Komatsu 3D78-1A(S/N 00101-UP)
Maintenance schedule, common problems & OEM parts breakdown
The Komatsu 3D78-1A(S/N 00101-UP) is a naturally aspirated, 3-cylinder diesel engine displacing 1.232 liters and rated at roughly 16.2 kW (21.7 hp), using mechanical fuel injection with no electronic control module. There is no onboard engine computer or fault-code system — the only feedback to the operator comes from analog gauges and warning lamps for oil pressure, charging, and coolant temperature on the host machine's instrument panel. This 00101-UP serial range was the factory power plant for the Komatsu WA20-1 compact wheel loader, built from 1987 to 1989, with an operating weight around 1.73 t (3,814 lb), a 60.2 L fuel tank, and a 12.1 L cooling system. Komatsu used the broader 3D78 engine family across several small industrial and construction platforms of that era, but this specific 1A calibration and serial range is documented against the WA20-1 chassis.
Komatsu revised this engine line as the WA20 loader moved into its next generation: the WA20-1's 3D78-1A gave way to a 3D78N-1C calibration in the follow-on WA20-2 loader, the kind of mid-life fuel-system and tuning update Komatsu typically applied without redesigning the base engine block. Because the WA20-1 was a short, three-year production run and has been out of front-line service for decades, most surviving 3D78-1A engines now move through the used and parts market as running takeouts or core units for rebuilders rather than as current factory-supported components. Demand concentrates on genuine wear items with real service life left in them — injection pump and nozzle rebuild kits, head and gasket sets, water pumps, and starter or alternator exchanges — since the mechanical simplicity of this pre-electronic engine keeps it worth reviving for landscaping, agricultural, and light-industrial uses where compact WA20-1 loaders still turn up.
Below: full specifications, fluids & capacities, the factory service schedule, common service parts, verified fault codes, what owners discuss, attachment guidance, the complete assembly directory, and a serial-number reference. Complete parts lists with full OEM part numbers, exploded diagrams, quantities, and fitment data are available free in Heavy Parts AI.
Komatsu 3D78-1A(S/N 00101-UP) specifications
Engine
| Engine model | 3D78-1A (S/N 00101-UP), 3-cylinder inline, water-cooled, naturally aspirated diesel |
| Displacement / cylinders | 1.232 L (1,232 cc), 3 cylinders |
| Rated power | approx. 16.2 kW (about 22 PS / 21.7 hp) class rating |
| Emissions tier | Pre-emissions-regulation era engine (late-1980s production); no EPA/EU emissions tier documented |
| Application | Factory power plant for the Komatsu WA20-1 compact wheel loader, initial production run |
Weights
| Operating weight (host machine, S/N 00101-UP range) | approx. 1.73 t (3,814 lb) |
| Shipping weight | Not documented for this exact serial range — verify with your dealer |
| Ground pressure | Not published for this wheeled machine class |
Dimensions
| Transport length | 3.2 m (10.5 ft) |
| Transport width | 1.08 m (3.5 ft) |
| Transport height | 2.34 m (7.7 ft) |
| Bucket width | 1.35 m (4.4 ft) |
| Max bucket discharge height | 1.75 m (5.7 ft) |
| Turning radius (outside) | 2.92 m (9.6 ft) |
| Wheelbase / ground clearance | Not documented for this exact serial range — verify with your dealer |
Performance
| Max travel speed | 15 km/h (9.3 mph) |
| Bucket capacity | 0.22 to 0.26 cu m (0.29 to 0.34 cu yd) heaped, depending on bucket configuration |
| Gradeability / breakout / lift | Not documented for this exact serial range in available factory data — verify with your dealer |
Service capacities (summary)
| Fuel tank | Not documented for this exact serial range — verify with your dealer or shop manual |
| Hydraulic system | Not documented for this exact serial range — verify with your dealer or shop manual |
| Engine oil | Not documented for this exact serial range — verify with your dealer or shop manual |
| Cooling system | Not documented for this exact serial range — verify with your dealer or shop manual |
Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.
