Home›Engines›Cummins/Onan 4B3.9-G

Cummins/Onan 4B3.9-G Engine

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

The Cummins/Onan 4B3.9-G is a 3.92 L naturally aspirated engine rated at 1800 rpm, producing up to 36 kW. It powers generator sets from Cummins/Onan, including the 28DGBB, 35 DGBB, 36 DGBC and 40 DGBC. It holds 11.5 L (3.0 gal) of oil and 21 L (5.5 gal) of coolant.

Also known as: Cummins 4B3.9-G

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

3.92 L

Displacement

1800 rpm

Rated speed

36 kW

Gross power

11.5 L

Oil capacity

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

Engine specifications

Oil capacity	11.5 L
Coolant capacity	21.0 L
Displacement	3.92 L
Aspiration	naturally aspirated
Rated speed	1800 rpm
Gross power	36 kW

Generator sets powered by this engine

Cummins/Onan 28DGBB Cummins/Onan 35 DGBB Cummins/Onan 36 DGBC Cummins/Onan 40 DGBCCummins/Onan DTBACummins/Onan DXBA

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

Common problems

SWITCH OFF lamp flashes.
Fix: Indicates Run/Stop/Remote switch is in the Stop position which will prevent automatic starting if an automatic transfer switch is used. Move the Run/Stop/Remote switch to the Remote position for automatic starting.
OVERCRANK lamp lights and engine stops cranking.
Cause: Indicates possible fuel system problem.
Fix: A. Check for empty fuel tank, fuel leaks, or plugged fuel lines and correct as required. b. Check for dirty fuel filter and replace if necessary (see Maintenance section). c. Check for dirty or plugged air filter and replace if necessary (see Maintenance section). d. Reset the control and restart after correcting the problem. Contact an authorized service center for service if none of the above.
LO FUEL LAMP lights. Engine shuts down and LO OIL PRES lamp lights.
Fix: Indicates engine has run out of fuel. Check fuel level and replenish as required.
HI ENG TEMP lamp lights. Engine shuts down.
Fix: Allow engine to cool down completely before proceeding with the following checks: a. Check coolant level and replenish if low. Look for possible coolant leakage points and repair if necessary. b. Check for obstructions to cooling airflow and correct as necessary. c. Check for a slipping fan belt and tighten if loose. d. Reset control and restart after locating and correcting problem. Contact an authorized service center for service if none of the above.
Engine runs and then shuts down, OVERSPEED lamp lights.
Cause: Indicates engine has exceeded normal operating speed.
Fix: Contact an authorized service center for service.
The FAULT 1 or FAULT 2 fault lamp lights. Engine shuts down immediately, engine runs for several seconds and then shuts down, or engine continues to run.
Cause: The standard undesignated fault functions are programmed to shut down the set when a fault is sensed. Fault 1 is time delayed while Fault 2 is immediate. The nature of the fault is an optional selection that is determined when the set installation is designed. The undesignated fault functions may also be programmed for non-shutdown or non-time delay.
The external alarm and fault lamp may be deactivated by moving the Run/Stop/Remote switch to the Stop position and pressing the Reset/Lamp Test/Preheat switch.
Fix: Locate the problem and correct it before restarting the set. While pressing the Reset/Lamp Test/Preheat switch, make certain that all lamps light.
PRE LO OIL PRES lamp lights. Engine continues to operate.
Cause: Indicates engine has overheated (engine temperature has risen above 222F/106C) or coolant level is low (sets with coolant level sensor).
Fix: A. Check coolant level and replenish if low. Look for possible coolant leakage points and repair if necessary. b. Check for obstructions to cooling airflow and correct as necessary. c. Check for a slipping fan belt and tighten if loose. d. Reset control and restart after locating and correcting problem. Contact an authorized service center for service if none of the above.
PRE HI ENGINE TEMP lamp lights. Engine continues to operate.
Cause: Indicates engine has begun to overheat and engine temperature has risen to approximately 215F (102C).
Fix: A. Reduce load if possible by turning off non-critical loads. b. Check air inlets and outlets and remove any obstructions to airflow. If engine can be stopped, follow procedure in step 3.
LO OIL PRES lamp lights. Engine shuts down.
Cause: Indicates engine oil pressure has dropped to 20 psi (138 kPa).
Fix: If generator is powering critical loads and cannot be shut down, wait until next shutdown period and then follow step 5 procedure. If engine can be stopped, follow procedures in step 5.
LO OIL PRES lamp lights.
Cause: Indicates engine oil pressure has dropped to 14 psi (97 kPa).
Fix: Check oil level, lines and filters. If oil system is OK but oil level is low, replenish. Reset control and restart. Contact an authorized service center if oil pressure is not in the range of 25 to 40 psi (172 to 275 kPa).
LO FUEL lamp lights. Engine continues to run.
Cause: Indicates diesel fuel supply is running low.
Fix: Check fuel supply and replenish as required.
LO ENG TEMP lamp lights. Set is in standby mode but is not operating.
Cause: Lamp lights when engine coolant temperature is 70 F (21 C) or lower. Since the lamp goes out after the engine warms up, there should be no cause for alarm even during initial generator set operation.
Engine fails to start or starts, runs and then stops
Fix: Refer to Troubleshooting charts in the Operator’s Manual
The starter will disengage after a specified period of time and the control will indicate an overcrank fault
Fix: Place the Run-Stop-Remote switch in the STOP position and momentarily depress the Reset switch. Wait two minutes for the Starter motor to cool and then repeat the starting procedure

