Cummins/Onan 4BT3.9 Generator Set

Tools: Frequency meter or tachometer

1. Start the generator set.
2. Set no-load speed for desired droop (isochronous 60.0 Hz/1800 r/min).
3. With no load, turn GAIN clockwise until actuator lever oscillates; reduce until stable.
4. Reduce GAIN one division, then turn fully clockwise while observing lever.
5. Turn I adjustment counterclockwise until lever stable; it should oscillate up to 3 times.
6. Apply and remove loads to check response; recalibrate if unsatisfactory.

Tools: Screwdriver

1. Set frequency adjust (engine speed) potentiometer to mid position and tighten locking nut.
2. Set I potentiometer one division from zero.
3. Set Gain potentiometer at the third division from zero.
4. For isochronous operation turn Droop potentiometer fully counterclockwise.
5. Turn droop screw to approximately 40 for 3 percent droop.
6. Turn droop screw to approximately 80 for 5 percent droop.

Tools: Frequency meter or tachometer, Wrench

1. Use the frequency control meter (if equipped) or an accurate tachometer.
2. Loosen the speed adjustment screw locknut.
3. Start set, warm to operating temperature, then apply full rated load.
4. Adjust speed adjust screw in or out to reach required speed (60 Hz - 1800 r/min, 50 Hz - 1500 r/min).
5. Hold screw at proper setting and retighten locknut.
6. Remove and apply loads to check response; readjust if necessary.

⚠ Seated voltage regulator potentiometers are factory calibrated; further adjustments only by qualified service representative.

Tools: Screwdriver

1. Adjust voltage control rheostat R21 (if available) to mid position.
2. Open control panel doors to gain access to voltage regulator.
3. With set operating and voltage monitored, adjust Volts potentiometer to desired voltage.
4. Perform fine voltage adjustment (+/-3 percent) with rheostat R21, retighten locking nut.
5. Stop and restart generator set to confirm proper operation.

⚠ Contact with high voltage can cause severe personal injury or death. Do not touch exposed wiring or components.

Tools: Insulation mat or dry wood platform

1. Refer to order, installation wiring diagrams and unit wire routing to confirm signal source connections.
2. Apply appropriate signal voltage (12-VDC; 240-, 480-VAC); solenoid should trip breaker open.
3. If breaker did not trip open, remove signal source; check continuity of interconnect wiring and solenoid leads.

Tools: Insulation resistance meter (at least 500 volts)

1. Disconnect load and line conductors; place breaker in On position.
2. Use insulation resistance meter applying at least 500 volts to test leads.
3. Measure insulation resistance between each pole, and to ground.
4. Also test between line and load terminals with breaker in Off position.
5. Reading less than 100,000 ohms indicates a ground; clean case surfaces and retest.

⚠ Place Run/Stop/Remote switch in Stop position and disconnect battery negative (-) cable.

Tools: Ohmmeter

1. Disconnect the line and load wires from the circuit breaker.
2. Move breaker handle to On position and check resistance across each pole (line to load).
3. Resistance should be very low (near zero) and relatively equal across all poles.

⚠ Excessive dust or dirt on diodes causes overheating and eventual failure; keep assemblies clean.

Tools: Accurate ohmmeter

1. Disconnect all leads from assembly to be tested.
2. Connect meter lead to positive (X) stud; test CR1, CR2, CR3; record values.
3. Connect meter lead to negative (XX) stud; test CR4, CR5, CR6; record values.
4. Reverse meter leads and repeat; readings should be high in one test, low in other.
5. Replace assembly if any reading is high or low in both tests.

⚠ Ignition of explosive battery gases can cause severe personal injury; do not smoke while servicing batteries. Wear protective apron and goggles.

Tools: Hydrometer

1. Check that connections are clean and tight; apply light coating of non-conductive grease.
2. Check charge condition of starting batteries with a hydrometer.
3. Keep electrolyte at proper level above plates by adding distilled water.
4. Check specific gravity and re-charge if below 1.260.

⚠ The rotor assembly should under no circumstances be dismantled, or magnetic properties will be destroyed.

⚠ The highly magnetic rotor will attract the stator core; avoid contact that may damage windings.

Tools: Wrench, Hoist or lifting device

1. Disconnect negative (-) battery cable to prevent accidental starting.
2. Remove endbell cover and conduit box access covers.
3. Disconnect and withdraw leads P2, P3, P4 from auxiliary terminal block.
4. Remove bolts and clamps retaining exciter stator to endbracket.
5. Guide stator assembly from endbracket, avoiding contact with magnetic rotor.
6. Remove through-bolt and firmly pull complete rotor assembly from location.
7. Locate dowel pin on rotor shaft; remove and retain for reassembly.

⚠ Highly magnetic rotor will attract stator core; avoid contact which may damage windings.

Tools: Hammer, Torque wrench

1. Place dowel pin into locating hole at end of main rotor shaft; tap to secure fit.
2. Assemble permanent magnet to main rotor shaft using dowel pin; secure with through-bolt; torque 40 to 45 ft-lbs (54 to 61 N*m).
3. Place stator on endbracket with leads at 12 o'clock position; secure with bolts and clamps.
4. Route and connect VR21 and PMG leads P2, P3 and P4 to auxiliary terminal block.
5. Mount endbell cover with four M5 screws and lockwashers.

⚠ Do not apply heatsink compound to diode stud threads.

Tools: Proper size wrench, Torque wrench

1. Disconnect diode lead wire from stud terminal.
2. Use proper size wrench to unscrew diode from rectifier assembly base.
3. Apply heatsink compound to underside of new diode; not to stud threads.
4. Insert new diode into mounting hole; torque to 36-42 in-lbs (4-4.8 N*m).
5. Reconnect diode lead wire to stud terminal; torque nuts to 24 in-lbs (2.7 N*m).