Do the following in sequence after you have the ignition ON, engine OFF, shifter in 4th.

Push the button on the shifter. Listen for a click by the glove box and see if the green dash light comes on in the center of the instrument cluster.

Dash light on after the click was heard? Push the button once overy two seconds and listen under the left side of the auto for the clicking of the solenoid on the side of the OD. If you hear clicking under the auto with the dash light on, read the 700-900 FAQ's about changing the actuating piston seals(rebuild).

Dash light on, no cliking under the auto. Crawl under and clean and tighten the yellow wire terminals on the solenoid. Try pushing the button again. If solenoid clicks, drive auto and see if OD engages at about 55 MPH. If no OD rebuild OD.

Pushing button does not light green dash light. Pull shifter further to right and to rear anbd try again with the button. If dash light comes on and relay clicks, replace 4th gear switch.

Pulling to right and rear yields no result. Crawl under and on front right side of tranny, pull connector from 4th gear switch (red wire) and find a way to ground it to the tranny or body. Try button again. If dash light goes on with the 4th gear switch bypassed, replace 4th gear switch.

Doing all the above still yields no dash light. Replace relay.

While under the auto, check fluid level in tranny. BE carefull removeing the fill plug as it rounds off very easy if it has not been off for a while. Penetrating fluid and taps with hammer BEFORE trying to loosen helps much.

If I were you, i'd read up on the process of changing the actuating piston seals.AN 85 nwhich has had no OD for a long time will fail shortly after you fix any electrical problems simply because of age.