3D78-1A(S/N 00101-UP) fluids & capacities
| System | Capacity | Recommended fluid |
|---|---|---|
| Engine crankcase, with filter | not published in accessible sources for this exact serial range (00101-UP); sister three-cylinder diesels of this same 1.23L displacement class typically hold roughly 3.5 to 4.5 L (3.7 to 4.8 US qt) with filter change — confirm exact figure on the engine's own refill chart or with your dealer | Komatsu genuine EO-series diesel engine oil, API CD/CE service class. Use SAE10W-30 in colder ambient ranges and SAE15W-40 in warmer ranges, per the Komatsu oil viscosity-by-temperature chart |
| Cooling system (radiator and block) | 12.1 L (3.2 US gal) | Komatsu Super Coolant AF-NAC, pre-mixed antifreeze/anti-corrosion coolant meeting ASTM D3306 / JIS K2234 specifications. Do not mix coolant types |
| Fuel tank | 60.2 L (15.9 US gal) | No. 2-D diesel fuel. Switch to a winter-grade or No.1-D blend in cold ambient temperatures to avoid fuel waxing |
| Grease points (fittings, spec only) | per fitting, not tank-metered | Komatsu G2-LI multipurpose lithium EP2 grease, the era's standard one-grease-fits-all recommendation for external pins and linkages |
Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.
Komatsu 3D78-1A(S/N 00101-UP) maintenance schedule
| Service interval | Tasks |
|---|---|
| Every 50 h |
|
| Every 250 h |
|
| Every 500 h |
|
| Every 1,000 h |
|
| Every 2,000 h |
|
| Every 4,000 h |
|
Servicing the 3D78-1A(S/N 00101-UP) beyond the schedule
Predictive maintenance & fluid analysis
Because the 3D78-1A carries no ECM or onboard diagnostics, oil sampling is the main way to catch trouble early on this mechanical-injection three-cylinder. Send used oil for analysis at every oil change to track iron and copper wear trends from the naturally aspirated block and injection pump. Watch coolant condition closely too — the small 12.1 L cooling system in this size class concentrates contamination fast, so rust color or an oil sheen in the coolant is an early warning worth acting on before a head gasket or water pump fails outright.
Corrective & common repairs
Corrective work on this engine centers on the mechanical fuel system: injection pump wear, sticking delivery valves, and worn nozzles that show up as hard starting, white smoke, or a rough idle across the three cylinders. Starter and alternator faults are common on equipment this age — check wiring and grounds before swapping a component. Because the WA20-1 host is a compact loader, also inspect engine mounts and drive-belt alignment, since a worn mount vibrates into the fan belt and can be mistaken for a charging-system problem.
Overhaul & rebuild points
A full rebuild on the 3D78-1A(S/N 00101-UP) starts with the top end: cylinder liner wear, piston ring gap, and valve seat condition, since even a naturally aspirated three-cylinder accumulates real thermal cycling over 1.232 L of displacement across decades of service. Check crankshaft and connecting-rod bearing clearances if oil pressure has trended down or knock shows at idle. Parts support for this WA20-1-era engine is thin, so source injection pump and nozzle rebuild kits early — lead time, not labor, is usually the bottleneck on this job.
Seasonal & environment servicing
Cold starts are the main seasonal risk on this mechanical-injection engine — with no ECM, there is no automatic cold-start enrichment, so check decompression/starting aids and battery condition before winter and run the correct oil viscosity for ambient temperature. In hot, dusty environments typical of loader duty, shorten air cleaner element checks and watch the small-capacity cooling system for airflow blockage through the radiator fins. Drain the fuel/water separator more often in humid or wet storage conditions to keep water out of the mechanical injection pump.
3D78-1A(S/N 00101-UP) common service parts
| Part number | Part | Notes | |
|---|---|---|---|
| YM129100-55650 | Element,Fuel Filter | Fuel system service | Check fitment → |
| YM129100-55620 | Fuel Filter Ass'Y | Fuel system service | Check fitment → |
| YM129100-55621 | Fuel Filter Ass'Y | Fuel system service | Check fitment → |
| YM171301-92750 | Wrench,Engine Oil Filter | Lubrication service | Check fitment → |
| YM121120-12901 | Element | Scheduled service item | Check fitment → |
Always confirm against your machine's serial number — cross-check any part in Heavy Parts AI before ordering.
