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Cummins/Onan 4B3.9-G2 Engine

Name: Cummins/Onan 4B3.9-G2 Engine
Brand: Cummins/Onan

The Cummins/Onan 4B3.9-G2 is a 3.92 L 4-cylinder naturally aspirated engine rated at 1800 rpm, producing up to 41 kW. It powers generator sets from Cummins/Onan, including the C40 D6 and C45D5. It holds 7.2 L (1.9 gal) of coolant.

Also known as: Cummins 4B3.9-G2

Part of the Cummins/Onan 4B3.9 engine family — 4 variants share parts & fitment.

3.92 L

Displacement

Cylinders

1800 rpm

Rated speed

41 kW

Gross power

On this pageSpecs Generator sets Maintenance FAQ Service intervals Torque & specs Fluids Fault codes Procedures

Engine specifications

Coolant capacity	7.2 L
Displacement	3.92 L
Cylinders	4
Aspiration	Naturally aspirated
Rated speed	1800 rpm
Gross power	41 kW

Generator sets powered by this engine

Cummins/Onan C40 D6 Cummins/Onan C45D5

How to read your Cummins/Onan engine serial & model number →

Common problems

Weakness and Sleepiness
Fix: IF YOU OR ANYONE ELSE EXPERIENCEANY OF THESE SYMPTOMS, GET OUT INTO THE FRESH AIR IMMEDIATELY. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired.
Vomiting
Fix: IF YOU OR ANYONE ELSE EXPERIENCEANY OF THESE SYMPTOMS, GET OUT INTO THE FRESH AIR IMMEDIATELY. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired.
Oil Pressure Sensor OOR - Low Code 141
Oil Pressure Sensor OOR - High Code 135
Muscular Twitching
Fix: IF YOU OR ANYONE ELSE EXPERIENCEANY OF THESE SYMPTOMS, GET OUT INTO THE FRESH AIR IMMEDIATELY. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired.
Low Battery Voltage Code 441
Inability to Think Coherently
Fix: IF YOU OR ANYONE ELSE EXPERIENCEANY OF THESE SYMPTOMS, GET OUT INTO THE FRESH AIR IMMEDIATELY. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired.
High Battery Voltage Code 442
Fuel contact with hot engine or exhaust
Fix: Do not permit any flame, cigarette, pilot light, spark, arcing equipment, or other ignition source near the generator set or fuel tank. Fuel lines must be adequately secured and free of leaks. Use an approved flexible line for fuel connection at the engine. Do not use copper piping on flexible lines as it will become brittle if continuously vibrated or repeatedly bent. Ensure all fuel supplies have a positive shutoff valve.
Exhaust gases are deadly
Fix: Provide an adequate exhaust system to properly expel discharged gases away from enclosed or sheltered areas, and areas where individuals are likely to congregate. Visually and audibly inspect the exhaust system daily for leaks per the maintenance schedule. Ensure that exhaust manifolds are secured and not warped. Do not use exhaust gases to heat a compartment. Make sure the unit is well ventilated.
Engine Speed High (Shutdown) Code 234
Engine Oil Pressure Low (Warning) Code 143
Engine Oil Pressure Low (Shutdown) Code 415
Engine Fail to Start Code 359
Engine Coolant Temperature OOR Low (Warning) Code 144

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan 4B3.9-G2.

Maintenance schedule

Item	Spec	Capacity	Part #
Air filter			—
Fuel filter			—
Oil filter			—
Battery	Set mounted starting battery.		—
Belt			—
Coolant		17.3 liters	—
Oil		10.9 liters	—
Spark plug			—

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.

