Electrical-testing equipment can be intimidating, but it's really pretty easy to use. In this story, we describe a bucks-down way of testing gauges with three batteries and a length of wire. In this series, we have also made several references to a fused jumper wire.
A jumper wire is used to complete the circuit during testing. Alligator-clipped wires are available at Radio Shack, as are inline fuses. We bought a package of four jumpers, 20-inches long, and spliced a 20-amp inline fuse into the red wire. The cost was under seven bucks.
The best piece of electrical testing equipment is a multi-meter. Lighted-handle-type circuit testers and less expensive testers were common because, until recently, a dedicated multi-meter was pricey and not easily justified for a weekend enthusiast. However, multi-meters have become so affordable there's no reason not to have one. The Craftsman unit pictured here is a capable unit and costs $25-$35. There are more expensive models on the market, but this one suits our needs. We've also seen multi-meters under $15 that are also capable units--but the hard case on this Craftsman unit won us over. If you don't know how to use a multi-meter, Craftsman sells a good book for a few dollars that teaches you how to use one, and Radio Shack has a basic electrical testing book.
When It Doesnt It Work
When something doesn't work, the cause is usually one of six things; no power, an inoperable accessory (or bulb), no ground, a bad switch, a bad relay, or a wiring problem.
The following seven steps, when followed in order, should help you solve just about any electric problem in your car:
1. Check the power source. Your problem could be a bad battery or a blown fuse. Never overlook the obvious. Check both ends of the fuse with a circuit tester. We've seen fuses that looked OK, but had an internal break in continuity.
2. Check the accessory (or bulb) itself.
3. Test for power at the accessory.
4. Test the ground.
5. Test the switch(es).
6. Test the relay (if applicable).
7. If 1-6 are OK, its time to drag out the wiring diagram and start tracing wires.