Ford Repair: Explorer -96 XLT 4WD - Tachometer, lh side, ford dealers

Question
This always happens - when I first start engine, tach gives correct reading for a moment but then drops to zero for 30 - 90 seconds, then jumps to normal for 5 - 30 seconds, then back to zero for a short time. Then the needle does a few more "stutters" and then back to normal and operates fine for remainder of trip.  This occurs always during the first 2 - 3 minutes after starting the engine.  After shutting off vehicle and restarting (no matter how long it has been running or has been off, no matter the temperature or humidity) tach goes through the same routine. Is there a sending unit that needs to be replaced? If so, where is it located? Could it be a bad ground that improves with heat from engine? If so, where is ground for tach located?

Answer
Testing this component involves removing the instrument cluster and checking connectors and resistance of the wires, and for shorts. The ground for the tach is on the LH side under the hood, near or on the rear of the fender apron. (G100). Probably more than one wire, black with a tracer line of yellow or white.

In 20 years working at Ford dealers, I have to say we never had this particular problem.

The tach signal comes from the PCM. You might want to check for codes to see if that gives any clues. Chances are it could be a bad ground or poor connection.