Tach showing 800 RPM's low

[scrivyscriv]

1993 2wd Explorer Sport, 159xxx miles, M5R1 transmission.

When the weather here in Tennessee started to get chilly early November, my tach started losing RPM's.

It reads 800 RPM's too low until I've driven it for about fifteen miles; then it will jump up to its correct reading - usually after I make a complete stop.
When I turn off the engine and restart it warm, the tach reads fine.
If I start the engine cold, the tach reads incorrectly.

I've seen other threads with the speedometer giving erroneous readings at the same time, but mine has been consistent and correct.

Where's the best place to start?

 

[Michiganbear]

well, I would say the needle is sticking or dragging. after a few minutes of driving, the cold, stiff, lethargic parts these old vehicles have starts to loosen, and it works ok.

 

[Willard]

The needle is sticking or, more likely, the cable gets frozen and is sticking. I'm not sure you could do much to that besides replace the cable.

Edit: Oops, I'm talking about the speedometer, not the tach. So there's no cables there. In that case, yea you could have a sticking needle and/or failing gauge. You could try and go to a junkyard and pull one out of another truck and see if it helps.

 

[scrivyscriv]

Worth a shot.

I checked my Hayne's for electrical schematics but got nothing worthwhile there...
I'll post with the results after I R&R the cluster or parts therein.

Thanks guys!

 

[Michiganbear]

I dont have my manual handy (ford), but I recall the tach signal comes directly from the eec. If we had a dizzy in our explorers, the signal would come from the ign module.