Replace the Deck and Drive Belts on a John Deere S100

Belt problems are the most common repair on John Deere S100 series riding mowers. The deck belt wears from normal use and eventually needs replacement. The drive belt lasts longer but can fail too. This guide covers both belt types with routing diagrams and step-by-step replacement procedures.

Belt Types

The S100 series uses two main belts—one drives the mower deck and another drives the transmission.

Deck Belt (Blade Drive)

Model	Deck Size	Belt Part Number	Belt Length
S100	42”	GX20072	95.5”
S120	42”	GX20072	95.5”
S130	42”	GX20072	95.5”

Drive Belt (Transmission)

Model	Belt Part Number	Belt Length
S100	GX20006	89.5”
S120	GX20006	89.5”
S130	GX20006	89.5”

Note: Always verify part numbers with your specific model and serial number. John Deere made running changes during production.

Aftermarket Options

OEM John Deere belts cost more but fit perfectly. Aftermarket alternatives:

• Oregon 75-904 - Fits GX20072 deck belt
• Stens 265-164 - Fits GX20072 deck belt
• Oregon 75-903 - Fits GX20006 drive belt

Aftermarket belts work fine for most users. OEM belts may last slightly longer.

Identifying Replacement Signs for Your Belt

Deck Belt Symptoms

• Squealing when blades engage
• Slipping under heavy grass
• Blades won’t engage or engage slowly
• Visible damage - cracks, fraying, glazing
• Burned rubber smell during mowing

Drive Belt Symptoms

• Mower won’t move or moves slowly
• Jerky motion at low speeds
• Slipping when climbing hills
• Belt squeal when starting to move

Visual Inspection

Remove the belt and check for:

• Cracks on the inner V-surface
• Glazed or shiny spots (slipping)
• Frayed edges
• Missing chunks
• Stretched length (compare to new belt)

Any of these means replacement time.

Deck Belt Replacement

The deck belt replacement requires removing the mower deck. Plan for 30-45 minutes.

Tools Needed

• Socket set (10mm, 13mm, 15mm)
• Flat blade screwdriver
• Work gloves
• Block of wood (to prop deck)

Step 1: Prepare the Mower

1. Park on flat, level surface
2. Set parking brake
3. Lower deck to lowest cutting position
4. Turn off engine and remove key
5. Disconnect spark plug wire

Step 2: Remove the Deck

1. Remove the belt from the engine pulley
2. Disconnect the deck lift linkage (two pins)
3. Disconnect the blade engagement linkage
4. Slide the deck out from under the mower

Step 3: Remove Old Belt

With deck removed:

1. Locate belt routing path around pulleys
2. Take a photo for reference
3. Release belt tension by moving spring-loaded idler
4. Slip belt off pulleys one at a time
5. Thread belt off the deck

Step 4: Install New Belt

1. Route new belt around mandrel pulleys first
2. Work belt around idler pulleys
3. Pull back spring-loaded tensioner
4. Slip belt onto tensioner pulley
5. Release tensioner slowly
6. Verify belt sits in all pulley grooves

Important: The belt must sit inside all belt guides. Guides prevent the belt from jumping off during operation.

Step 5: Reinstall Deck

1. Slide deck under mower frame
2. Connect blade engagement linkage
3. Connect deck lift linkage pins
4. Route belt onto engine pulley
5. Verify deck moves up and down freely

Step 6: Test

1. Reconnect spark plug wire
2. Start engine and let idle
3. Engage blades at low throttle
4. Listen for squealing or rubbing
5. Check that blades spin at full speed

Drive Belt Replacement

The drive belt connects the engine to the transmission. This job takes about an hour.

Tools Needed

• Socket set (10mm, 13mm, 15mm)
• Needle-nose pliers
• Pry bar
• Work gloves

Step 1: Access the Belt

1. Remove the mower deck (see above)
2. Locate belt covers/shields
3. Remove any shields blocking belt access

The drive belt runs from the engine pulley down to the transmission pulley.

