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Komatsu 3D78AE-3A(S/N 00715-UP)

Maintenance schedule, common problems & OEM parts breakdown

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The Komatsu 3D78AE-3A(S/N 00715-UP) is the Yanmar-built diesel that powers the Komatsu WA20-2, a sub-2-tonne mini/compact wheel loader built for tight-access site work, nurseries, and municipal yards. It's a naturally aspirated, mechanically governed and mechanically injected 3-cylinder engine, 78 x 84 mm bore/stroke, 1.204 L displacement, rated 16.2 kW (21.7 hp) at 2,600 rpm with 73.5 N.m peak torque at 1,500 rpm. The WA20-2 runs 1,735 kg with an open canopy or 1,885 kg with an enclosed cab, carries a 0.28 m3 standard bucket, and moves up to 15 km/h through a torque-converter, powershift (Torqflow) drivetrain rather than a hydrostatic transmission. Komatsu built the WA20 nameplate across several generations: the WA20-1 opened the series in the late 1980s, the WA20-2 covered here followed through the end of that decade and into the mid-1990s, and later WA20-3, WA20-5, and current WA20-6 generations (into the 2020s) carried the concept forward.

Across WA20 generations the biggest change sits under the hood: the WA20-1 used a Komatsu-branded 4-cylinder diesel, while the WA20-2 switched to this smaller Yanmar-sourced 3-cylinder unit, reflecting Komatsu's practice of outsourcing small-displacement power for its mini construction line. Later generations moved on again to engines meeting progressively tighter emission standards, eventually adding electronic governing that the 3D78AE-3A never had. That simplicity is exactly why this generation still matters in the used and parts market: no ECM, no fault codes, and a mechanical fuel system a competent diesel shop can service without OEM diagnostic tools. Parts commonality with other Yanmar-powered Komatsu compact equipment of the same era keeps aftermarket support alive, making the WA20-2 a low-cost, low-complexity option for buyers who want a small loader without electronics to fail.

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 3D78AE-3A(S/N 00715-UP) specifications

Engine

ModelKomatsu 3D78AE-3A diesel engine, S/N 00715-UP — vertical, water-cooled, 4-cycle, 3-cylinder direct-injection diesel; factory-built by Yanmar and badged by Komatsu
Rated (flywheel) power16.2 kW (21.7 hp) at 2,600 rpm, as fitted to its confirmed host machine
Max. torque73.5 N·m (7.5 kgf·m / 54 lb-ft) at 1,500 rpm
Displacementapprox. 1.204 L (73.5 cu in), 3 cylinders, per the factory shop manual covering this engine family
Bore x stroke78 mm x 84 mm (3.07 in x 3.31 in), per the same engine-family shop manual
Starting motor12V 1.8 kW
Alternator12V 40A
Battery12V, 60 Ah x 1
Emissions certificationNot documented for this exact serial range in available sources — this engine predates modern nonroad emissions-tier certification for its power class; verify current status with your dealer
Series notes3D78AE-3A, -3B, and -3C share the same base engine architecture; the letter suffix reflects the injection/governor calibration set for a given application rather than a different engine block

Weights

Host-machine operating weight, canopy config.1,735 kg (3,825 lb) — figure is for the confirmed host wheel loader, not the bare engine; includes a 55 kg (121 lb) operator
Host-machine operating weight, enclosed-cab config.1,885 kg (4,156 lb), includes a 55 kg (121 lb) operator, same application
Ground pressureNot published in available sources
Standalone engine dry weightNot published in available sources for this exact model/serial — verify with your dealer

Dimensions

Host-machine overall lengthapprox. 3,360 mm (132.3 in), without bucket teeth — reported for this host loader in independent equipment listings; not found as machine-readable text in the primary factory operation manual
Host-machine overall widthapprox. 1,350-1,355 mm (53.1-53.3 in), bucket-width reference, same caveat as length
Host-machine overall heightapprox. 1,740 mm (68.5 in), without cab roof, same caveat as length
Wheelbase / treadNot documented in available sources
Ground clearanceNot documented in available sources
Standalone engine dimensionsNot published in available sources for this exact model/serial — verify with your dealer