Service intervals

Item	Action	Hours	Months	Notes
Exhaust system	Inspect	8 hr		Visually and audibly inspect the exhaust system daily for leaks per the maintenance schedule; check for corrosion, obstruction, and leaks every time you start the generator set and every eight hours when you run it continuously
Air cleaner restriction indicator	Check	24 hr		If service indicator shows red, replace air cleaner elements and reset indicator.
Coolant heater	Check	24 hr		Verify power and operation; check for leaks.
Coolant level of radiator(s)	Check	24 hr		Top up with Cummins recommended coolant mix if low.
Engine oil level	Check	24 hr		Top up with recommended oil if low.
Fuel level in fuel tank	Check	24 hr		Refill as required.
Fuel lines and hoses	Check	24 hr		Check for leaks, worn or damaged hoses.
Generator set enclosure	Inspect	24 hr		Walk-around; ensure no inlets/outlets covered.
Operator panel operation	Check	24 hr		Check display; replace component if not functioning.
Air intake system for leaks	Inspect	50 hr		Visually inspect; check audibly when running.
Battery connections	Check	50 hr		Verify connections are secure.
Charge alternator	Check	50 hr		Check visually and audibly when running.
Coolant lines and radiator hoses	Check	50 hr		Check for leaks, wear, and cracks.
Cooling fan blades	Inspect	50 hr		Visually inspect through guarding.
Drive belt condition and tension	Check	50 hr		Check for wear or slippage.
Exhaust components and hardware	Check	50 hr		Visually inspect; check audibly when running.
Radiator air flow	Inspect	50 hr		Check for blockage, debris, wear or damage.
Engine oil and filters	Replace	200 hr		First service — After the initial 50 hour interval and every 200 hours thereafter (footnote 3).
Fuel filter and water separator	Replace	500 hr
Cooling system coolant	Replace		24 mo

