One of the tractors here, a Cub Cadet 129, was slowly leaking oil and I decided to investigate one day. This tractor had a peculiar case of accelarating without operator input. It would also slowly wander away. Both of these are symptoms of a worn hydrostatic trunnion. This little piece has a slot that holds a spring and is the link between the operator and the hydrostatic swashplate.

The oil seemed to be dripping from the drain plug, although that is the lowest point it could mean a bad plug, crush washer, or worse cracked oil pan. After I removed the plug and drained the oil I realized it was the oil pan. A crack had developed along the drain plug threads, the drain plug had split apart the aluminum oil pan. Luckily there were a couple of spare oil pans available so I swapped a better one in. This is something that can be repaired, but I did not feel like trying that. I did need to remove the engine in order to do this. Since it was out I did some other things, clean the spark plug, adjust starter tension, fix an exhaust leak, miscellaneous wiring, etc.

Next up are the hydrostatic issues. I had to take the seat and rear fenders off as the little access panel in front of the seat is of little use for this repair. The picture below shows the bracket that holds the spring. Over the tractor’s lifetime the spring wore away the metal holding it in, and the spring would get stuck in the grooves and misbehave.

Another smaller issue were the check valves, seen in the top left of the image above. These control the hydraulic oil flow and if these valves are stuck or sticky, the transmission won’t operate properly. I managed to free these up by wiping them off and working oil into them.

To fix the trunnion bracket I welded a little filler into the corners and followed up with some filing. Once everything was back together the transmission operated much better than before. There is always more to work on with these old tractors, but they are workhorses and a lot of fun to use.