Chasing down the cause of an electrical problem stress you out? Keep you awake at night? Let’s see if we can make things a little easier for you with a short selection of tips and techniques you can take back to the bay immediately!
Applying voltage drop
This is a topic we’ve tackled a number of times, and I invite you to search the term “voltage drop” on the Motor Age website and YouTube page for a ton of information that can help you fully understand this valuable testing method. The idea behind the test is simple enough – applied voltage will “drop” across every resistance present in a circuit. The only real source of resistance in a circuit should be the load, or the component that is performing the “work” the circuit was designed to do. Therefore, system voltage should be almost entirely present going in and should be almost entirely gone coming out.
Now understand that every part of the circuit has some resistance. Wire has resistance, switch contacts have resistance, and even the fuse protecting the circuit has some resistance. That explains why you won’t see all the applied voltage on the positive side of the load and why you should never see a perfect 0.0 volts on the ground of the same load.
All of this is based on the premise that you’re taking all your measurements with your positive meter lead and leaving your negative meter lead on the negative battery post. Maintaining your reference at the battery is the only way to insure that you are testing the entire circuit path. Move that ground meter lead to a spot in the trunk, for example, to test a component located there is going to leave a large part of the path back to the battery unchecked – and that may be exactly where it’s hiding!
Additionally, the circuit you’re testing must be “on” when tested. If no current is flowing, voltage won’t drop across anything. That means someone has to hold down the horn button if you’re testing the horn, or keep their foot on the brake pedal when testing the brake lights. You get the idea.