Performance

Bucket capacity, standard0.28 m³ (0.37 cu yd), struck, on the confirmed host wheel loader
Bucket capacity, optional large/ranch buckets0.4 m³ (0.52 cu yd) and 0.5 m³ (0.65 cu yd), listed as factory options for this host machine
Rated (normal) load4.4 kN (450 kgf / 992 lb)
Travel speed, forward/reverse15.0 km/h (9.3 mph), single speed, both directions
Max. rimpull (drawbar pull)16.7 kN (1,700 kgf / 3,750 lbf)
Min. turning radius2,540 mm (8 ft 4 in) outside of chassis; 2,950 mm (9 ft 8 in) at center of outside tire
GradeabilityNot published in available sources

Service capacities (summary)

Engine oil pan (drain and refill)min. 3.1 L (0.82 US gal), for this host machine; a larger 4.7 L figure in the same factory table applies to bigger loader siblings in the same family, not this host machine
Fuel tankreported at approx. 25 L (6.6 US gal) in an equipment listing; not confirmed against the factory manual's own capacity table — verify with your dealer
Hydraulic tankNot documented in available sources — verify with your dealer
Cooling systemCapacity not documented; coolant spec is a long-life antifreeze-type coolant mixed at 30% or more with water
Transmission case oil (type only, no capacity found)SAE 10W-30, API classification CD
Front/rear axle oil (type only, no capacity found)Komatsu-spec AXO75-equivalent axle oil

Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.

3D78AE-3A(S/N 00715-UP) fluids & capacities

SystemCapacityRecommended fluid
Engine crankcase (with filter)3.6 L total oil pan capacity (0.95 US gal); refill after a routine oil-and-filter change runs about 3.1 L (0.82 US gal), since some oil stays in the filter and passages after draining.Diesel engine oil, API CD/CE service class or equivalent. Select SAE grade by ambient temperature: SAE10W in the coldest range (down to roughly -30C), SAE10W-30 through the mid range, stepping up to SAE15W-40 or SAE30 above about 20C.
Cooling system4.2 L (1.19 US gal) for the full machine circuit (engine plus radiator). The engine side of the circuit alone holds about 1.8 L (0.48 US gal); the remainder is radiator, hoses and header tank.Water with a corrosion-inhibited, permanent-type antifreeze added whenever ambient temperature can drop below 0C; plain water with corrosion inhibitor is acceptable in non-freezing climates. Never use hard or untreated water. Drain and refill on the standard interval (about 4,000 hours or 2 years).
Fuel tank25.0 L (6.6 US gal).No. 2-D diesel fuel (ASTM D975 No. 2, or equivalent to ISO 8217 DMA / BS 2869 A1-A2, minimum Cetane No. 45) at ambient temperatures of about 10C and above; switch to a No. 1-D winter-grade diesel (ASTM D975 No. 1) in colder ambient to prevent fuel waxing.
Transmission / transfer case1.4 L (0.37 US gal).Same viscosity ladder as the engine crankcase: SAE10W-30 through the cold-to-mid ambient range, stepping up to SAE15W-40 above about 20C.
Axles, front and rear (each)4.0 L (1.06 US gal) per axle housing.SAE10W-30 in cold-to-mid ambient, stepping up to SAE15W-40 above about 20C, matching the transfer case fluid selection.
Hydraulic system and tank22.0 L (5.8 US gal) tank/system capacity.Anti-wear hydraulic oil, viscosity selected by ambient temperature band consistent with the engine-oil chart (lighter grade in cold climates, heavier grade in hot climates).
Grease (spec only)Not tank-metered - applied per fitting.NLGI No. 2 multipurpose lithium-base grease, used across the full ambient temperature range.

Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.

