OK, I still have no manual for that mower, but there are other mowers with "auto drive" and they may use the same system. The auto drive in those seems to just be a single pedal control that operates forward and reverse friction clutches, driven by a belt, like the Greenfield system. The mechanism seems to be fairly simple - if it doesn't work it can only be clutch wear, linkage wear or damage, or a need for adjustment. However I haven't found an auto drive that has a "lever under the seat" to engage and disengage the drive, though they do sometimes have a parking brake lever low on the left side. To get further with this we'd need to have some pictures of the mechanism and work it out that way. Alternatively there may be an Outdoorking member who is familiar with your mower and already knows the answer.