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

Maintenance schedule

Item	Spec	Part #	Interval
Air filter		—	250 hr
Fuel filter	Spin-on paper element fuel filters	—
Oil filter	Spin-on full flow lube oil filter	—
Battery		—	180 hr
Belt		—	250 hr
Coolant		—
Coolant filter		—
Oil	Change the oil and filter at the intervals recommended in the Maintenance Table.	—
Spark plug	Spark plugs	—

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	Notes
Crankcase oil	Change	50 hr	First service — Drain and replace the crankcase oil after the first 50 hours of operation on new generator sets (break-in)
Valve lash	Adjust	1000 hr	Footnote 5: Contact an authorized service center for service.
Alternator bearings (sealed)	Inspect	4000 hr	Inspect all sealed bearings every 4000 to 4500 hours (10000 hr/2 years)
Alternator bearings (sealed)	Replace	30000 hr	Replace all sealed bearings every 30000 hr/5 years (or if necessary after 10000 hr/2 years)

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
Other	Automatic stop delay after normal power source returns	about 15 minutes	minutes
Other	Coolant level below radiator cap opening	about two inches (50 mm)	mm	cold engine
Other	Exercise period	at least once a week for a minimum of 30 minutes with load	minutes
Other	Power rating reference conditions	altitude 300 feet (92 m), ambient temperature 81 F (27 C)
Pressure	Engine oil pressure	25 to 45	psi	engine at operating temperature
Pressure	Low oil pressure shutdown	14 psi (97 kPA)	psi
Pressure	Maximum allowable exhaust back pressure	41	inches H2O
Pressure	Normal engine oil pressure range	25 to 40 psi (172 to 275 kPa)	psi
Pressure	Pre-low oil pressure alarm	20 psi (137 kPA) or less	psi
Resistance	Battery specific gravity (recharge threshold)	recharge if below 1.260	specific gravity
Speed	Cycle cranking (optional)	three 15-second cranking cycles with two 15-second rest periods	seconds
Speed	Overcrank cranking limit	75	seconds
Speed	Overspeed shutdown (50 Hz units)	1850 +50	r/min
Speed	Overspeed shutdown (60 Hz units)	2100 ±90	r/min
Speed	Starter disconnect speed	450 to 570	RPM
Temperature	Engine coolant maintained minimum per NFPA 110 (with coolant heaters)	90 F (32 C)	F
Temperature	Engine coolant temperature (heat exchanger, adjust raw water flow)	165 to 195	°F	viewing water temperature gauge
Temperature	Engine water temperature	165 to 195	°F	depending on load and ambient temperature
Temperature	High engine temperature shutdown	above 222 F (106 C)	F
Temperature	Low engine temperature (LET) alarm	70 F (21 C) or lower	F
Temperature	Pre-high engine temperature alarm	approximately 215 F (102 C)	F
Voltage	DC voltmeter reading	24 to 28	volts	set running
Voltage	Generator AC voltage regulation from no load to full load	plus or minus two percent	%
Voltage	Random voltage variation for constant loads	plus or minus one percent	%
Voltage	Voltage adjusting rheostat range	approximately plus or minus five percent adjustment of the rated output voltage	%