3D78-1A(S/N 00101-UP) fault codes & troubleshooting
| Code | Meaning | Likely cause | What to do |
|---|---|---|---|
| Charge (battery) pilot lamp | Alternator not charging the battery circuit | Loose or broken alternator belt, worn brushes, or a faulty alternator/regulator | Check belt tension and wiring, then test alternator output; replace if out of spec |
| Oil pressure pilot lamp | Engine lubrication pressure has dropped below the safe switch setting | Low oil level, clogged oil filter/strainer, worn oil pump, or a failed pressure switch | Stop the engine immediately, check oil level and filter, then verify pressure with a mechanical gauge before restarting |
| Water temperature gauge in red zone | Engine coolant temperature has exceeded normal operating range | Low coolant, clogged radiator, slipping or broken fan belt, or thermostat failure | Idle down and stop safely, let engine cool, check coolant level and belt, then inspect radiator and thermostat |
Codes and remedies are general guidance for this model family — always confirm with diagnostic tooling and your dealer before major repairs.
All 3D78-1A(S/N 00101-UP) assemblies by section
Every catalogued assembly group for the Komatsu 3D78-1A(S/N 00101-UP). Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.
Cooling System
Komatsu : 3d78-1a(S/N 00101-Up) : Cooling System : Water Pump And Cooling Fan
| YM***02 | Bolt Plated | 2 |
| YM***20 | Gasket,Water Pump | 1 |
| YM***02 | Screw | 1 |
Cylinder Block
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Camshaft And Tappet
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Crankshaft
| YM***00 | Gear,Crank | 1 |
| YM***01 | Ball,Steel | 3 |
| YM***10 | Spring Pin | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Cylinder Block(#00101-00765)
| YM***02 | Screw | 1 |
| YM***00 | Liner,Cylinder | 3 |
| YM***00 | O-Ring (Kit) | 2 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Cylinder Block(#00766-)
| YM***02 | Screw | 1 |
| YM***00 | O-Ring (Kit) | 2 |
| YM***20 | Plug,Drain(Without Oil Cooler Type) | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Flywheel And Flywheel Housing
| YM***00 | Gear,Ring | 1 |
| YM***02 | Screw | 6 |
| YM***02 | Screw | 4 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Front Cover And Gear
| YM***10 | Washer,Seal | 9 |
| YM***51 | Cap, Lub. Oil Supply | 1 |
| YM***52 | Bolt | 4 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Oil Pan
| YM***52 | Screw | 6 |
| YM***02 | Screw | 18 |
| YM***52 | Screw | 2 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Piston And Connecting Rod
| YM***00 | Screw | 2 |
| YM***80 | Piston Ass'Y | 3 |
| YM***00 | Ring,Snap | 6 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Block : Power Take Off
| YM***50 | Bracket | 1 |
| YM***70 | Cover,(For Shipping) | 1 |
| YM***80 | Gasket | 1 |
Cylinder Head
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Head : Air Cleaner
| YM***41 | Buffle | 1 |
| YM***01 | Element | 1 |
| YM***21 | Body | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Head : Air Intake Manifold
| YM***02 | Screw | 2 |
| YM***00 | Manifold,Air Intake | 1 |
| YM***52 | Bolt | 2 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Head : Cylinder Head
| YM***80 | Retainer, Spring | 6 |
| YM***40 | Seal,Valve? Exhaust (Kit) | 6 |
| YM***10 | Valve,Exhaust | 3 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Head : Cylinder Head Cover
| YM***20 | Cover, Breather | 1 |
| YM***51 | Cap, Lub. Oil Supply | 1 |
| YM***00 | Clip,Breather Hose | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Head : Exhaust Manifold
| YM***10 | Manifold, Exhaust | 1 |
| YM***02 | Nut | 4 |
| YM***22 | Stud | 4 |
Komatsu : 3d78-1a(S/N 00101-Up) : Cylinder Head : Rocker Arm
| YM***02 | Screw | 6 |
| YM***80 | Spring | 3 |
| YM***00 | Plug | 2 |
Electrical System
Komatsu : 3d78-1a(S/N 00101-Up) : Electrical System : Alternator
| YM***63 | Regulator Ass'Y | 1 |
| YM***BM | Bearing,Ball | 1 |
| YM***62 | Brush | 2 |
Komatsu : 3d78-1a(S/N 00101-Up) : Electrical System : Alternator And Mounting
| YM***30 | Plate,Adjust | 1 |
| YM***02 | Bolt | 1 |
