Overdrive Faults
Fully 90% of overdrive problems that are not electrical faults are traceable to either the gear oil level being tool low, or an improper adjustment of the solenoid, so we'll skip these and concentrates on the others.
An overdrive that fails to engage can sometime be traced to dirt between the check ball and seat in the pressure regulator valve, or sometimes to a badly worn pump. Both are quite rare, though possible, so don't overlook them. Worn or broken rings on the operating pistons or the accumulator piston can make for a lazy-shifting or slipping unit.
However, the real winner is a partially or completely blocked bypass port in the operating valve. The valve, which is activated by the solenoid through the operating shaft and lever, has a small hole bored in it (about .018") which is very easily blocked by dirt, etc. The results of blockage can be really fun to find. FIrst, partial or intermittent blocking results in slow engagement, but most noticeable is that there is no compression-assisted slowdown. The unit appears to hang between overdrive and direct drive and feels like it is not in gear. Basically, what is happening is that the partially blocked valve will not allow the oil to return to the sump fast enough, and the pressure build-up above the valve tries to engage the overdrive while the unit's springs try to engage directly, and there we hang beween the two. The fun begins when the bypass port becomes completely blocked. Above a certain speed, the pressure above the valve becomes high enough to engage overdrive, no matter what you do or where the switch is.
Fortunately, the valve blockage is easy to repair. With the transmission tunnel removed, the access to the valve is on the right side of the top of the overdrive unit and is under a 7/16" plug. Under the plug is a spring and check ball assembly which can be removed with a small magnet. After it is cleaned, it can simply be set back in place, hole-in end facing up, followed by the ball and spring assembly. It is very straightforward and easy to remedy once you find it.
Ken Gillanders
Temple City, CA
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