Part of the John Deere Repair Manuals.
Wrenching on a 1970-model 120? This 54-page John Deere 120 service manual PDF covers the hydrostatic tractor from carburetor to differential. Inside: wiring diagrams for the charging and ignition circuits, hydraulic schematics tracing the gerotor charge pump, check valves, and implement relief valve, plus full hydrostatic drive teardown steps covering swashplates, valve plates, and cylinder blocks. You also get brake, axle, and differential procedures right down to ring gear pattern checks and backlash adjustment with a dial indicator. Torque the charge pump housing cap screws to 50-55 ft. lbs. and set the spark plug gap at 0.025 inch; those are factory numbers, not forum guesses. Your tractor is down and the clock is ticking. Bookmarked sections and a real text layer let you search any spec and get back to wrenching.
What's Inside This Manual
| System | Pages | Key Topics |
|---|---|---|
| General | 3-4 | Introduction, Specifications, Predelivery |
| Engine | 5-7 | Throttle Cable Adjustment, Governor Adjustment, Tachometer Usage |
| Fuel System | 8-9 | Idle Mixture Adjustment, Idle Stop Screw Adjustment, Carburetor Service |
| Electrical System | 10-21 | Meter Polarity Adjustment, Ampere Reading, Charging System Service |
| Specifications | 22 | Dimensions, Wheel Tread, Tire Sizes, Engine Specifications, Transmission |
| Driveshaft Servicing | 22-23 | Roll Pins, Drive Shaft Yokes, Flexible Disk, Clutch Cone, Engine Mounting Bolts |
| Hydrostatic Drive | 24-38 | Piston Operation, Hydrostatic Drive Pistons, Swashplate Operation, Operation of Hydrostatic, Reversing in Variable Pump Circuits, Circuit Diagram, Hydrostatic Removal, Testing and Disassembly, Charge Pump Service, Implement Relief Valve, Check Valves, Hydrostatic Disassembly, Center Section Housing Removal, Valve Plate Inspection, Valve Plate Cleaning, Center Section Needle Bearings, Cylinder Block Removal, Piston Cleaning, Piston Inspection, Slipper Retainer Inspection, Cylinder Block Disassembly, Motor Swash Plate Removal, Transmission Motor Shaft Removal, Motor Shaft Bearing Extension, Trunnion Shaft Assembly Removal, Thrust Plate Inspection, Trunnion Bearing Installation, Pump Shaft Seal Installation, Reassembly Procedures, Piston Assembly Handling |
| Brake Service | 39-40 | Seal Retainer Centering, Brake Shoe Inspection, Brake Drum Installation |
| Axle Shaft, Seal and Bearing Service | 41 | Axle Shaft, Seal or Bearing Service |
| Differential and Final Drive | 42-52 | Differential Removal, Differential Disassembly, Pinion Shaft Removal, Pinion Gear Removal, Ring Gear Replacement, Thrust Washer Installation, Pinion Gear Alignment, Pinion Shaft Installation, Differential Bearing Removal, Bearing Installation, Spur Gear Removal, Pinion Assembly Removal, Pinion and Spur Gear Assembly, Outer Bearing Race Replacement, Inner Bearing Race Replacement, Assembling Pinion and Differential Group, Pinion and Spur Gear Installation, Outer Bearing Pressing, Differential Assembly Installation, Ring Gear Pattern Check, Desirable Pattern, Toe Pattern, Backlash Check, Welch Plug Installation, Rear Housing Installation, Hydraulic |
Quick Reference Specifications
| Specification | Value | Page |
|---|---|---|
| Battery capacity | 40 amp-hr. | p. 10 |
| Spark plug gap | 0.025 in. | p. 10 |
| Spark plug torque | 15 to 20 ft. lbs. | p. 17 |
| Charge relief valve plug torque | 15-17 ft. lbs. | p. 28 |
| Charge pump housing cap screws torque | 50-55 ft. lbs. | p. 31 |
| Charge pressure | 75-110 PSI | p. 27 |
| Implement relief valve pressure | 500-550 PSI | p. 31 |
| Wheel Base | 46 in. | p. 3 |
| Curb Weight | 736 lbs. | p. 3 |
| Engine Crankcase Capacity | 3 US pints | p. 5 |
| Fuel Tank Capacity | 1.75 US Gallons | p. 8 |
| Breaker Point Gap | 0.020 in. | p. 10 |
