A simple voltmeter and some basic electrical circuit knowledge can go a long way toward eliminating those nagging electrical problems.
Electrical snafus are among the most annoying problems with older cars. Unlike tires or bodywork, there's something about electrical systems that seems to baffle many car crafters. We could blame it on 12-volt voodoo or the curse of mummy wiring, but the best way to get past all those superstitions is with some good old-fashioned electrical knowledge. Most direct-current conundrums can be solved with basic diagnostic work that will point to a simple repair. Dig out that test light and multimeter because we'll use them as the tip of our wiring attack spear to root out all those nasty electrical gremlins.
Voltage Drop Testing
Many common electrical difficulties can be diagnosed by using a simple voltage drop test to discover the root of the problem. The voltage drop test is a dynamic test intended for use mainly with high-current draw systems, such as the starting system, charging system, or items such as a heater fan, electric fuel pump, or electric cooling fans. Performing this test will require a digital multimeter. The number displayed on the multimeter during the test will indicate the amount of voltage lost to resistance through a connection or a cable in the circuit. The beauty of this test is that we can break down a complex circuit into individual components to home in on the real problem.
The one electrical diagnostic tool that any good car crafter should have is a handheld multimeter from Actron or this Equus meter. They can measure voltage, ohms resistance, DC current, as well as rpm, dwell, and other goodies. This model 3340 Equus retails for $96.99, but basic units go for $25. Other essential tools include wire strippers, crimp pliers, and a quality test light.
A voltage drop test uses small voltage readings measured within a circuit as an indication of resistance. Most high-current flow, low-resistance circuits-like a simple length of battery cable-should generate a 0.10- to 0.20-volt drop during a dynamic test. Dynamic testing means this voltage drop can only be measured when a load is applied to the circuit, as when the engine is cranking.
If the cable measures 0.75 volt, for example, this would represent excessive circuit resistance. Making the connections for the test is easy. You set the meter on volts and place the negative lead to one end of the cable and the positive lead to the other end of the battery cable.
Not too long ago, we ran into a hard-start issue with a '68 small-block Camaro. The car had a new battery, cables, and starter but still suffered slow-cranking woes. The new cables were suspicious, so we checked the voltage drop for the battery cables by setting our multi-meter on the lowest scale (some models, like our Equus, do this automatically). Next, we pulled the coil wire out of the distributor cap and grounded it so the engine would not start. We placed the multimeter test leads on the positive battery post end of the positive cable and the other lead on the positive cable connection at the starter motor. Then we had a friend crank the engine while we read the meter. We discovered that brand-new discount-store battery cable measured a 0.91-volt drop (roughly four times the normal voltage drop), which means it had a tremendous amount of resistance. On the ground side, the voltage drop (resistance) was even greater at a 1.1-volt drop, adding even more resistance to the circuit. We swapped both cables for high-quality, 1/0 multistrand welding cable, available through either MAD Enterprises or Painless. Retesting resulted in only a 0.20-volt drop in each cable. The car cranked over like it had 24 volts because the battery could now deliver all its cranking amps to the starter instead of losing it to resistance.
This photo simulates a voltage-drop test on the positive battery cable. Place the positive voltmeter test lead on the battery side of the cable and the negative lead on the starter motor cable connection. If the voltmeter reads more than 0.4 volt, there is excessive resistance in the cable. Remember, voltage drop will only work with the engine cranking. Do not crank the starter for more than 10 seconds to avoid overheating and damaging the starter.>>>>>>