there are 2 main wires on the coil...1 is the power...the other is the ignition sense wire that also is the tach wire....the tach works off the pulses of the ignition

if the inside of the truck has been real dusty then it is possible that the instruments have also gotten dusty if that is the case the tach needle is very sensitive to dirt...try and clean it...

 

it only is faulty when it is cold

 

Cleaning the dust is a good idea. Maybe it has to warm up before the contacts will pass the current because of it.

 

My tach will stop sometimes when I am driving I can stop and cut ignition off and restart engine and tach starts working.I believe problem is in cluster panel because it does not effect the auto transmisson on changing gears when it is supposed to.