My Smart Drive started slipping and then wouldn't pull at all - over the course of one day.
Simply fixed as follows:
1 - Diagnostic - With the controllever adjusted to the lowest thumb position, pull the mower backward and engage the smart drive lever slowly. If the smart drive doesn't stop (brake) the mower before the lver us straight down, the cable is too loose.
2 - Solution for loose cable - loosen lock nut (against adjustment sleeve (near the hinge on the mower handle)), and loosen (lengthen) the adjuster until the diagnostic above shows the cable is not too loose. Tighten the lock nut against the adjustment sleeve.
Celebrate! unless this loose cable thing was not your problem
sns
"I lost track in Dynamics when we got to planetary gears and instantaneous center of zero acceleration - so I became a civil engineer - our stuff doesn't really even move - let alone spin!"