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Honestly, I'm not a fan of checking a circuit’s resistance over a voltage drop test. The main reason is because when you’re doing a voltage drop test, you get to see what's happening on a live circuit. Resistance checks are done on open circuits that are isolated from the rest of the circuit.

When doing resistance checks, you must remove what you’re checking from the circuit first in order to do your resistance check; otherwise, you might damage the circuit or your meter. It's for that reason I find resistance tests to be inconclusive sometimes. Sure, there are times when testing resistance is the only way to test a circuit, but when diagnosing a problem in a circuit, I prefer a voltage drop test.

Case in point: ignition coil testing. Manufacturers list the resistance you should see at the primary and secondary windings of an ignition coil. I've never found a bad ignition coil this way. I spoke to an electrical engineer about this once. He told me that a coil can have good resistance readings and still be bad. The reason for this is that the windings inside the coil are made of very thin wire. You can have a small break in this wire and the resistance will still indicate everything is fine, but the minute the coil is under load, it fails because of the open or short in the windings. On the other hand, if you're checking a coolant temp sensor, resistance testing is the way to go. My point: Be sure to use resistance checks wisely. But, if you can do a voltage drop test over a resistance check, go for it.

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