Komatsu D20/D21 (1986-1994)/PC300-5 (6D125) 24V 40A Generator - Installation Method & Precautions

I. Generator Installation Method (Standardized Steps for Both Models)

Preparations

1. Turn off the main power switch of the whole machine and remove the negative terminal of the battery (for 24V models, remove the negative terminal first to prevent short circuit due to grounding).

2. Clean oil stains, rust and dust on the generator mounting position (engine block/bracket), pulley and tensioner, and check for deformation of bolt holes and locating pins on the mounting bracket.

3. Prepare special tools: torque wrench (30-100N·m), socket set, flathead/cross screwdriver, belt pry bar, and spare flat washers/spring washers (Komatsu original factory specification for anti-loosening).

Step 1: Lifting and Preliminary Positioning of the Generator Body

1. Place the generator into the locating slot/locating pin of the mounting bracket on the engine side, ensuring the mounting lugs of the generator fit the bracket without gaps (D20/D21 adopt side cantilever mounting, while PC300-5 (6D125) adopts front gear chamber side mounting of the engine, both with locating pins to prevent offset).

2. Insert 2 main mounting bolts (coarse thread, M12/M14 depending on the model), put on flat washers and spring washers, and screw them by hand until they fit (do not tighten, leave adjustment allowance for subsequent belt tensioning).

3. Install the adjusting arm bolt (tension side) of the generator and also screw it by hand until it fits, keeping the generator capable of slight swing.

Step 2: Installation and Tensioning of the Drive Belt (Key Adaptation, Universal for Both Models)

1. According to the engine belt routing diagram (pasted on the engine side cover for D20/D21 and on the gear chamber cover for PC300-5), fit the special V-belt (multi-rib belt) for 24V models onto the generator pulley, crankshaft pulley and tensioner, ensuring the belt is seated in the groove without deflection.

2. Gently pry the generator body with a pry bar (towards the side far from the crankshaft) to tension the belt. Belt Tension Standard: Press the middle section of the belt (between the two pulleys) with fingers, the depression amount shall be 10-15mm (adapted tension for 24V 40A generator; loose belt will slip and burn, over-tightened belt will wear bearings).

3. After tensioning, first tighten the adjusting arm bolt, then tighten the main mounting bolts to complete the belt tensioning and fixation.

Step 3: Electrical Circuit Connection (Absolute Polarity Correctness for 24V Models, Critical!)

• B+ Terminal (thick terminal, M10/M12 with insulating sheath): Positive output of the generator, connected to the positive terminal of the battery/main power busbar.

• D+ Terminal (thin terminal, M6): Excitation signal wire, connected to the voltage regulator/instrument panel charging indicator light.

• G Terminal: Ground wire terminal, connected to the engine block/frame (ensure good grounding; poor grounding of 24V models will cause insufficient charging).

• Plug-in circuit: Komatsu original factory adopts waterproof plugs, divided into 2-core/3-core types, insert according to the plug buckle alignment to prevent loosening.

1. First connect the ground wire (G Terminal): Crimp the ground wire terminal firmly, put on a spring washer, tighten the bolt (torque 8-12N·m), and ensure close fit with the metal surface of the engine block without paint/oil stains.

2. Then connect the excitation wire (D+ Terminal): Wrap the thin wire terminal with an insulating sleeve, tighten the bolt (torque 6-8N·m) to avoid terminal grounding.

3. Finally connect the positive output wire (B+ Terminal): A Komatsu original anti-loosening washer must be added to the thick cable terminal, tighten the bolt (torque 25-30N·m), and put on an insulating sheath after connection to prevent grounding with the engine block/bracket.

4. Inspect all circuits: No winding, no contact with high-temperature components (exhaust pipe, engine block), fix the wire harness on the engine bracket with buckles, and reserve 5-10mm of movement allowance (prevent wire breakage caused by engine vibration).

Step 4: Battery Reset and Preliminary Inspection

1. Refit the negative terminal of the battery, tighten the bolt (torque 15-20N·m), and close the main power switch.

2. Manually rotate the crankshaft pulley to check if the generator pulley rotates smoothly without jamming or abnormal noise, and the belt has no eccentric wear or off-groove.

