Tach works, cruise works, air works, tach quite they quit!
 

What is going on? They must be tied in together! i could be driving down the road and suddenly I look down and the tach is working and the cruise works and the air works. i took the truck to a serve yourself car wash with the tach running and left the motor run as I washed the truck (94 Dodge Cummins, 5 speed.) and when i got in the tach quit while I was washing the truck. Any ideas?

it sounds like your crank sensor is starting to go bad or the wiring to the sensor is corrided