The Short Version: One of the belts had jumped off the transaxle pulley. After putting the belt back on the pulley (easily accessed beneath the battery) it moved forward and backwards without any problems.
Last weekend I decided to work on the lawn mower so I could get some yard work done. I replaced a tire and bought a battery charger to recharge a dead battery. After getting all of this fixed I started up the mower, parked it near the back door for a moment, went into the house to get a couple of things, and then came back out and put the mower in reverse.
It didn’t work. The mower wouldn’t move forward or backward. The engine was running just fine but when I tried to change gears or move it simply didn’t do anything.
I shut down the mower, put it into “freewheel”, and pushed it back to its parking spot. A few days later I went back out to see if I could figure out what the problem is. I started by checking the freewheel control, watching the linkage but nothing seemed wrong.
While I was down on the ground I happened to look up into the area beneath the battery compartment and that’s when I discovered the cause. Here’s a photo of the same section from the top, with the battery removed:
The belt that connected to the transaxle pulley had jumped off (left). I put it back on the pulley
(right) (below, out of view), moved it until the belt tensed up, and then cranked up the mower.
It moved forward and in reverse just like it used to. I may go ahead and replace the belt soon since it’s probably lost tension (its about three years old) but at the moment it works just fine.
Updated 06/07/2012: The last time I had the battery compartment open I went ahead and cleaned out that mess of debris that is visible in the photo above.
Updated 09/09/2013: While working on fixing a couple of problems that I created I also discovered that the belt may not have been properly routed when we had to have the original seller service the mower (for this very problem, though at the time I hadn’t figured it out). Specifically, I believe the belt was not routed through an additional pulley toward the rear of the mower. Admittedly, this was not entirely the fault of the service folks. The pulley does not appear on the diagram underneath the mower for this belt. Having since put the old belt back on, and routed it through this pulley, it hasn’t jumped loose. At the moment I’m convince that my belt wasn’t as stretch as I thought it was.