1990 F250 7.3 DIESEL
 

I have a 1990 f250 with a 7.3 diesel with a auto trans with overdrive and the other day i was driving and i push the overdrive button to off everything was working fine then i push the button to on after a bit i notice the tack was not working and the yellow light stays on the button the trans will not pull out in second unless i put the shifter in second and will not go into overdrive it wants to pull out in drive. i change tack sensor checked fuses change cluster in dash i put a new rebuilt trans in a week before this:-banghead

inspect the wires closely going into the tach sensor.
there are also relays in the power distribution box to look into.