On some models, a drive belt is attached to a drive disc. When the drive handle is engaged, a pulley will apply tension to the drive belt which rotates the drive disc, engages the friction ring and drives the wheels. If the drive belt is worn stretched or broken, the wheels will not be able to engage until you replace the belt with a new one.
02 - Worn friction ring or wheel
The friction ring or friction wheel can also be worn out and unable to rotate the wheels. You can replace the friction ring or wheel to solve the problem.
03 - Broken traction control cable
The snowblower traction control cable applies tension to the drive plate. When the handle lever is depressed, a miss adjusted or broken control cable will prevent the wheels from engaging. Replace the cable if necessary but be sure to properly adjust the tension. So the drive plate engages when the lever is depressed.