BCB gave you the way to test this. There are two issues, volts and amps. Volts is driven by the battery. Amps are limited by the resistance in the wiring between the battery and the demand. If you see 12 volts, that means you have continuity between the battery and the measuring point. If the bulb won’t light, that means you have excessive resistance in the circuit somewhere. This can happen in several ways:
(1) poor connection at the battery or some intermediate terminal strip. Clean the crud off and even add a little electrical contact grease to prevent it from re-forming.
(2) bad wire. This can be in the form of a wire that has flexed so long that metal fatigue has caused most of the strands of wire to break internally, leaving just one or two strands intact and they are high resistance since they are almost fatigued apart themselves. This can also happen when battery acid wicks up into the wire, under the insulation. This will turn bright copper wire into a green mess that won’t conduct very well.
(3) bad switch. Contacts can corrode when exposed to water, and corrosion leads to high resistance through the switch.
Ohm’s law will kill you for any of the above. Just adding 100 ohms of resistance limits the max current to 12 / 100 amps, which is about 120 milliamps. Not going to make that bulb glow very brightly. You can, if you want, use an ohmmeter to measure the resistance thru each wire. It ought to be in the .00x ohm range.
Simple way to isolate is to hook a new wire to the + battery post. See if the bulb will light using the new + wire and the original – wire. If not, the – wire is bad and needs to be repaired or replaced. Repeat for the – battery post. Note that it is rare but possible that both wires are bad.
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2008 Pantera Classic
2014 Mercury Pro XS 200
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