TACH needle jumps around while cranking

I believe most vehicles with a tach do the same thing. When you turn the engine the tach sensor wire is hot in start position.

On stock EFI Mustangs, the tach signal is picked up from the TFI module and is called the IDM Sense signal. It uses computer pin # 4, a Dark green/Yellow wire: the wire has a 22K Ohm resistor inside the wiring harness between the TFI module and the computer.

If you have an aftermarket ignition, your setup may be different.

Diagram courtesy of Tmoss & Stang&2birds


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

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http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
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FoMoCo, I know this is an old thread, but mine does the same thing. Did you every figure out the issue with the tach bouncing on crank?