Komatsu 3D78AE-3A(S/N 00715-UP) maintenance schedule

Service intervalTasks
Every 50 h
  • Check engine oil level in the pan before starting; top up to the mark, don't overfill.
  • Check coolant level in the radiator reserve tank and inspect hoses/clamps for leaks or chafing.
  • Check fuel level and top off at end of shift to limit tank condensation.
  • Check tire inflation pressure and wheel-nut torque.
  • Empty the air cleaner dust cup and tap out accumulated dust before it packs the element.
  • Wipe down articulation-joint grease fittings and check for play; contaminated grease here accelerates pin and bushing wear.
Every 250 h
  • Grease articulation-joint, bucket-link, bucket, and lift/dump cylinder pins with NLGI No.2 grease.
  • Check and clean the air cleaner element; replace if torn or heavily clogged.
  • Check hydraulic tank oil level and top up with the specified hydraulic fluid.
  • Check fan belt tension and adjust to spec.
  • Check battery electrolyte level and clean/tighten terminals.
  • Check the glow-plug timer relay circuit and glow-plug operation; a sticking timer relay is a common cold-start complaint on this engine.
Every 500 h
  • Change engine oil and replace the oil filter cartridge (pan holds 3.6 L total, roughly 3.1 L on a filter-change refill).
  • Replace the fuel filter element and bleed air from the fuel system.
  • Check transmission (Torqflow) oil level and condition; discolored or burnt-smelling oil is the leading cause of harsh or delayed shifts on this drivetrain.
  • Check axle and transfer-case oil levels, top up as needed.
  • Check parking brake holding force and service brake pedal free travel.
Every 1,000 h
  • Change hydraulic tank oil and replace the hydraulic filter element.
  • Change transmission oil and clean the transmission oil strainer/screen.
  • Change axle oil and transfer case oil.
  • Check and adjust engine valve clearance.
  • Inspect the injection pump governor/flyweight-advance mechanism for wear or sticking; a worn advance shows up as rough idle or hunting at part throttle.
  • Purge and repack articulation-joint and steering-cylinder pins with fresh grease, flushing out old contaminated grease.
Every 2,000 h
  • Recheck and adjust engine valve clearance and injection timing.
  • Test injector nozzles for spray pattern and opening pressure; recondition or replace as needed.
  • Flush and refill the cooling system with fresh coolant (full circuit approx. 4.2 L, engine-only approx. 1.8 L).
  • Inspect starter motor, alternator, and glow-plug timer relay for wear; service or replace as needed.
  • Inspect torque converter and powershift clutch performance via pressure/stall check.
Every 4,000 h
  • Run a compression test on all three cylinders; investigate excess wear if readings vary beyond spec between cylinders.
  • Pull and inspect/rebuild the injection pump assembly, including the flyweight-advance and governor linkage.
  • Evaluate the torque converter and powershift clutch packs for rebuild if shifting stays harsh after a transmission oil change.
  • Inspect articulation and steering pins/bushings against service-limit wear and rebush as needed.
  • Inspect engine top end (rings, liners, main and rod bearings) if oil consumption or blow-by has increased over time.

Servicing the 3D78AE-3A(S/N 00715-UP) beyond the schedule

Predictive Maintenance & Fluid Analysis

For the 3D78AE-3A, oil-sample trending is more useful than fixed calendar changes given its simple mechanical fuel system. Watch iron and copper trends in engine oil to catch injection-pump flyweight-advance wear early, and check transmission oil condition at every interval; discoloration or a burnt smell signals internal Torqflow wear before shift harshness shows up. Cold-start white smoke that clears within a minute is normal on this high-hour design; smoke that lingers points toward injector or advance-mechanism wear worth sampling for.

Corrective & Common Repairs

Typical fixes on this engine center on the glow-plug timer relay, a known wear point that causes hard cold starts or no-start conditions; test and replace it before condemning the glow plugs themselves. Injection-pump flyweight-advance sticking shows up as rough idle or hunting at part throttle and usually needs pump service rather than injector work. On the WA20-2 chassis, harsh or delayed shifts trace back to transmission oil condition first, not the powershift clutches; change the oil before opening the transmission.

Overhaul & Rebuild Points

Because the 3D78AE-3A is mechanically injected with no fault codes, overhaul timing relies on compression testing and injector inspection rather than diagnostics. Rebuild candidates include the inline injection-pump governor/flyweight assembly, injector nozzles, and the glow-plug timer relay circuit. On the WA20-2 driveline, plan torque-converter and powershift clutch-pack inspection alongside engine overhaul, since both wear on similar hour cycles. Articulation and steering pins/bushings also warrant inspection at major overhaul, especially on machines with a history of contaminated grease.

Seasonal & Environment Servicing

Cold climates demand extra attention to the glow-plug timer relay and battery condition, since this naturally aspirated 3-cylinder relies entirely on glow plugs for cold starts with no electronic assist. Some cold-start white smoke is normal on this design and isn't itself a fault. In wet, muddy, or abrasive-yard environments, purge and repack the articulation joint more often than the standard interval; contaminated grease at this joint is the leading cause of premature pin and bushing wear on the WA20-2 chassis.

