While this question is probably more at home on EE.SE, the implications can be quite profound when troubleshooting automotive problems.

If I want to test for continuity on a 12 V circuit with next to no load, can I safely do so with the multimeter set to resistance mode?

An example use case would be when testing for fuel injector signal:

  • if resistance mode behaves more like current mode (negligible resistance), in the absence of the injector a live circuit can generate excessively high current that could burn the ECU.

  • if resistance mode behaves like voltage mode (very high resistance), there should be no problems with using it for this particular application.

  • 1
    are you asking if the current delivered by the meter could be a source of damage? Mar 9 '16 at 22:18

In "measure resistance" mode, the DMM (Digital Multi Meter) uses an op-amp (Operational Amplifier) to put a constant current across the probes. It then measures the voltage drop that results, and by Ohms law (V = IR, voltage equals current x resistance) you then know the resistance. This assumes that the voltage present is only that from the DMM. I don't think the DMM is going to handle having a the car put a voltage across the probes. You could conceivably pop the meter if there is enough voltage across the probes. I don't recommend doing this.

I don't think the load on the circuit is going to matter, if there is enough potential difference between the two DMM probes, you could put enough current through it to pop the meter. The fact that the in car circuit does not have a lot of current flowing in it is only important as the large current draw could cause a bigger voltage differential across the probes. This just seems like a really dicey thing to do. And I have done some very spectacular meter melting.

  • 2
    "spectacular meter melting" to me means, "I've failed a bunch in the process of learning and becoming successful." +1 Mar 9 '16 at 23:22
  • Exactly! I actually kept the Craftsman screwdriver I used as a probe. I melted the blade into a nice sphere.. :) Learned a lot from that one. Three words: "Shower of sparks". 'Nuf said.
    – cdunn
    Mar 10 '16 at 16:17

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