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Let's say I cut my headlight harness off.
Inside the wire loom, there are three orange wires, like this:

enter image description here

I know one of them is for low beam, one for high beam and one for ground.
I have an analog multimeter I just bought. How can I use it to determine which wire is for low,
which is for high, and which is ground?

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Don't try measuring current and don't allow any of the three wires to short or you'll blow a fuse or melt a wire...

Don't try measuring ohms. Set the meter to read DC volts (not AC)

Connect one lead of your multimeter to the battery ground terminal (presumably the negative terminal) and turn your lights on as if to activate the normal beam (low).

One of the three wires should read +12V dc. Check that the other ones don't because if they do then you may have to try a different approach.

Hopefully one wire is registering +12V. Next put lightswitch onto full beam and check which one of the three wires registers positive 12V - it shouldn't be the same as the wire that went positive on the low beam test.

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  • \$\begingroup\$ Great answer. I was under the impression that some cars, older ones in particular, turned on high and low beams at the same time when you switch to high beams. If this is true, the full beam setting may liven up two of the three wires. \$\endgroup\$ – mkeith Mar 16 '15 at 21:08

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