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Service specifications

Type	Item	Value	Unit	Condition
Gap	MPU sensor tip to ring gear teeth clearance	0.5 - 0.6	mm
Other	AFLEX-H (shake table test datum)	<=3.200	g	AC156, tri-axial, 5% damping SRS
Other	AFLEX-V (shake table test datum)	<=1.333	g	AC156, tri-axial, 5% damping SRS
Other	ARIG-H (shake table test datum)	<=2.400	g	AC156, tri-axial, 5% damping SRS
Other	ARIG-V (shake table test datum)	<=0.533	g	AC156, tri-axial, 5% damping SRS
Other	Flywheel teeth (Teeth Pulses Per Revolution)	110		PCC setting
Other	Glow plug preheat time	5		glow plug calibration
Other	Glow plug total glow plug time	60		glow plug calibration
Other	Ground snow load limit	30	psf	For indoor (covered) and outdoor (exposed) installation
Other	Horizontal design seismic coefficient (z/h = 0.0)	1.440	g	Certified seismic design level, IBC Importance IP <= 1.5, Soil Classes A-E, Risk Categories I-IV, Design Categories A-F, SDS <= 2.000 g
Other	SDS (g) at z/h=1 for C10 D6 - C60 D6	2.0	g	Certified SDS, z/h = 1
Other	SDS (g) at z/h=1 for C50 D6C - C125 D6C	2.5	g	Certified SDS, z/h = 1
Other	Speed/frequency mismatch threshold	2.0	Hz	PCC setting
Other	Speed/frequency mismatch threshold time	1.5	seconds	PCC setting
Other	ZPA-H (shake table test datum)	<=2.160	g	AC156, tri-axial, 5% damping SRS
Other	ZPA-V (shake table test datum)	<=0.480	g	AC156, tri-axial, 5% damping SRS
Resistance	Battery ground connection resistance	more than 1	Ohm	indicates repair needed
Resistance	Coolant temperature sensor resistance	540 Ohms at -40 F to 1100 Ohms at 100 F; 1100 Ohms at 100 F to 1800 Ohms at 230 F	Ohm
Resistance	Diode reverse direction resistance (good)	above 20	MOhm	reverse direction at 1000 VDC
Resistance	Starter solenoid wiring continuity (relay 87 to SW)	less than 5	Ohm
Resistance	Temperature sender harness lead resistance	less than 0.5	Ohm	each lead from connection to PCC
Resistance	Varistor resistance (good)	greater than 100	MOhm
Resistance	Winding test conditions	20	C	winding temperature for insulation measurement
Resistance	Windings minimum insulation resistance (in-service)	5	MOhm	1 minute at 500 V test voltage
Resistance	Windings minimum insulation resistance (new)	10	MOhm	1 minute at 500 V test voltage
Speed	Frequency-to-speed gain select	30	RPM/Hz	PCC setting
Speed	Overspeed Trip Level (60 Hz)	2250		PCC threshold
Speed	Secondary Overspeed Trip Speed (60 Hz)	2250		PCC threshold
Speed	Starter Disconnect Speed	400	RPM	InPower setting
Speed	Starter Disconnect Speed (code 5134)	450	RPM	InPower setting
Temperature	High coolant temperature shutdown threshold	230 F / 110	C	shutdown
Temperature	High coolant temperature warning threshold	220 F / 105	C	warning
Temperature	Low coolant temperature warning threshold	70 F (21 C)		4BT3.3 engines
Torque	Air cleaner strap clamp	3.4 to 4.5	Nm
Torque	Alternator drive disc bolt and adaptor bolt (section 12.5)	52 +/- 5	Nm
Torque	Alternator drive disc bolts and adaptor bolts	47 to 57	Nm
Torque	Bolts on engine mounting brackets to engine	47 to 57	Nm
Torque	Bottle bolt to bracket	3 to 4	Nm
Torque	CAC inlet and outlet connections	10 to 12	Nm
Torque	Coolant reservoir bottle to fan guard	10 to 12	Nm
Torque	Diode into rectifier plate	2.6 to 3.1	Nm
Torque	Enclosure top panel bolts	9.8 to 11.9	Nm
Torque	Equipment grounding lug (AC load connections)	13.8	Nm (120 in-lb)
Torque	Equipment grounding lug (circuit breaker)	13.8	Nm (120 in-lb)
Torque	Exhaust connections, engine	3.4 to 4.5	Nm
Torque	Fan to hub bolt	24 to 29	Nm
Torque	Generator set enclosure panel fasteners	9.8 to 11.9	Nm
Torque	Ground lug to terminal box	4.7 to 5.7	Nm
Torque	M6 hex head screws	9.8 to 11.9	Nm
Torque	Muffler clamps (three)	24 to 29	Nm
Torque	Muffler end bolts (two each end)	24 to 29	Nm
Torque	NDE/terminal covers torque #8 star head bolt	4 to 5	Nm
Torque	Neutral lug (AC load connections)	31.1	Nm (275 in-lb)
Torque	Neutral lug (circuit breaker)	31.1	Nm (275 in-lb)
Torque	Neutral lug to terminal box	12.3 to 15	Nm
Torque	Nuts to isolator stud (engine/alternator isolators)	82 to 100	Nm
Torque	Radiator side bracket bolts	24 to 29	Nm
Torque	Terminal box cover panels	4.7 to 5.7	Nm
Torque	Terminal box mounting bolts (6 each side)	12.3 to 15	Nm
Voltage	12V Charging Alternator Disconnect Voltage	9	VDC	InPower setting

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Recommended fluids

Fluid	Grade	Standard	Capacity	Notes
Coolant	50/50 coolant solution (50% pure water and 50% ethylene glycol)
Coolant	50/50 coolant solution (50% water / 50% ethylene glycol)
Engine oil	15W40		7.9 L (8.35 qt)	Refer to the Engine Operator Manual
Fuel	ASTM number 2D fuel	ASTM 2D
Grease	Dow Corning silicone heat sink compound type 340 or similar			Apply to base of replacement diode only