3D78AE-3A(S/N 00715-UP) common service parts

Part numberPart
YM119833-55620Fuel Filter Ass'YCheck fitment →
YM102103-55520O-Ring,(For Fuel Filter)Check fitment →
YM119005-35100Oil Filter Ass'YCheck fitment →
YM129350-12900ElementCheck fitment →

Always confirm against your machine's serial number — cross-check any part in Heavy Parts AI before ordering.

All 3D78AE-3A(S/N 00715-UP) assemblies by section

Every catalogued assembly group for the Komatsu 3D78AE-3A(S/N 00715-UP). Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.

Cooling System
Komatsu : 3d78ae-3a(S/N 00715-Up) : Cooling System : Water Pump And Cooling Fan(#00119-)
YM***02Bolt Plated2
YM***01Thermostat1
YM***20O-Ring,(Not Shown)1
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Cylinder Block
Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Block : Camshaft And Tappet(#00119-)
YM***01Gear Ass'Y,Camshaft1
YM***00Rod,Push6
YM***50Bearing, Thrust1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Block : Crankshaft(#00119-)
YM***00Gear,Crank1
YM***80Parallel Pin1
YM***50Pulley,(For Variable Pump)1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Block : Cylinder Block(#00119-)
YM***00O-Ring (Kit)2
YM***30Thrust Metal Ass'Y (Std)2
YM***20Plug,Drain(Without Oil Cooler Type)1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Block : Flywheel And Flywheel Housing(#00119-)
YM***50Oil Seal Case, Assembly1
YM***00Gear,Ring1
YM***80Seal? Rear1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Block : Front Cover And Gear(#00119-)
YM***02Bolt Plated2
YM***51Cap, Lub. Oil Supply1
YM***40O-Ring (Kit)2
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Block : Oil Pan(#00119-)
YM***52Screw22
YM***40Plug,Drain1
YM***30Housing Assembly., Gear1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Block : Piston And Connecting Rod(#00119-)
YM***50Ring Set (+0.25 mm)1
YM***00Connecting Rod, Assembly3
YM***00Screw2
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Cylinder Head
Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Head : Air Cleaner(#00119-)
YM***41Buffle1
YM***00Element1
YM***01Air Cleaner Ass'Y1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Head : Air Intake Manifold(#00119-)
YM***02Screw2
YM***60Silencer1
YM***52Screw4
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Head : Cylinder Head Cover(#00119-)
YM***20Cover, Breather1
YM***51Cap, Lub. Oil Supply1
YM***00Clip,Breather Hose1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Head : Cylinder Head(#00119-)
YM***80Retainer, Spring6
YM***00Guide,Valve? Exhaust6
YM***00Plug2
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Head : Exhaust Manifold And Muffler(#00119-)
YM***02Stud2
YM***21Muffler1
YM***22Stud4
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Cylinder Head : Rocker Arm(#00119-)
YM***50Rocker Arm Ass'Y6
YM***02Screw6
YM***80Spring3
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Electrical System
Komatsu : 3d78ae-3a(S/N 00715-Up) : Electrical System : Alternator (40a)(#00119-02682)
YM***50Brush Ass'Y1
YM***00Retainer,Bearing1
YM***70Diode Ass'Y1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Electrical System : Alternator (40a)(#02683-)
YM***30Front Bracket Ass'Y1
YM***40Alternator A. (40a)1
YM***00Rotor Ass'Y1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Electrical System : Alternator And Mounting (40a)(#00119-02682)
YM***02Nut1
YM***30Plate,Adjust1
YM***50Bolt1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Electrical System : Alternator And Mounting (40a)(#02683-)
YM***40Screw1
YM***60Nut1
YM***30Adjuster, Belt1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Electrical System : Safety Relay And Glow Plug Timer(#00119-)
YM***20Timer,Solenoid1
YM***10Relay,Stop Solenoid1
YM***00Relay, Safety3
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Electrical System : Starting Motor (1.8kw)(#00119-)
YM***40Bolt,Through2
YM***50Pinion Ass'Y,Pinion1
YM***10Bearing,Ball1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Electrical System : Starting Motor And Mounting (1.8kw)(#00119-)
YM***52Bolt Plated2
YM***20Cover1
YM***02Screw2
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Fuel System
Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Filter And Piping(#00119-)
YM***70Union1
YM***40Lever1
YM***00O-Ring (Kit)1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2) (Stock Raising Spec.)(#02683-)
YM***70Nut1
YM***40Lever,Control1
YM***30Spacer,Lever Shaft1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump (Governor) (1/2)(#00119-)
YM***70Nut1
YM***40Lever,Control1
YM***30Spacer,Lever Shaft1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2) (Stock Raising Spec.)(#02683-)
YM***01Weight Assembly., Governor1
YM***50Angleich Ass'Y1
YM***00Washer,Spring1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump (Governor) (2/2)(#00119-)
YM***01Weight Assembly., Governor1
YM***50Angleich Ass'Y1
YM***00Washer,Spring1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump (Pump) (1/2) (Stock Raising Spec.)(#02683-)
YM***50Kit, Shim1
YM***10Plate? Caution1
YM***90Key,Woodruff1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump (Pump) (1/2)(#00119-)
YM***50Kit, Shim1
YM***10Plate? Caution1
YM***02Nut1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump (Pump) (2/2) (Stock Raising Spec.)(#02683-)
YM***70Retainer, Spring3
YM***00Plunger W/Barrel3
YM***02Screw2
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump (Pump) (2/2)(#00119-)
YM***70Retainer, Spring3
YM***00Plunger W/Barrel3
YM***02Screw2
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump And Piping (Stock Raising Spec.)(#02683-)
YM***20Retainer, Pipe1
YM***60O-Ring,Fuel Injection Pump1
YM***20Pipe,Injection? No.21
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Injection Pump And Piping(#00119-)
YM***20Retainer, Pipe1
YM***60O-Ring,Fuel Injection Pump1
YM***20Pipe,Injection? No.21
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Fuel System : Fuel Nozzle And Spill Pipe(#00119-)
YM***20Spring1
YM***00Pipe, Assembly1
YM***00Fuel Injection Nozzle, Assembly1
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Lubricating Oil System
Komatsu : 3d78ae-3a(S/N 00715-Up) : Lubricating Oil System : Lubricating Oil Filter(#00119-)
YM***50Nut1
YM***00Valve,Relief1
YM***02Bolt Plated3
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Lubricating Oil System : Lubricating Oil Pump And Piping(#00119-)
YM***50Oil Pressure Transmitter1
YM***30Bolt, Assembly1
YM***02Seal,Ring (Kit)2
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Tool And Engine Kit
Komatsu : 3d78ae-3a(S/N 00715-Up) : Tool And Engine Kit : Gasket Kit(#00119-)
YM***60O-Ring,Fuel Injection Pump1
YM***40O-Ring (Kit)2
YM***01Gasket (Kit)1
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Komatsu : 3d78ae-3a(S/N 00715-Up) : Tool And Engine Kit : Tool(#02683-)
YM***20Flex Horn (Option.)1
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3D78AE-3A(S/N 00715-UP) serial number reference

