TACH needle jumps around while cranking

[FoMoCo17]

1990 Mustang - Supercharged

When I start my car during "Cranking", the Tach needle bounces all around ... once running, it doesn't do it but am wondering if I have some sort of ignition related problem? I don't recall the needle bouncing all the way up to 3,000 rpm and back down repeatedly a while back ... any ideas? Only acts erratic while cranking the engine.

 

[2000xp8]

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.

 

[jrichker]

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.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

 

[FoMoCo17]

Yes, I know all of this ... the 22K resistor is there and is measured. I have a MSD Billet distributor w/stock internals and MSD coil. Just was wondering why the tach was jumping around while cranking ... anyone know?

 

[FoMoCo17]

Should it bounce up to 3,000 rpm and back down when starting and to lower levels like 2,000 rpm then back down to 0 continually while starting. It looks very erratic, and I do not recall if it always did this?

 

[PoweredByAFR185]

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?