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finding a (+) brake light wire
Most vehicles use a (+) brake light circuit. The (+) brake light wire is often found near
the brake pedal.
How to find a (+) brake light flash wire with your multimeter:
1.
Set to DCV or DC voltage (12V or 20V is fine).
2.
Attach the (-) probe of the meter to chassis ground.
3.
Probe the wire you suspect of being the brake light wire.
4.
Press the brake pedal. If your meter shows (+)12V, release the pedal and make
sure it goes back to zero.
5.
If it does return to zero, this is the correct brake wire.
finding the accessory/heater wire
An accessory/heater wire will show +12V when the key is in the accessory and run
positions. It will not show +12V during the cranking cycle. There will often be more
than one accessory wire in the ignition harness. The correct accessory wire will power
the vehicle's climate control system. Some vehicles may have separate wires for the
blower motor and the air conditioning compressor. In such cases, it will be necessary
to add a relay to power the second accessory wire.
© 2003 directed electronics, inc.
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