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Fault codes

Code	Name	Meaning	Severity	Action
121	Loss of Speed Sense	No engine speed signal detected from crankshaft speed or camshaft position.	Shutdown	Inspect MPU sensor and harness; set MPU tip clearance 0.5-0.6 mm; verify calibration.
1311	Customer Input #1	Configurable customer input 1 fault.	Warning	Check TB1-12; check external wiring; verify fault function settings.
1312	Customer Input #2	Configurable customer input 2 fault.	Warning	Check TB1-14; check external wiring; verify fault function settings.
1317	Low Coolant Level	Optional coolant level sensor indicates low coolant level.	Warning	Add coolant; check coolant level sensor connections and operation; verify calibration.
1318	Low Fuel	Optional low fuel level switch indicates low fuel.	Warning	Add fuel; verify switch operation; check wiring; verify settings.
135	Oil Pressure Sensor OOR - High	Engine oil pressure sensor signal out of range - shorted high.	Warning	Inspect sensor connections; check supply 4.75-5.25 VDC and signal voltage.
141	Oil Pressure Sensor OOR - Low	Engine oil pressure sensor signal out of range - shorted low.	Warning	Inspect sensor connections; check supply and signal voltage.
1417	Power Down Failure	The PCC has failed to go to sleep.	Warning	Verify calibration; cycle B+ power; replace baseboard if fault persists.
143	Engine Oil Pressure Low	Engine oil pressure below low oil pressure warning threshold (40 psig for 5 seconds).	Warning	Check oil level, external leaks, sensor accuracy, oil cooler, regulator, oil pump.
1433	Local E-Stop	State of the Local Emergency Stop has been changed.	Shutdown	Reset local E-Stop; check E-Stop switch and wiring.
1434	Remote E-Stop	State of the Remote Emergency Stop has been changed.	Shutdown	Reset remote E-Stop; install jumper TB1-16 to TB1-15; check switch and wiring.
1435	Low Coolant Temperature	Engine coolant temperature below low coolant temperature warning threshold (70 F / 21 C).	Warning	Verify LCT threshold; check coolant heater operation; check ambient temperature.
1438	Fail to Crank	Engine failed to crank after the generator control received a start signal.	Shutdown	Check battery, starter, starter solenoid, starter relay, CAM/Crank sense.
144	Engine Coolant Temperature OOR Low	Engine coolant temperature signal out of range - shorted low.	Warning	Inspect sensor connections; check sensor resistance; verify calibration.
1442	Weak Battery	Battery voltage dropped below weak battery voltage threshold (8 V for 2 seconds) while cranking.	Warning	Check battery, connections, charging voltage, DC alternator, harness.
1446	High AC Voltage	One or more phase voltages exceeded high AC voltage threshold (110% of nominal, 10 seconds).	Warning	Check threshold, PCC AVR circuit, generator output connections, PMG, EBS.
1447	Low AC Voltage	One or more phase voltages dropped below low AC voltage threshold (85%, 10 seconds).	Shutdown	Check AVR settings, rotating rectifier diodes, overload, residual magnetism, connections.
1448	Underfrequency	Frequency dropped below underfrequency threshold (6 Hz) for 10 seconds.	Shutdown	Check overrides, threshold, overload, engine governor.
1449	Overfrequency	Frequency went above overfrequency threshold (6 Hz) for 10 seconds.	Shutdown	Verify overfrequency threshold and time delay; check engine governor.
145	Engine Coolant Temperature OOR High	Engine coolant temperature signal out of range - shorted high.	Warning	Inspect sensor connections; check sensor resistance; verify calibration.
146	Engine Coolant Temperature Above Normal	Engine coolant temperature exceeded warning threshold (220 F / 105 C).	Warning	Check ambient temperature, coolant level, cooling components, sensor, thermostat, water pump.
1469	Speed/Hz Mismatch	Perceived engine speed and generator set output frequency do not match.	Shutdown	Check flywheel teeth (110), MPU signal, thresholds (2.0 Hz, 1.5 s), gain (30 RPM/Hz).
1471	High AC Current	Generator output current exceeded warning limit threshold (110% for 60 seconds).	Warning	Check for short, overload, CTs and CT setup, rating setup, fault override.
