Holley Instructional Video: Identifying 12-volt Power Sources

Not as trivial as you think--especially on older cars!

Transcript: One of the things your going to need to do when you're wiring fuel injection into a vehicle is find a switched ignition power source and possibly other sources for power. There are multiple ways to find this. Something many people use is this, a good old test light. Although it tells you if there's voltage somewhere, it doesn't tell you how much. It's a poor idea to use a test light to find voltage because you don't know if you have 12 volts or if you have three volts. You just see a light. What you want to do is purchase a volt meter. This is a high-quality volt meter but you can buy a volt meter that will do the job for less than ten dollars, so there is no excuse not to have one. A volt meter will tell you exactly how much voltage you have at a source. When you put fuel injection in a car that has forty year-old wiring, there's a good change that theres worn, deteriorated wiring. By probing points with a volt meter you'll find out if you have 12 volts or if you have nine volts.

What's actually the case with this car is, this particular car has the ballast resister left in it. If we raise the RPM up on this car you're going to see the voltage drop even further, causing potential problems. So, that's an example of why a test light is bad to use and why you want to use a volt meter.