Grasshopper 721D Mower Transmission Troubleshooting and Drive Belt Replacement

Step 1: Troubleshoot the Transmission and Drive Belts

Check hydraulic fluid, plugs, and left belt are sound; prove the right belt slips under load.

Watch this step (3:28)

Parts in this step: (A) G2 hydrostatic transmission, (C) Hydraulic pump drive belt

1. 1.1 Check the hydraulic fluid level
   • Lift the mower seat to reach the G2 hydrostatic transmission (A) rectangular hydraulic reservoir.
   • Unscrew and pull the dipstick cap from the reservoir top.
   • Check hydraulic fluid reaches the dipstick full mark before suspecting pump failure.
   • Thread the dipstick cap snug into the reservoir.
2. 1.2 Inspect the transmission valves
   • Find the recessed Allen-head plugs atop the left and right hydraulic pumps.
   • Check both plugs and tow bypass valves are closed and not vibrated loose.
   • Top Allen plug: pressure diagnostic port, not tow bypass valve.
   • Allen wrench size is unstated; use a wrench that fits without play.
3. 1.3 Compare left and right pump belt tension by hand
   • Shut off the engine; reach between radiator frame and transmission.
   • Pull left pump belt up and down: a healthy belt stays taut without sliding.
   • Pull the right hydraulic pump drive belt (C) by hand.
   • Hand-rotate the right pump cooling fan to check the belt slides loosely in the pulley.
4. 1.4 Run the engine and engage the drive lever to confirm belt slip
   • Tilt the seat forward (seat safety switch bypassed) and stand clear of both rear tires.
   • Start engine; check center pulley and both pump cooling fans spin at idle.
   • Feather the right steering lever slightly forward to load the right hydraulic pump.
   • Watch right pump pulley and cooling fan slow and slip in the moving belt.

Key measurements

• Hydraulic fluid level: at reservoir dipstick full line

• Tip: Check hydraulic fluid, bypass valves, and belt tension before condemning an $800 to $900 pump motor.
• Tip: Compare the suspect right belt to the taut left belt by hand-pulling each.
• Heads up: Keep hands and fingers clear of the pulley compartment while the engine runs.
• Heads up: Stand clear of tires; lightly feather the steering lever with the seat switch bypassed.

Before you move on: Reservoir full, top plugs tight, and right belt visibly slipping under load.

Step 2: Unbolt the Drive Coupling and Remove the Old Belt

Check steering linkages are tight, open the drive shaft coupling gap, and remove the stretched belt.

Watch this step (5:50)

Parts in this step: (A) G2 hydrostatic transmission, (B) Drive shaft coupling, (C) Hydraulic pump drive belt

You'll need: Socket wrench, Combination wrench, Drive shaft coupling bolt

1. 2.1 Locate the drive shaft coupling bolts and check drive linkages
   • Shut off the engine; park in a shaded, level workspace.
   • Wiggle the steering control linkage rods atop G2 hydrostatic transmission (A) to check for play.
   • Verify no linkage bolts are loose or disconnected where the rod meets the right pump arm.
   • Find the two drive shaft coupling (B) bolts directly behind the radiator.
2. 2.2 Remove the two drive shaft coupling bolts
   • Fit a socket wrench and combination wrench onto both coupling bolts and nuts.
   • Bolt head size is not stated: pick the socket and wrench that fit snugly.
   • Remove both drive shaft coupling bolts.
   • Separate the coupling faces slightly to open a center drive shaft gap.
3. 2.3 Walk the worn belt off the pulley and out through the coupling gap
   • Grab the loose right hydraulic pump drive belt (C) one-handed without releasing the belt tensioner.
   • Pull the belt toward the outer pulley rim while hand-turning the pulley.
   • Walk the stretched belt off the pump and center pulleys.
   • Slide the old belt out through the unbolted drive shaft coupling gap.

Key measurements

• Drive shaft coupling fasteners: 2 bolts behind radiator

• Tip: A stretched belt walks off the pulley rim by hand without loosening the tensioner.
• Tip: Check the old belt back for stamped part number 381540.

Before you move on: Both coupling bolts out, steering linkage tight, and old belt clear of the shaft.

Step 3: Install the New Belt and Reconnect the Coupling

Seat the new OEM belt on both pulleys, re-bolt the drive shaft coupling, and check engagement.

Watch this step (7:26)

Parts in this step: (A) G2 hydrostatic transmission, (B) Drive shaft coupling, (C) Hydraulic pump drive belt

You'll need: Socket wrench, Combination wrench, Hydraulic pump drive belt, Drive shaft coupling bolt

1. 3.1 Route and seat the new belt onto the pulleys
   • Compare old belt and new hydraulic pump drive belt (C), OEM part 381540, for length and thickness.
   • Pass the new belt through the open drive shaft coupling (B) gap.
   • Seat the belt V-profile fully in the right pump pulley groove first.
   • Stretch and roll the belt onto the center pulley, keeping fingers clear of the rim.
2. 3.2 Reinstall and tighten the two drive coupling bolts
   • Align the two drive shaft coupling flange bolt holes.
   • Insert both coupling bolts through the flange holes and hand-thread the nuts.
   • Hold each nut with the combination wrench; firmly tighten both bolts with the socket wrench.
   • Torque is not stated: tighten until the coupling faces cannot shift.
3. 3.3 Start the engine, verify belt engagement, and lower the seat
   • With the seat tilted up, start the engine and watch the new belt run on G2 hydrostatic transmission (A).
   • Push the right steering lever slightly forward and back to check immediate wheel engagement without belt slip.
   • Fold the mower seat down over the transmission compartment.

Key measurements

• Replacement OEM belt: Grasshopper part 381540
• Drive shaft coupling fasteners: 2 bolts tightened firmly

• Tip: Pass the belt through the coupling gap before bolting the shaft, or it cannot reach the center pulley.
• Heads up: Keep fingers out of the pinch point between the tight new belt and pulley groove.
• Heads up: Brace your hand when pulling wrenches near radiator fins so a slipping tool does not cut your knuckles.

Before you move on: New belt sits squarely in both pulley grooves; both drive shaft coupling bolts are tight.

Step 4: Test Drive and Hill Climb Verification

Check the new belt restores balanced tracking and full forward and reverse climbing power.

Watch this step (8:09)

Parts in this step: (A) G2 hydrostatic transmission, (C) Hydraulic pump drive belt

1. 4.1 Test zero-turn spins and straight tracking on gravel
   • From the operator seat, drive onto level gravel.
   • Push both steering levers evenly forward and reverse; check G2 hydrostatic transmission (A) tracks straight.
   • Do tight zero-turn spins both ways to check right wheel power matches left.
2. 4.2 Test forward and reverse hill climbing on a steep incline
   • Drive up a steep dirt incline and stop midway.
   • Engage the right drive motor from the incline standstill to check hydraulic pump drive belt (C) holds without slipping.
   • Reverse all the way up the steep incline to check full hydraulic load capacity.

• Tip: Stopping mid-hill and backing up a steep incline maximizes right pump belt load.

Before you move on: Mower climbs a steep incline forward and reverse with steering levers even.