Grasshopper 721D Mower Transmission Troubleshooting and Drive Belt Replacement
Step-by-step guide made from Indy Farm Life's YouTube video.
Diagnose a loss of drive power on a Grasshopper 721D zero-turn mower equipped with a G2 hydrostatic transmission by inspecting the hydraulic fluid level, relief valves, drive linkages, and pump belts. Replace a stretched, slipping right-side hydraulic pump drive belt to restore full forward, reverse, and hill-climbing power.
The full step-by-step Spetz Specs for this project (every part, size, diagram and step, free): Grasshopper 721D Mower Transmission Troubleshooting and Drive Belt Replacement Spetz Specs.
- Difficulty: beginner
Safety
- Keep hands and fingers clear of spinning fans, belts, and pulleys when running the engine with the seat lifted to observe belt slippage.
- Bypassing the seat safety switch allows the mower to run and engage drive motors while standing beside it. Use extreme caution so the machine does not move unexpectedly and run you over.
- Watch your fingers for pinch points between the belt and pulley rim when rolling the new belt onto the pulley.
Must know before you build
- Rule out a $20 belt (part #381540), linkage, or fluid before replacing an $800+ pump motor.
- Test belt tension by pulling the belt up and down by hand in front of the radiator; if the belt slides easily and turns the pump cooling fan, the belt has lost tension.
- Do not mistake the G2 transmission top Allen diagnostic plug for the tow/bypass relief valve.
- Unbolt the drive shaft coupling using a ratchet and wrench to route the new belt into place.
- Verify your Grasshopper has a G2 transmission (G2 front decal, single hydraulic filter, and separate pump and drive motor) rather than an older Gemini unit (two white oil filters on top).
Tools
- Socket wrench (hand ratchet with socket)
- Combination wrench (open-end / box-end hand wrench)
Materials
- Hydraulic pump drive belt (1, OEM V-belt, Grasshopper / Mitsuboshi, Part # 381540)
Step by step
Step 1: Troubleshoot the Transmission and Drive Belts
Check the hydraulic fluid dipstick level, verify the transmission valves are closed, compare left and right pump belt tension by hand, and run the engine under load to confirm the right belt is slipping.
- Check the hydraulic fluid level. Unscrew and pull the dipstick from the rectangular hydraulic fluid reservoir under the seat to verify the fluid reaches the full line.
- Tip: Always check the hydraulic fluid reservoir level first before assuming a transmission component is broken.
- Inspect the transmission valves. Locate the Allen-head plugs on top of the left and right G2 transmission pumps and verify they are tight and not backed out.
- Tip: Ensure the tow/transport bypass valves are fully closed so the transmission builds full hydraulic pressure.
- Common mistake: Viewer note: The Allen-head plug pointed out on top of the pump is actually a diagnostic plug rather than the bypass/relief valve.
- Compare left and right pump belt tension by hand. Reach between the radiator frame and the transmission to pull on the left hydraulic pump belt, then grab the right side belt and test if the belt and cooling fan slip by hand.
- Tip: Compare the suspect belt against the known-good side; a properly tensioned belt won't slide around the pulleys when pulled by hand.
- Run the engine and engage the drive lever to confirm belt slip. Start the engine with the seat tilted forward, push the right steering lever slightly forward to put a load on the right pump, and watch the right pulley and cooling fan stall and slip.
- Tip: Briefly engaging the drive lever while watching the pump pulley confirms whether the belt slips under load.
- Watch out: Stand clear of the tires and only feather the drive lever slightly if testing with a bypassed seat switch so the mower does not run you over.
Step 2: Unbolt the Drive Coupling and Remove the Old Belt
Inspect the drive linkages for play, remove the two bolts on the drive shaft coupling to create a gap, and walk the loose belt off the pulley.
- Locate the drive shaft coupling bolts and check drive linkages. Locate the two bolts on the drive shaft coupling directly behind the radiator, and wiggle the steering control linkage rods on top of the pump to verify nothing is loose or disconnected.
- Tip: If a drive motor has sloppy or no movement, wiggle and inspect the steering drive linkages to make sure a loose bolt isn't the culprit.
- Remove the two drive shaft coupling bolts. Use a socket wrench and combination wrench to remove the two bolts securing the drive shaft coupling so there is a clean gap in the shaft.
- Walk the worn belt off the pulley and out through the coupling gap. Pull the loose right-side belt toward the rim of the pulley while rotating the pulley by hand to walk the belt off, then slip it out through the unbolted drive shaft gap.
- Tip: A stretched belt can often be walked right off the pulley by hand without releasing the belt tensioner.
Step 3: Install the New Belt and Reconnect the Coupling
Route the new drive belt through the shaft gap onto the pulleys, reinstall and tighten the two drive coupling bolts, and start the mower.
- Route and seat the new belt onto the pulleys. Pass the new belt through the gap in the drive shaft coupling, seat it into the right pump pulley groove, and stretch it onto the center drive pulley.
- Tip: Compare the old belt side-by-side with the new OEM belt (Grasshopper Part # 381540) before installing to confirm whether the old belt has stretched or thinned and to verify the new belt matches the heavy-duty belt on the opposite side.
- Watch out: Keep fingers clear of the pinch point between the tight new belt and the pulley groove when rolling the belt into place.
- Reinstall and tighten the two drive coupling bolts. Align the drive shaft coupling holes, thread the two bolts back in, and tighten them firmly using a socket wrench and combination wrench.
- Watch out: Brace your knuckles when pulling hard on the wrench behind the radiator fins so a slipping wrench doesn't cut your hand.
- Start the engine, verify belt engagement, and lower the seat. Start the mower, push the right steering lever forward and backward to verify the right drive wheel engages without the belt slipping, and fold the seat back down.
Step 4: Test Drive and Hill Climb Verification
Test the mower's responsiveness on level gravel and drive up and down a steep dirt incline in forward and reverse to verify full power is restored to the right drive motor.
- Test zero-turn spins and straight tracking on gravel. Sit on the mower and work the right and left steering levers in forward, reverse, and zero-turn spins on gravel to verify both drive wheels respond equally.
- Test forward and reverse hill climbing on a steep incline. Drive the mower up a steep dirt berm, stop mid-hill and resume climbing forward from a standstill, and back all the way up the incline in reverse to confirm full hydraulic power under heavy load.
- Tip: Test the mower on a steep incline in both forward and reverse to put maximum load on the hydraulic pump belt and confirm it no longer slips.