The engine model/serial plate sits on or near the valve (cylinder head) cover and reads the engine model (3D78AE-3A) followed by the serial digits, with 00715 and up marking this production run; if the plate is missing or worn, the serial is also stamped into the block. On the host WA20-2 wheel loader, the machine serial plate sits on top of the engine cylinder head cover, and the chassis PIN is stamped into the front frame's right side (facing direction of travel) behind the fender. Small compact Komatsu loaders of this era use a plain numeric serial run rather than a 3-letter alpha prefix ahead of the number.

PrefixIdentifies
No documented 3-character alpha PIN prefixKomatsu WA20-2 compact wheel loader (confirmed host machine for this engine/serial block)

Frequently asked questions

What engine powers the Komatsu 3D78AE-3A(S/N 00715-UP)?

It's a Yanmar-built, Komatsu-badged 3-cylinder direct-injection diesel, naturally aspirated, mechanically governed and mechanically injected with no electronic controls. Bore/stroke is 78 x 84 mm for 1.204 L displacement, rated 16.2 kW (21.7 hp) at 2,600 rpm with 73.5 N.m peak torque at 1,500 rpm. It's fitted to the Komatsu WA20-2 compact wheel loader.

What is the operating weight of the machine this engine powers?

The Komatsu WA20-2 host loader runs 1,735 kg with an open canopy or 1,885 kg with an enclosed cab, depending on configuration.

What replaced the Komatsu WA20-2 and its 3D78AE-3A engine?

