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Why is 1994 GMC truck not going into overdrive?

Possible causes:

* Low transmission fluid. Check the transmission fluid level and add more if necessary.

* Dirty transmission fluid. Change the transmission fluid and filter.

* Faulty shift solenoid. Replace the shift solenoid.

* Faulty transmission control module (TCM). Replace the TCM.

* Wiring issues. Check the wiring to the transmission and TCM.