The tach gets its signal from the Coil on the drivers fender well. Its only 1 harness with 4 or 5 wires in it, check and see if maybe a wires is loose???? Not a hundred percent on this but i think the green wire is the tach signal
Aftermaket tachs read the signal coming off the coil.
The stock tach reads the IDM signal on pin 4 of the computer (dark green/yellow wire). There is a 22K ohm resistor in line with the wire. If the wire or resistor breaks or goes bad you will get a code 16 and the tach will not work. Check the wiring connector at the TFI module for damaged dark green/yellow wire, loose or dirty connections.
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 Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.
Thanks for the replys... So I found the resistor and it looks good from what I can see.. Buy is it possible to just replace to, or is there away to check if its still good?!
An ohmmeter or DVM can be used to check the resistor. Be sure to read the instuction manual for the meter and understand it clearly before you check out the resistor. Check it before you replace it. If it checks good, then it is not the source of the problem
Radio Shack has the resistor - 99 cents, Model: 271-1128.
update: checked the resistor and its good. dunno what else it could be? also I had to take the bulb out of the engine oil light because it would stay on.. I have oil, so dunno why would the tach not work.. hmm do you think getting another cluster would work?