Preventive maintenance on the D type OD is whatever you cannot get to easily with the OD in the auto. This is based upon the OD has no noise as it is rotated. You will not know if the planetary gears are OK with the unit out of the auto unless you open the unit up, or have had the OD in overdrive and functioning before the unit was removed.

Make sure the 4th gear switch on the transmission works...In 4th gear it should stay closed even if the shift lever is wiggled all over the place. Or replace just to be sure. A common failure item.

Bare minimum, remove overdrive from the transmisson adapter and replace the actuating piston O-rings. This about $12.00, 1/2 hour max effort now will save you from having to remove the OD later to replace them. And

Carefully remove, clean, and reinstall the operating valve.

Solenoid adjustment requires an amp meter. Draw to initial pull can be as high as 13 amps, holding draw should be about 2 amps. If holding draw does not drop, solenoiod will burn out.

E-mail me for some instructions on how to for any of the above and feel free to ask any other questions.

Duane