1472	High AC Current	Generator output current exceeded shutdown set limit (150% for 10 seconds).	Shutdown	Check for short/overload, CTs and CT setup, rating setup, fault override.
151	Engine Coolant Temperature High	Engine coolant temperature exceeded shutdown threshold (230 F / 110 C).	Shutdown	Check cooling system, coolant level, sensor, thermostat, water pump.
1853	Annunciator Input 1 Fault	Customer fault 1 (input 1, LED 1) on the universal annunciator is active.	Warning	Check condition and wiring at TB1-1; verify configuration; replace annunciator if faulty.
1854	Annunciator Input 2 Fault	Customer fault 2 (input 2, LED 2) on the universal annunciator is active.	Warning	Check condition and wiring at TB1-2; verify configuration; replace annunciator if faulty.
1855	Annunciator Input 3 Fault	Customer fault 3 (input 3, LED 3) on the universal annunciator is active.	Warning	Check condition and wiring at TB1-3; verify configuration; replace annunciator if faulty.
1944	Annunciator Configuration Error	More than one network device configured to activate one annunciator output relay.	Warning	Check network configuration and devices for duplicate use of the same relay output.
2335	Excitation Fault (Loss of AC Sense)	Control lost voltage sensing or lost zero cross sensing.	Shutdown	Check configuration, fuse F5 (10 Amp), harness connections, residual magnetism, excitation coil, control board.
234	Engine Speed High	Engine speed signals indicate engine speed greater than shutdown threshold.	Shutdown	Check MPU sensor and installation; verify overspeed trip level and flywheel teeth (110).
2676	Alternator Frequency Conflict	Alternator line frequency and alternator excitation frequency do not match.	Shutdown	Check alternator line frequency gain; check J22 and J18 connections; check harness and control board.
2677	Fail to Stop	Generator set continues to run after receiving shutdown command from controller.	Shutdown	Check stuck fuel actuator/fuel shutoff valve; check governor module and wiring.
2729	IO Module Lost	Intermittent data link between I/O module and PCC (no input fault levels set to Shutdown).	Warning	Check AUX 101 wiring to PCC 1302; verify System IO Board Enable is disabled.
2731	IO Module Lost	Intermittent data link between I/O module and PCC (at least one input fault set to Shutdown).	Shutdown	Check AUX 101 wiring to PCC 1302; verify I/O settings.
2897	Factory Memory Block Corrupt	The control detected a corrupted memory block.	Warning	Connect with InPower 10.0+; select Initial Calibration (Recover Device).
2898	Periodic or Fault Memory Block Corrupt	The control detected a corrupted memory block.	Warning	Connect with InPower 10.0+; select Initial Calibration (Recover Device).
2899	User Memory Block Corrupt	The control detected a corrupted memory block.	Warning	Connect with InPower 10.0+; select Initial Calibration (Recover Device).
2911	Trim Memory Block Corrupt	The control detected a corrupted memory block.	Warning	Connect with InPower 10.0+; select Initial Calibration (Recover Device).
2972	Field Overload	Field AVR Duty Cycle operated at maximum output longer than Max Field Time parameter.	Shutdown	Check voltage sensing into baseboard; check AVR output; check application.
359	Engine Fail to Start	Engine failed to start after expiration of last crank time.	Shutdown	Check flywheel teeth setting (110), starter disconnect speed (400 RPM), fuel supply, glow plugs.
415	Engine Oil Pressure Low	Engine oil pressure below low oil pressure shutdown threshold (35 psig).	Shutdown	Check oil level, external leaks, sensor accuracy, oil cooler, oil pump.
441	Low Battery Voltage	Battery voltage is low.	Warning	Check battery cables, battery condition, ground connection, DC alternator, charger.
442	High Battery Voltage	Battery voltage above high battery threshold (16 V for 60 seconds).	Warning	Verify charger set to 12 VDC; check fault threshold; check engine DC alternator.
5134	Unknown Shutdown	PCC detected that engine RPM decreased to zero while not in run at rated mode.	Shutdown	Check calibration, air intake/exhaust restriction, MPU signal, start disconnect, fuel supply.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Procedures