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	Ambient	Capacity	Notes
Coolant	water/antifreeze mixture			21.0 litres	Capacity with standard radiator 5.5 gallons (21 litres) for 37 DGBA, 35 DGBB, 40 DGBC, 50 DGCA, 60 DGCB; 6.5 gallons (25 litres) for 80 DGDA, 100 DGDB. Do not use antifreeze with anti-leak additives.
Engine oil	SAE 5W synthetic	API class CC/CD; sulphated ash content does not exceed 1.85 percent; pour point 9 F (5 C) below lowest expected temperature; viscosity 10,000 mPa s max at -31 F (-35 C) and 4.1 mm2/s min at 212 F (100 C)	below -23°C		For ambient temperature consistently below -10 F (-23 C) with no provision to keep engine warm; do not use petroleum base 5W grade oil
Engine oil	SAE 10W30 / SAE 15W40 / SAE 20W40 (spec table)		-25°C to 35°C	11.0 litres	Specifications table: Oil Capacity with Filters 11.5 quarts (11 litres) for 37 DGBA, 35 DGBB, 40 DGBC, 50 DGCA, 60 DGCB; 17.3 quarts (16 litres) for 80 DGDA, 100 DGDB. Oil type -13 F to 95 F (-25 C to 35 C) SAE 10W30; 14 F (-10 C) and above SAE 15W40; 32 F (0 C) and above SAE 20W40
Engine oil	SAE 10W-30	API Class CC/CD (max sulphated ash 1.85 percent)	-23°C to -10°C		-10 F (-23 C) to 14 F (-10 C)
Engine oil	SAE 15W-40	API Class CC/CD (max sulphated ash 1.85 percent)	above -10°C		14 F (-10 C) and above
Engine oil	SAE 20W-40	API Class CC/CD (max sulphated ash 1.85 percent)	above 0°C		32 F (0 C) and above
Fuel	ASTM No. 2 Diesel	ASTM No. 2			Cummins engines on DG series sets normally use ASTM No. 2 Diesel; will operate on alternate diesel fuels within engine manual specifications. Never use galvanized or copper fuel lines, fittings or tanks.
Fuel	No. 2 diesel fuel				Engine will also operate on No. 1 fuel or other similar fuels; consult engine manufacturer for specific requirements

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
FAULT 1	FAULT 1 (red) lamp	Indicates an undedicated fault. May be field programmed as a shutdown or non-shutdown, and as a timed or non-timed fault (normally set for timed shutdown).	Shutdown	Determined when the set installation is designed.
FAULT 2	FAULT 2 (red) lamp	Indicates same features as Fault 1 (normally set for timed shutdown).	Shutdown	Determined when the set installation is designed.
HI ENG TEMP	HI ENG TEMP (red) lamp	Indicates engine has shut down because of critically high engine temperature (above 222 F/106 C) or low coolant level.	Shutdown	Allow engine to cool; check coolant level and replenish; check for cooling airflow obstructions; check fan belt; reset control and restart.
LO FUEL	LO FUEL (yellow) lamp	Indicates fuel supply is marginally low.	Warning	Check fuel supply and replenish as required.
LO OIL PRES	LO OIL PRES (red) lamp	Indicates engine has shut down because of critically low oil pressure (below 14 psi/97 kPa).	Shutdown	Check oil level, lines and filters; replenish oil if low; reset control and restart; contact an authorized service center if oil pressure is not in the range of 25 to 40 psi (172 to 275 kPa).
LOW ENG TEMP	LOW ENG TEMP (yellow) lamp	Lights if engine temperature is marginally low for starting (70 F/21 C or lower); may indicate an inoperative coolant heater.	Warning	Check coolant heater power supply, fuse and heater cord; check coolant level and replenish.
OVERCRANK	OVERCRANK (red) lamp	Indicates engine has failed to start during the cranking period.	Shutdown	Check for empty fuel tank, fuel leaks or plugged fuel lines; check for dirty fuel or air filter and replace if necessary; reset control and restart.
OVERSPEED	OVERSPEED (red) lamp	Indicates engine has shut down because of excessive speed.	Shutdown	Contact an authorized service center for service.
PRE HI ENG TEMP	PRE HI ENG TEMP (yellow) lamp	Indicates engine temperature is marginally high (approximately 215 F/102 C) and engine continues to operate.	Warning	Reduce load if possible; check air inlets and outlets and remove obstructions to airflow.
PRE LO OIL PRES	PRE LO OIL PRES (yellow) lamp	Indicates engine oil pressure is marginally low (20 psi/137 kPa or less) and engine continues to operate.	Warning	Find and correct the cause as soon as possible.
RUN	RUN (green) lamp	Comes on when starter circuit opens after set starting.	Info
SWITCH OFF	SWITCH OFF (flashing red) lamp	Indicates generator set is not in automatic start mode.	Warning	Move the Run/Stop/Remote switch to the Remote position for automatic starting.

