Testing for voltage drop is probably the third most useful electrical test that you can perform to diagnose a vehicle's electrical issues. This is extreamly beneficial as you can determine if a load is taking too much or if there are unintentional resistance elsewhere in the circuit.
Unintentional excessive resistance can be caused by several things, such as corrossion in wiring or connectors and bad connections. The theory behind voltage drop testing follows the same rules as the other two major electrical tests. Actually, all the tests kind of go together to diagnose the basic functions of the circuit. Voltage drop is testing how much voltage is lost across a load in a circuit.
The first thing you will need is the proper tools for the job. There are several specialty tools out there for you to pick from. Each tool type has its perks and each brand offers other distinct strategic advantages. I have provided a compiled list of tools that you could use for this test, a short description of each one, and a link to buying one on Amazon. I have found that Amazon offers the best prices on about anything you may need for any project. They are listed from most useful in this situation to least useful.
- The Power Probe 3 - This is basically a logic probe, a voltmeter, and a jumper built into one. It will allow you to read voltage, find grounds, find power sources, and even apply power/ground to the circuit with the flick of a switch. Click on the name to be directed to an article I have written about the Power Probe 3.
- Voltmeter - A meter that allows you to read voltage. They come in different designs such as a analog voltmeter, digital voltmeter, and DVOM (digital volt and ohm meter). This test requires only the voltmeter function, but a DVOM is the best. Click the link above for a cheaper model, but if you like the better brand check out the Fluke 115.