In the off chance it might help somebody in the future, I'm going to briefly document my transmission repair.

Background: 10 year old HRB216TDA. Most everything still original parts (cables, drive belt, transmission, wheels)

Symptoms: Wheels lacked drive. They would turn when the mower

was lifted off the ground but would not pull the mower under load.

Results: Tried the easy stuff first: Adjusted cables. Then replaced drive belt and cables. Symptoms continued. Purchased transmission online relatively cheaply ($125 shipped to my door and spent a better part of a morning removing the old one and replacing it with the new one. Got it all back together and it works like a champ.

Recommendations:

1. Find the parts list online. It helps to reference that to figure out the order to remove and replace parts.

2. Use a good pair of snap ring pliers. There are two between each wheel and the deck that you'll need to remove to be able to get the transmission (with axles attached) out. There's also a 3rd snap ring inside the deck that holds the height adjustment lever and the metal sleeve that helps to hold the axle. You'll need to remove that one from the side that has the longer part of the axle to be able to angle the transmission out of the mower. This one is a little larger than the others, so you may need a different set of snap ring pliers for it.

3. For getting the spring reattached to the transmission, I wore heavy leather gloves and eye protection and used a couple of heavy duty zip ties to lever the spring back on.

4. Make sure you don't lose the small springs or the slot keys that make sure the pinion gear works correctly.

5. The slot keys go in such that the flat side is up. Also, the pinion gears go in such that the key acts to stop it from rotating toward the front of the mower.

Post Mortem: I took apart the old transmission to see if I could figure out what the problem was. It appears to me that there are two plastic discs that are part of the clutch assemble that wear over time (they appear melted) that causes this problem. For the most part, the gears themselves showed very little wear. The first ring gear (attached to the drive worm gear) showed a bit of wear.

I've posted some pictures. I hope this proves useful to somebody.

Here is a link that might be useful: Some pictures.