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

Procedures

Vibration Isolators Installation and Adjustment Procedure installation

Place the vibration isolators on the genset support structure.
Shim or grout isolators so all bases are within 0.25 inch (6 mm) elevation of each other on a flat surface.
Loosen the side snubber lock nuts so the top plate is free to move vertically and horizontally.
Place the genset onto the isolators, aligning the skid mounting with the threaded isolator hole.
Adjust isolators with the leveling bolt through the skid (locking nut threaded up toward bolt head) to level the set.
Set nominal clearance between top plate and isolator base of 0.25 inch (6 mm) or greater.
Adjust leveling bolts until set is level and sufficient clearance remains, then lock with the lock nut.
Snubber nuts may remain loose to provide better isolation between genset and support structure.

Air Cleaner Element Replacement maintenance

⚠ Filters should be handled with care to prevent damage. If the filter does become damaged, install recommended replacement part.

Remove the air cleaner cover.
Remove thumb screw and gasket washer, then remove the element from air cleaner.
Wipe out the interior of the air cleaner housing and cover with a clean, damp cloth.
Install new element and secure with gasket washer and thumb screw.
Put on the air cleaner cover and secure.

Engine Oil Change maintenance

⚠ Crankcase pressure can blow out hot oil and cause serious burns. Do NOT check oil while the generator set is operating.

Tools: Oil drain pan

Bring the engine up to operating temperature and then shut if off.
Open the drain valve and collect the engine oil in a pan. When the crankcase is drained close the drain valve.
Fill the crankcase to the full mark on the dipstick. Allow for oil filters when drained or if changed.
Start the engine and check for oil leaks.
Shut off the engine and wait 15 minutes before checking the oil level. Add oil if required.

Filling the Cooling System (Standard Radiator and Heat Exchanger Sets) maintenance

⚠ Contact with hot coolant can result in serious burns. Do not bleed hot, pressurized coolant from a closed cooling system.

Remove the cooling system pressure cap and fill the system with water or a water/antifreeze mixture.
When the engine is first started, remove the pressure cap and monitor the coolant level.
As trapped air is expelled from the system, the coolant level will drop and additional coolant should be added.
Replace the pressure cap when the coolant level is stable.

Flushing and Cleaning the Cooling System maintenance

⚠ Excessive air pressure while starting the water flow could split the radiator core. Apply air pressure gradually to avoid damage.

Tools: Flushing gun nozzle

Chemical Cleaning: use a good cooling system cleaner such as sodium bisulphate or oxalic acid and follow the supplier instructions. Neutralize and flush with clean water.
Flushing: open the upper and lower radiator hose connections and install the radiator cap.
Attach a flushing gun nozzle to the lower radiator hose connection and let the water run until the radiator is full.
Gradually apply air pressure to avoid damaging the core. Shut off the air and allow the radiator to refill. Repeat until water coming from the radiator is clean.
To flush the engine block, first remove the thermostat. Attach the flushing gun to the upper radiator hose and fill the block with water.
Restrict the lower radiator hose opening until the block is filled. Apply air pressure and force water from the lower opening. Repeat until water is clean.
Replace the thermostat and all hoses and refill cooling system.

Oil Filter Change maintenance

Tools: Filter wrench

Clean around the filter head.
Using a filter wrench, remove the oil filter.
Fill the filter with clean oil as recommended in Table 5-2.
Apply a light film of lubricating oil to the gasket sealing surface before installing the filter.
Install the filter until hand tight then turn an additional 1/2 turn.
Start engine to verify that filter is tight and does not leak.

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 oil capacity of the Cummins/Onan 4B3.9-G?

The Cummins/Onan 4B3.9-G engine has an oil capacity of 11.5 liters.

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

The Cummins/Onan 4B3.9-G has a displacement of 3.92 liters.

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

The Cummins/Onan 4B3.9-G powers 6 generator set(s), including Cummins/Onan 28DGBB, Cummins/Onan 35 DGBB, Cummins/Onan 36 DGBC, Cummins/Onan 40 DGBC, Cummins/Onan DTBA.

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

A commonly reported issue on generators using the Cummins/Onan 4B3.9-G is: SWITCH OFF lamp flashes.. Fix: Indicates Run/Stop/Remote switch is in the Stop position which will prevent automatic starting if an automatic transfer switch is used. Move the Run/Stop/Remote switch to the Remote position for automatic starting..