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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.
IMPORTANT!
Do not test tachometer wires using a test light or logic probe. The vehicle will be
damaged.
How to find a tachometer wire with your multimeter:
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 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.
To test and determine the polarity of this wire:
1. Set your multimeter to DCV or DC voltage (12 or 20V is fine).
2. Attach the (+) probe of the meter to (+)12V.
3. Probe the wire that you suspect leads to the bulb with the (-) probe of the meter.
4. Turn the ignition switch to the ON position.
5. If the meter indicates 12 volts until the light goes out you have isolated the correct wire and the wire's polar-
ity is negative (ground while the bulb is on).
6. If the meter reads zero volts until the light goes out and then reads 12 volts, you have isolated the correct
wire and the wire's polarity is positive.
finding the wait-to-start bulb wire for diesels