Disabling the AMF Feature adjustment

⚠ Must be performed by a qualified technician.

Tools: InPower service tool

Connect to the PCC 1.1 or PCC 2.3 via InPower.
Navigate to Adjustments->Features->Automatic Transfer Switch folder.
Select the Auto Mains Failure Enable parameter.
Double-click on Enabled in the Value field.
Select Disabled from the pop-up choices.
Select Device->Save Adjustments from the top menu bar.
Click Save on the confirmation pop-up.
Disconnect InPower from the PCC1302 after confirmation.

Fuel Injection Pump Speed Lever Adjustment adjustment

Make sure the generator set is off and the battery is disconnected.
If replacing pump or engine, discard speed adjust screw and tamper proof wire and seal.
Replace the low idle screw; reuse throttle screw nut and acorn nut.
Pull throttle lever back and screw new low idle screw until it contacts boss.
Continue tightening until throttle lever is locked in place.
Tighten the throttle lock nut.
Install tamper proof wire and seal.
Set the throttle lever as shown and lock it using seal wire.

AC Load Connections (Circuit Breaker) installation

⚠ Improper installation can lead to electrocution and property damage.

Tools: Insulated wrench, Torque wrench

Shut down and disable the generator set (Off then E-Stop, cool down).
Turn off battery charger AC and disconnect negative battery cable.
Remove enclosure side panel to access main circuit breaker box.
Place circuit breaker handle in OFF position.
Remove four bolts holding circuit breaker cover.
Connect conductors to load-side terminals, neutral lug, and grounding lug.
Torque circuit breaker terminals per breaker label.
Torque neutral lug to 31.1 Nm (275 in-lb).
Torque equipment grounding lug to 13.8 Nm (120 in-lb).
Fill stub-up openings with duct seal or mastic tape.
Install circuit breaker cover.

Connect Battery installation

⚠ Do not connect battery cables until installation is complete to prevent accidental startup.

Connect the positive battery terminal.
Connect the negative battery terminal.
Make sure black and red battery cable boots are in place.

Connecting the Battery installation

⚠ Accidental or remote starting of the generator set can cause severe personal injury or death.

Press the generator set's O (Off) button and allow it to cool.
Turn off and disconnect the battery charger from the AC source before disconnecting battery cables.
Disconnect the negative (-) cable from the battery.
Connect the positive battery terminal.
Connect the negative battery terminal.
Make sure black and red battery cable boots are in place.

Adding Oil maintenance

⚠ Contact with hot engines can cause severe burns.

Ensure the oil fill cap area is clean.
Add the appropriate amount of oil.
Recheck the engine oil level.
Clean up and dispose of any oil per local regulations.

Changing Engine Oil and Oil Filter maintenance

⚠ Make sure the generator set is shut down and disabled.

⚠ Avoid skin contact and breathing of vapors.

Operate generator set until water temperature is approximately 60 C (140 F).
Shut down and disable the generator set; disconnect negative battery cable.
Drain the oil into an appropriate container.
Remove the oil filter and clean the filter mounting surface.
Apply a thin film of clean oil to the new filter gasket.
Install new filter until gasket touches, then turn 1/2 to 3/4 more.
Close drain valve and add the appropriate amount of oil.
Run engine at idle; check and repair any leaks.

Checking Engine Oil Level maintenance

⚠ Crankcase pressure can blow out hot oil and cause severe burns.

⚠ Do not check oil while the engine is operating.

Make sure the generator set has not run for approximately five minutes.
Clean off the area surrounding the dipstick port.
Pull out the dipstick and wipe it clean.
Reinsert and fully seat the dipstick.
Remove the dipstick and check the oil level.
Reinsert and fully seat the dipstick.

Cleaning Radiator Cores Using Pressurized Water Equipment maintenance

⚠ Do not use cleaners containing ammonia.

⚠ Wear PPE when cleaning with a pressure washer or compressed air.

Tools: Industrial pressure washer, PPE

Remove enclosure end panel(s) to assist cleaning.
Inspect radiator exterior for obstructions.
Pressure wash in the opposite direction to the airflow.
Keep lance at a right angle to core, minimum 18 inches (450 mm) distance.
Replace the end panel(s) where necessary.

