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© 2004 Directed Electronics, Inc. Vista, CA
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 brake pedal and make sure it goes back to zero.
5. If it does return to zero, this is the correct brake 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.
To test for a tachometer wire, a multimeter capable of testing AC voltage must be used. The tachometer wire will
show between 1V and 6V AC. In multi-coil ignition systems, the system can learn individual coil wires. Individual
coil wires in a multi-coil ignition system will register lower amounts of AC voltage. Also, if necessary, the system
can use a fuel injector control wire for engine speed sensing. Common locations for a tachometer wire are the
ignition coil itself, the back of the gauges, engine computers, and automatic transmission computers.
IIM
MP
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OR
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AN
NTT!! Do not test tachometer wires with a test light or logic probe. The vehicle will be
damaged.
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1. Set to ACV or AC voltage (12V or 20V is fine).
2. Attach the (-) probe of the meter to chassis ground.
3. Start and run the vehicle.
4. Probe the wire you suspect of being the tachometer wire with the red probe of the meter.
5. If this is the correct wire the meter will fluctuate with the rpm of the motor and read between 1V and 6V.
In diesel vehicles it is necessary to interface with the wire that turns on the WAIT TO START light in the dash-
board. This wire illuminates the bulb until the vehicle’s glow plugs are properly heated. When the light goes out
the vehicle can be started. This wire is always available at the connector leading to the bulb in the dashboard.
It can also be found at the Engine Control Module (ECM) in many vehicles.
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