Ok been battling all week an put almost 600 bucks into it. Tach will not work until i drive for about 15 min. I have replaced the following parts. alternator, eec relay, trailer tow relay, selinoid, neutral safety switch , throttle positon sensor, tach sensor, cluster for all guages. batteries . Trans is in safe mode until tach starts working please help

 

I'd check all the wiring between the tach sensor and the pcm. (I've had a connector that was sensitive to engine compartment temperature before.)

 

what year is your truck too shane?

 

Its a 1990.

 

Still nothing is helping have spent 900 bucks on all kind of relays an sensors. Have looked all over for broken wires an found nothing wrong with them. Still in limp mode

 

Stop throwing money/parts at it and TEST. See post #2; have you checked continuity of the wires from the tach sensor connector to the TCM, under both conditions (tach working / not working)? Have you measure the resistance of the tach sensor when cold, and again after warming up? Something is happening in that 15-minute period, as suggested above, probably temperature-related. It's most likely something you can measure, but swapping parts won't provide that information.