Draining Oil maintenance

Detach the oil drain hose from the side of the engine.
Place the end of the drain hose into an appropriate container.
Open the oil drain valve to release oil.
Close the oil drain valve when sufficient oil drained.
Wipe the oil drain valve clean and re-attach the drain hose.

Heavy Duty Air Cleaner Element Removal maintenance

⚠ Do not clean paper-type air cleaner elements.

Wipe dirt from the cover and upper portion of air cleaner.
Lift tab (3) and turn end cover (4) counterclockwise.
Pull the end cover (4) away from the housing (1).
Remove the air filter element (2) from the housing.
Dispose of the dirty element per local environmental requirements.

Locking the Generator Set Out of Service maintenance

⚠ Accidental or remote starting of the generator set can cause severe personal injury or death.

Tools: Insulated wrench

Press Off switch, then press E-Stop button.
Thoroughly ventilate the generator set before disconnecting any leads.
Turn off and disconnect the heater (where fitted) from AC source.
Turn off and disconnect the battery charger (where fitted) from AC source.
Turn off the fuel supply to the engine.
Disconnect the battery negative cable first using an insulated wrench.
Place warning notices at each location.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Parts cataloged across the Cummins/Onan 4B3.9 family

Part #	Description	Category
982-0200	Air Filter	air_filter
300-3606	Voltage Regulator Installation (PMG-Excited Generators), Rev. P	avr
300-3607	Voltage Regulator Installation (Self-Excited Generators), Rev. G	avr
220-4289-31	Main stator assembly (broad range generators)	other_service
220-4289-32	Main stator assembly (broad range generators)	other_service
220-4289-33	Main stator assembly (broad range generators)	other_service
220-4289-34	Main stator assembly (broad range generators)	other_service
220-4289-35	Main stator assembly (broad range generators)	other_service
220-4298-31	Main stator assembly	other_service
220-4298-32	Main stator assembly	other_service
220-4298-33	Main stator assembly	other_service
220-4298-34	Main stator assembly	other_service
220-4298-35	Main stator assembly	other_service
220-4298-36	Main stator assembly	other_service
300-4111	Auxiliary Relay Board (ARB), Rev. E	other_service
319-0969	QS-Series Power Distribution Panel Wiring, Rev. G	other_service
612-6487	7-Light DC Control Wiring Diagram, Sh 1 and 2 of 2, Rev. V	other_service
612-6488	12-Light DC Control Wiring Diagram, Sh 1 and 2 of 2, Rev. W	other_service
612-6489	AC Control Wiring Diagram (Without Meters), Rev. S, modified 6/2001	other_service
612-6490	AC Control Wiring Diagram (With Meters), Rev. ZB, modified 1/2002	other_service
625-2164	Generator Reconnection Diagrams, Sh 1 and 2 of 2, Rev. K	other_service
625-2256	QS-Series Generator Connections When The Voltage Selector Switch Is Provided, Rev. B	other_service
965-0520	Cooling Fan	other_service
625-2141	Day Tank Pump Control Wiring, Rev. P	pump

Parts cataloged across all variants of this engine family — verify the exact part fits your specific variant and serial number before ordering.

Common questions

What is the displacement of the Cummins/Onan 4B3.9-G2?

The Cummins/Onan 4B3.9-G2 has a displacement of 3.92 liters with 4 cylinders.

Which generators use the Cummins/Onan 4B3.9-G2 engine?

The Cummins/Onan 4B3.9-G2 powers 2 generator set(s), including Cummins/Onan C40 D6, Cummins/Onan C45D5.

What is a common problem with the Cummins/Onan 4B3.9-G2 engine?

A commonly reported issue on generators using the Cummins/Onan 4B3.9-G2 is: Weakness and Sleepiness. Fix: IF YOU OR ANYONE ELSE EXPERIENCEANY OF THESE SYMPTOMS, GET OUT INTO THE FRESH AIR IMMEDIATELY. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired..