518 behind a 318 ....... overderive trouble

[burnt_servo]

working on my 1991 1/2 ton 4x4 , 318 with 4 speed auto ....

anyway , it shifts in and out of ovderive by it's self fine , but when i use the button on the dash to take it out of overdrive manually , nothing happens .

i'm about to tear into it this weekend , but thought i'd ask for some input as to where to start to look , hopefully to save myself a bit of time .

the computer was replaced with a factory rebuilt one last nov .

i'm thinking maybe the switch on the dash might be faulty .

it stopped working when it hit -40 for 2 weeks here , so who knows what may have stopped working .

[cummins king]

check the switch and wiring first