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

i have a 94 Dodge Cummins, with a 5 speed. If the tach works so does the cruise and the air conditioning compressor but if the tach quits so does the cruise and air. I could be driving down the road and the tach could come on with out warning or not come on. The other day it worked and I took the truck to a serve yourself car wash and left the truck running while i washed it. The tach was working when i got out, but had quit while i was washing the truck. Somehow they are tied in together. Any ideas?

I should add that this problem is ongoing.