Like most things in life, it's the little things that can trip you up and make you miserable. Eliminating those pesky electrical gremlins is about knowing how to do the job right the first time. Apply a little bit of electrical savvy to your next wiring job, and you won't have to pay the consequences of being stranded alongside the road with a melted harness.
Drop ItOne of the most valuable and easy tests you can run on any electrical circuit is a voltage-drop test. All you need is a digital voltmeter and a little bit of time. The voltage-drop test measures the amount of voltage that is lost in the circuit due to high-resistance connections, bad cables, weak switches, or a poorly functioning component.
The simplest test is to measure the amount of voltage drop that occurs between the alternator and the battery. Start the engine, and using your digital voltmeter, measure the voltage at the back of the alternator by placing the red lead on the alternator output terminal and the black lead on the alternator case for a ground. Then measure the voltage at the battery with the red lead on the positive and the black lead on the negative battery terminal. On our test Camaro, the alternator cranked out 14.4 volts, but the reading at the battery was only 13.5. This meant there was a 0.9-volt drop between the alternator and the battery. An acceptable voltage drop is more like 0.4 to 0.5 volt. We either have a wire with high resistance or a bad connection. We've seen as much as a 1.5-volt drop on an early Chevy musclecar traced to an undersized fusible link.
Remember that voltage-drop tests must be performed while the system is operating. Another popular voltage-drop test is with the starter circuit. GM cars are famous for "heat-soak" problems that occur because resistance increases with temperature. Again, the test can only be performed with the starter in operation. This produces a current flow and therefore a voltage in the circuit. To perform our test, we disabled the ignition, set the digital voltmeter on the millivolt (0.001 volt) scale, and tested each series of connections. For example, we tested the voltage drop for the positive battery cable between the solenoid and the positive battery terminal on a '68 Camaro. While the positive battery cable looked fine, we discovered a massive 1-volt drop in this cable alone! In order to record these high readings, we had to switch to the 2-volt scale on our multimeter. Combined with the smallish negative cable, the two cables alone produced a massive 1.6-volt drop in the starting circuit. The owner noted that the car had always had difficulty starting when it was hot. An acceptable level might have been around 0.20 to 0.40 volt for each cable.