How do you tell which wire is the positive and negative and which is for the high and low beam on a 2001 Montero Sport LS?

Testing Polarity on wires

The basic way to determine polarity on a wire is to probe it with a digital voltmeter. Modern vehicles use many solid state controls for various functions and circuits so ONLY use a digital voltmeter/multimeter.

Firstly, go purchase a cheap digital multimeter (from radio shack or harbor freight, starting at about $5 ). Put the meter in dc volt mode, 12 volts (or more) range. Hold the black meter probe on a metal body/engine ground-point and probe the wire in question with the red probe while the circuit is powered on.

If the wire has 12v + power on it you will get a volt reading. If the wire is open or has ground on it then there will be no 12volt reading.

Next, if there was no reading, repeat the test with the red probe attached onto the cars battery at the PLUS terminal (+), then probe the wire in question with the black probe (with the circuit powered up). If there is a 12volt reading then that wire has ground on it, no response means an open wire or the wire has B+ power on it.

Your electical system should be safe if you use a digital volt meter for this operation.

WARNING! Use the info following at your own risk! Improper use of a testlight can DESTROY your controller circuits!

If the circuit under test is a high power circuit (enough amperage normally goes thru the wire in question to light up a 12 volt lightbulb or headlight for instance) then a digital voltmeter CAN give a false indication of 12 volt power as that type of meter draws virtually no power to speak of. In other words, the meter can indicate power but a high resistance connection in the circuit could still prevent enough current flow to light up a bulb.

You can test this type of circuit with a load generating tester know as a 12 volt testlight. Here is where the danger lies; If in fact you are probing into a digital control circuit with a testlight (instead of the power circuit you thought you were working on) you will short it out and most likely destroy the solidstate control mechanism.

You can test for ground by clipping the testlight cord to B+ befor probing the wires or you can test for power by clipping the testlight cord to ground befor probing a powered on circuit. You can also test a bulb by grounding one contact and probing another with a testlight clipped to B+, if the testlight comes on at all no matter how dimly then the bulb filament connected to the terminal you probed is good.

To determine the highbeam wire unplug the connector from the headlight and probe the connections carefully for B+ with the circuit powered up. The one that has power only when the high beams are turned on (blue light on dashboard lit up) is for the highbeams. The one with no power ever is the ground and the other is for the lowbeams.

without being too confusing.