Komatsu carried the WA20 nameplate forward through later generations, including WA20-3 and eventually WA20-5 and WA20-6, each stepping up to engines meeting tighter emission standards with electronic governing the 3D78AE-3A never had. The mechanically injected WA20-2 generation was phased out as those emissions-compliant engines became standard, though the mini wheel loader concept it established continues in Komatsu's current lineup.

What 3D78AE-3A(S/N 00715-UP) owners discuss

What engine is actually under the hood, and does it match Komatsu's own nameplate?
The confirmed host, the WA20-2 compact wheel loader, carries the 3D78AE-3A, a small three-cylinder direct-injection diesel built by Yanmar for Komatsu under their long-standing supply arrangement for compact equipment. Parts buyers find the fuel lift pump, starter, oil pump, and glow-plug timer cross-listed under both Komatsu and Yanmar catalog numbers for this engine. Two related codes sit in the same family: 3D78AE-3B and 3D78AE-3C. Those pair with small excavators, not this loader. When sourcing a long block or major fuel-system parts, match the full code and serial prefix stamped on the block rather than just the base number, since the wrong suffix points to excavator-spec hardware.
Why is it hard to start cold, and is that normal?
Slow cranking and white smoke on a cold start that clears once the engine settles is normal behavior for this small direct-injection diesel once it has hours on it, and it is one of the most repeated complaints for the family. The troubleshooting order mechanics use: test each glow plug for correct resistance first, then check the glow-plug timer relay if the plugs read good, then check for air in the fuel lines or a weeping injector. A weak battery makes the symptom worse since it drops cranking speed and glow-plug voltage at the same time.
What fuel-system wear shows up as the hours climb?
The mechanical inline injection pump uses a flyweight advance mechanism to shift timing with engine speed. On high-hour engines in this family it can stick or wear, showing up as black smoke, power loss, or harder starting. Fuel restriction is the other repeat theme: a clogged tank pickup, or fuel that sat long enough to grow contamination, starves the lift pump and injection pump alike and shows first as inconsistent power under load rather than a flat no-start. Flush the tank and lines and renew filters before condemning the injection pump.
Is the drivetrain hydrostatic or a torque converter?
This size and generation of Komatsu wheel loader runs a torque-converter powershift drivetrain (Komatsu's Torqflow line): a water-cooled torque converter feeding a planetary, multi-disc transmission, not the hydrostatic drive Komatsu introduced on later wheel loader generations. Hesitation on takeoff, shudder on directional shifts, or a transmission that runs hot point mechanics straight at transmission oil level and condition before anything mechanical, since degraded oil is the most repeated root cause reported for loaders this size.
Where does this machine wear first, since it has wheels and not track undercarriage?
For a wheel loader, the equivalent wear point is the center articulation joint. The wear cause cited almost without exception is dirt worked into the grease because the fitting wasn't wiped clean before greasing, so the trapped grit acts like grinding paste between pin and bushing. Vertical play felt at the joint during a turn, especially under load, signals the pins and bushings are past service limits. Final drive and axle hub seals on loaders this size are the other spot to check for oil weep at similar hour counts.
What electrical gremlins come up most on machines this size?
Recurring complaints on this class of Komatsu loader are dash resets, false alarms, hard starting, and unexplained shutdowns, and mechanics trace most of these to a weak battery, corroded connectors, or chafed wiring rather than a bad control module. On this engine family specifically, the glow-plug timer relay is a known wear item shared across several 3D78 variants. A no-start with power everywhere else often comes down to the seat or neutral-start safety interlock switch losing its ground path rather than a wiring fault at the starter itself. Because that switch is part of the starting safety circuit, have your dealer verify correct interlock operation rather than bypassing it.
What should I check before buying a used WA20-2 with this engine?
Confirm the engine serial prefix stamped on the block falls in the 3D78AE-3A range and matches the loader's own records. This block is shared with small excavators under different suffixes, and mismatched or swapped engines do turn up. Cold-start it from dead cold and watch smoke color and how quickly it clears. Raise the loader arm, hold it, and watch for drift over several minutes, ideally under a load. Cycle the transmission through forward and reverse under light load and listen for shudder or delay. Check the articulation joint for vertical play at full lock. Look for corrosion at the fuse box and glow-plug relay, and confirm the seat or neutral-start interlock actually stops the starter when tripped. The hydraulic hold test and the interlock check are both safety items. Have your dealer verify them during a pre-purchase inspection.

Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.

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