3. Inspect the appearance of the generator: All bolts and circuit terminals are not loose, and the insulating sheath is not damaged.

Step 5: Start-up Test (Verify Installation Effectiveness)

1. Start the engine and run at idle speed (600-700r/min), observe the charging indicator light on the instrument panel: It shall go out immediately after start-up; if it stays on, there is a circuit/excitation fault.

2. Measure the battery terminal voltage with a multimeter: ≥26V at idle speed, and the voltage is stably 27-28V when accelerating to 1500r/min without load (standard charging voltage of 24V 40A generator, controlled by Komatsu original voltage regulator).

3. Load test (optional): Turn on the lighting, air conditioning and other electrical equipment of the whole machine, the voltage remains above 26.5V without obvious drop, indicating the generator works normally.

II. Core Installation Precautions (Divided into Universal Precautions & Model-Specific Precautions)

(I) Universal Precautions (Must Be Followed for All Models, Related to Equipment Safety)

1. Electrical Safety (Top Priority, Avoid Generator Burnout/Electric Shock)

•  It is strictly forbidden to connect wires with power on: The negative terminal of the battery must be disconnected when installing/dismantling circuits, otherwise the generator voltage regulator will be burned, the fuse will be blown, and even the battery will explode.

•  It is strictly forbidden to directly ground the B+ terminal: Even short-time grounding will burn the generator stator/rotor coil; the short-circuit current of 24V models is extremely large, which will also ablate the circuit.

•  Absolute polarity correctness: Connect all circuits in the order of "ground → excitation → positive"; reverse connection will directly damage the generator and voltage regulator (Komatsu original regulator is non-repairable).

2. Mechanical Assembly (Reach Standard Torque, Anti-loosening and Anti-vibration)

• All mounting bolts must use spring washers/lock nuts (Komatsu original factory adopts galvanized anti-loosening parts, ordinary washers are prohibited to replace); engine vibration will cause bolt loosening, leading to belt slipping and generator falling off.

• Strictly reach the standard torque: Main mounting bolts (M12) 35-40N·m, (M14) 45-50N·m; adjusting arm bolts 20-25N·m. It is strictly forbidden to violently tighten with a wrench, which will cause thread slipping/deformation of the mounting bracket.

• Locating pins must be installed: The locating pin between the generator and the bracket is the key to ensuring the coaxiality of the pulley; the lack of locating pins will cause eccentric wear of the belt and early damage to the generator bearing.

3. Belt and Tensioning (Affect Generator Service Life and Charging Efficiency)

• Must use Komatsu original/adapted specification belts: The adapted belt models for D20/D21/PC300-5 (6D125) 24V 40A models are 3PK/4PK-850/900 (depending on the model); non-original belts are easy to stretch and slip, leading to insufficient charging.

• Do not over-tension the belt: Over-tightened belt will cause excessive stress on the generator bearing and crankshaft bearing, resulting in early abnormal noise and jamming, and the service life will be shortened by more than 50%.

• Regularly inspect the belt groove: If the generator/crankshaft pulley groove is worn or frayed, it must be replaced in time, otherwise the belt is easy to come off the groove.

4. Grounding and Heat Dissipation (Key for 24V Models)

• The ground must be directly connected to the engine block, and it is forbidden to connect to the frame/covering parts; under the high current of 24V, indirect grounding will cause excessive contact resistance, insufficient charging and circuit heating.

• No obstruction to the generator heat dissipation air inlet: The rear end of the generator is a heat dissipation fan; during installation, ensure a gap of ≥30mm is reserved with surrounding components (such as air filter, oil pipe) to avoid oil stains entering the heat dissipation air inlet and causing overheating and burning of the coil.

• References：

1. Komatsu Original Factory Service Manual: KOMATSU D20/D21 Series Service Manual (SEBM026700, 1994 Edition), PC300-5 Hydraulic Excavator Service Manual (SEBM019800, 6D125 Engine Chapter).

2. Komatsu Generator Accessory Technical Specification: KOMATSU Electrical Parts Catalog (600 Series, 24V Generator Section).

3. Construction Machinery Industry Standard: JB/T 8128-2010 Technical Conditions for Generators for Construction Machinery (adapted to 24V 40A specification).