| YM***02 | Nut | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Electrical System : Safety Relay And Thermostart Timer
| YM***20 | Timer,Thermostart | 1 |
| YM***00 | Relay,Safety | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Electrical System : Starting Motor
| YM***60 | Bearing,Ball | 1 |
| YM***60 | O-Ring | 4 |
| YM***20 | Cover | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Electrical System : Starting Motor And Mounting
| YM***02 | Nut | 2 |
| YM***02 | Nut,Rock | 1 |
| YM***50 | O-Ring | 1 |
Fuel System
Komatsu : 3d78-1a(S/N 00101-Up) : Fuel System : Fuel Filter(#00101-00765)
| YM***00 | O-Ring (Kit) | 1 |
| YM***70 | Lever | 1 |
| YM***80 | Cup, Strainer | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Fuel System : Fuel Filter(#00766-)
| YM***00 | O-Ring (Kit) | 1 |
| YM***70 | Lever | 1 |
| YM***80 | Cup, Strainer | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Fuel System : Fuel Injection Pump
| YM***70 | Retainer, Spring | 3 |
| YM***50 | Kit, Shim | 1 |
| YM***10 | Plate? Caution | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Fuel System : Fuel Injection Pump And Piping
| YM***20 | Retainer, Pipe | 1 |
| YM***60 | O-Ring,Fuel Injection Pump | 1 |
| YM***80 | Screw | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Fuel System : Fuel Nozzle And Spill Hose(#00101-01291)
| YM***20 | Spring | 1 |
| YM***91 | Joint | 1 |
| YM***50 | Check Valve Ass'Y | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Fuel System : Fuel Nozzle And Spill Hose(#01292-)
| YM***70 | Union | 1 |
| YM***20 | Spring | 1 |
| YM***51 | Check Valve Ass'Y,Joint | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Fuel System : Governor
| YM***00 | Spring, Regulator | 1 |
| YM***01 | Weight Assembly., Governor | 1 |
| YM***00 | Lever Ass'Y | 1 |
Lubricating Oil System
Komatsu : 3d78-1a(S/N 00101-Up) : Lubricating Oil System : Lubricating Oil Filter
| YM***50 | Nut | 1 |
| YM***05 | Bracket | 1 |
| YM***02 | Bolt Plated | 3 |
Komatsu : 3d78-1a(S/N 00101-Up) : Lubricating Oil System : Lubricating Oil Pump And Suction Pipe
| YM***80 | Pipe,Inlet | 1 |
| YM***50 | Oil Pressure Transmitter | 1 |
| YM***40 | Gasket,Inlet Pipe | 1 |
Tool And Kit
Komatsu : 3d78-1a(S/N 00101-Up) : Tool And Kit : Gasket Kit
| YM***30 | Gasket,Cylinder Head | 1 |
| YM***91 | Gasket Kit | 1 |
| YM***10 | Gasket,Flywheel Housing | 1 |
Komatsu : 3d78-1a(S/N 00101-Up) : Tool And Kit : Tool
| YM***50 | Wrench,Engine Oil Filter | 1 |
| YM***20 | Flex Horn (Option.) | 1 |
| YM***60 | Fuel, Wrench, Strainer | 1 |
Frequently asked questions
What engine is the Komatsu 3D78-1A(S/N 00101-UP)?
It is a naturally aspirated, 3-cylinder diesel engine with 1.232 L displacement, rated at roughly 16.2 kW (21.7 hp), using mechanical fuel injection with no electronic controls. This serial range (00101-UP) was the factory engine fitted to the Komatsu WA20-1 compact wheel loader.
What is the operating weight of the machine this engine powers?
The WA20-1 wheel loader that carries this engine has an operating weight of roughly 1.73 t (3,814 lb), with a 60.2 L fuel tank and a 12.1 L cooling system.
What replaced the Komatsu 3D78-1A(S/N 00101-UP)?
Komatsu built the WA20-1 through a short 1987-1989 run, then moved to the WA20-2 loader, which used a revised 3D78N-1C calibration of this same engine family rather than a completely new design.
What 3D78-1A(S/N 00101-UP) owners discuss
Does this engine share parts with other manufacturers small diesels?
The engine smokes black under load and loses some power - what's usually behind that on this era of mechanical-injection engine?
Any known cold-start quirks with this generation of engine?
What wear patterns show up on the host wheel loader this engine powers, since the engine itself has no undercarriage?
Is this engine prone to overheating or head gasket failure?
What electrical issues come up most on machines this old running this engine?
What should I check before buying a used WA20-1 fitted with this 3D78-1A engine?
Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.
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