuit using larger 10-gauge wire.

Voltage-Drop TestThis one simple test will tell you more about any specific electrical component than any other test you can perform. The idea is to measure the amount of voltage that is lost while the circuit is in operation. What we're really doing is using the voltmeter to learn the amount of resistance in the circuit without having to remove any components, but current must be flowing at the time of the test.

To begin, set the multimeter on the lowest voltage setting-most often the 20-volt scale. Let's say we're going to test the amount of resistance in the negative battery cable. A high-quality cable with a good connection should only have about a 0.1-volt drop across its length. To test it, place one probe on the battery end of the negative cable and the other probe on the end of the cable where it's bolted to the engine. Disable the ignition system so the engine doesn't start, then have a helper crank the starter while you watch the multimeter. During cranking, current will flow through the circuit, and the voltmeter will indicate a voltage. If you measure more than 0.1 volt, there is high resistance in the circuit. You can also narrow your focus as much as you wish. For example, to find the resistance in just the connection between the battery and the cable, place one probe on the battery post and the other on the cable end, then crank the starter. If you see more than 0.05 volt then the connection is poor.

A higher voltage measured with the voltage-drop test means greater resistance. Increased resistance means the electrical system is delivering less power (current) to the load, like the starter motor, headlights, or electric fan. The more power you can deliver to the load, the more efficiently it will operate. It's just that simple.

Component Acceptable Volt Drop
Battery Cable 0.1 to 0.2 volt
Switch 0.2 to 0.3 volt
Alternator output to Battery* 0.4 to 0.5 volt
Wire to Electric Fuel Pump 0.1 to 0.2 volt

The voltage-drop spec between the alternator and the battery is the textbook maximum. However, many factory wiring systems get around this spec by using a main "buss-bar" for central power distribution. This version routes the alternator output and the voltage-regulator sensing wire directly to the buss-bar. In this layout, electrical system performance relies upon voltage level at the buss-bar not at the battery or alternator. This means a voltage drop greater than allowed by the above spec may not indicate a problem. Chevy musclecar-era systems were equipped with this layout with a higher acceptable voltage-drop spec. There's more specific information on this topic at

MAD Enterprises
Dept. 5.0
P.O. Box 675
CA  93265
Painless Wiring
9505 Santa Paula Dr.
Fort Worth
TX  76116
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