Step 2: Release Tension

1. Locate the drive belt tensioner
2. Use a pry bar to release tension
3. Hold tensioner back while removing belt
4. Note the routing around all pulleys

Step 3: Remove and Replace

1. Slip belt off transmission pulley
2. Work belt off engine pulley
3. Remove old belt completely
4. Route new belt onto engine pulley first
5. Work belt around idlers following original path
6. Pull tensioner back and slip belt on
7. Release tensioner slowly

Step 4: Verify Routing

Double-check that the belt:

• Sits in all pulley grooves
• Runs inside all belt guides
• Has proper tension (slight deflection when pressed)

Step 5: Reinstall Components

1. Replace belt shields
2. Reinstall mower deck
3. Test drive forward and reverse
4. Check for squealing or slipping

Belt Routing Diagrams

42-Inch Deck Belt Routing

The deck belt follows this path:

Engine Pulley (top)
 ↓
Idler Pulley (spring-loaded)
 ↓
Left Mandrel Pulley
 ↓
Center Mandrel Pulley
 ↓
Right Mandrel Pulley
 ↓
Fixed Idler Pulley
 ↓
Back to Engine Pulley

The belt wraps around the outside of mandrel pulleys and inside of idler pulleys.

Drive Belt Routing

Engine Pulley (top)
 ↓
Spring-Loaded Tensioner
 ↓
Stationary Idler
 ↓
Transmission Input Pulley

The drive belt is simpler with fewer pulleys in the path.

Common Belt Problems and Solutions

Belt Keeps Coming Off

Causes:

• Belt guides bent or missing
• Pulley bent or wobbling
• Debris packed in pulley groove
• Wrong belt size
• Worn tensioner spring

Fixes:

1. Clean all pulleys thoroughly
2. Check each pulley spins true (no wobble)
3. Verify all belt guides are present and positioned
4. Replace tensioner if spring is weak
5. Confirm correct belt part number

Belt Squeals But Looks Good

Causes:

• Belt glazed (slick surface)
• Tensioner weak
• Pulley bearing failing
• Oil or grease on belt

Fixes:

1. Replace belt (glazed belts don’t grip)
2. Replace tensioner assembly
3. Spin each pulley by hand to check bearings
4. Clean pulleys with brake cleaner

Blades Won’t Engage

Causes:

• Belt broken
• Belt off pulley
• PTO clutch failure
• Engagement cable broken

Fixes:

1. Visually inspect belt condition
2. Check belt routing
3. Test PTO clutch with multimeter
4. Inspect engagement cable for breaks

Belt Wears Quickly

Causes:

• Misaligned pulleys
• Bent deck
• Running belt too loose
• Running belt too tight
• Debris damage

Fixes:

1. Check deck for damage or bends
2. Verify all pulleys align
3. Replace tensioner if it’s not holding proper tension
4. Keep deck clean of debris

Essential Maintenance Tips for Your Mower

Extend Belt Life

• Keep deck clean and debris-free
• Don’t engage blades at full throttle
• Avoid hitting obstacles
• Check belt tension monthly
• Lubricate pivot points annually

Inspection Schedule

Interval	Check
Monthly	Belt condition and tension
50 hours	Clean pulleys and guides
100 hours	Consider replacement if worn
Annually	Replace if signs of wear

Storage

Before winter storage:

• Clean the deck thoroughly
• Inspect belts for wear
• Release belt tension if storing long-term (optional)
• Store in dry location

More Maintenance Resources

• John Deere S100 Problems
• John Deere Lawn Mower Parts Diagram
• John Deere S100 Oil Change
• Riding Mower Won’t Start

Summary

Belt replacement on the John Deere S100 is a manageable DIY job. The deck belt (GX20072) wears fastest and needs replacement every 100-150 hours or when showing damage. The drive belt (GX20006) lasts longer but still needs periodic inspection. Proper routing and tensioning prevent premature failure. Take photos before removal to make reinstallation easier.