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Cummins/Onan Onan Diesel 4-cyl Engine

The Cummins/Onan Onan Diesel 4-cyl is a 1.97 L 4-cylinder natural engine rated at 1800 rpm. It powers generator sets from Cummins/Onan, including the 12.5 RDJC and 15.0 RDJC. It holds 5.7 L (1.5 gal) of oil and 11.36 L (3.0 gal) of coolant.

Also known as: Onan Onan Diesel 4-cyl

1.966 L

Displacement

Cylinders

1800 rpm

Rated speed

5.7 L

Oil capacity

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

Engine specifications

Oil capacity	5.7 L
Coolant capacity	11.36 L
Displacement	1.966 L
Cylinders	4
Aspiration	Natural
Rated speed	1800 rpm

Generator sets powered by this engine

Cummins/Onan 12.5 RDJC Cummins/Onan 15.0 RDJC

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

Common problems

Engine does not start (little or no exhaust smoke; engine is not getting fuel)
Cause: Fuel system not primed
Fix: Prime fuel system; observe fuel flow from return line
Engine will not crank
Cause: Possible fault with control or starting system: fault lamp on
Fix: Correct fault and reset control
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 FAULT 1
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
SWITCH OFF lamp flashes
Cause: Run/Stop/Remote switch is in the Stop position which will prevent automatic starting if an automatic transfer switch is used
Fix: Move the Run/Stop/Remote switch to the Remote position for automatic starting
Stop circuit fails
Fix: Push governor arm down to shut off fuel in injection pump
PRE LO OIL PRES lamp lights. Engine continues to operate
Cause: 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
PRE HI ENGINE TEMP lamp lights. Engine continues to operate
Cause: Engine is operating near its capacity and temperature has risen to approximately 220 °F (104 °C) or coolant level is low on optionally wired sets
Fix: Reduce load if possible by turning off non-critical loads; check air inlets and outlets and remove any obstructions to airflow; if engine can be stopped, follow procedure in step 3
OVERCRANK lamp lights and engine stops cranking, or engine runs, shuts down, and LO OIL PRES lamp lights
Cause: Possible fuel system problem
Fix: Check for loss of engine prime, empty fuel tank, fuel leaks, or plugged fuel lines and correct as required; check for dirty fuel filter and replace if necessary (see Maintenance section); check for dirty or plugged air filter and replace if necessary (see Maintenance section); reset the control and restart after correcting the problem; contact an authorized service center for service if none of the above
No AC output voltage
Cause: Breaker is tripped
Fix: Reset breaker; contact an authorized service center if voltage buildup causes breaker to trip
LO OIL PRES lamp lights. Engine shuts down
Cause: 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 20 psi (138 kPa) or lower
LO FUEL LAMP lights. Engine shuts down and LO OIL PRES lamp lights
Cause: Engine has run out of fuel
Fix: Check fuel level and replenish as required
LO FUEL lamp lights. Engine continues to run
Cause: 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. (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.)
Cause: Engine coolant heater is not operating, is not circulating coolant, or ambient temperature has exceeded capability of heater
Fix: Check for the following conditions: coolant heater not connected to power supply — check for blown fuse or disconnected heater cord and correct as required; check for low coolant level and replenish if required, look for possible coolant leakage points and repair as required; contact an authorized service center if none of the above
HI ENG TEMP lamp lights. Engine shuts down
Cause: Engine has overheated (engine temperature has risen above 230 °F/110 °C) or coolant level is low
Fix: Allow engine to cool down completely before proceeding with the following checks: check coolant level and replenish if low, look for possible coolant leakage points and repair if necessary; check for obstructions to cooling airflow and correct as necessary; check for a slipping fan belt and tighten if loose; reset control and restart after locating and correcting problem; contact an authorized service center if none of the above
Green RUN lamp lights following engine startup
Cause: All engine systems are normal
Fix: No corrective action required

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan Onan Diesel 4-cyl.

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Oil filter	Full Flow Type		—
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	Notes
Exhaust system	Inspect	8 hr
Fuel supply	Check	8 hr	See Note 1; if daily inspection shows water or excessive dirt in sediment bowl, check fuel handling and storing facilities
Generator set	Inspect	8 hr
Oil level	Check	8 hr	Check oil level indicator after every 8 hours of operation; maintain at FULL mark
Air cleaner	Check	50 hr	Note 3: perform more often in extremely dusty conditions; remove and clean filter element after every 50 hours
Cylinder head bolts and valve clearance	Adjust	50 hr	First service — Note 2: tighten head bolts and adjust valve clearance after first 50 hours on new and overhauled engines, then adjust valve clearance each 500 hours thereafter
Battery	Inspect	100 hr	Check battery condition
Crankcase oil	Change	100 hr	First service — Or annually; perform more often in extremely dusty conditions (Note 3)
Fuel transfer pump sediment bowl and filter	Clean	100 hr	See Note 1
Governor linkage	Clean	100 hr	Carefully pull neoprene governor ball joints apart and clean them; do not lubricate
Oil filter	Replace	100 hr
Battery electrolyte level	Check	200 hr	Check level of electrolyte at least every 200 hours of operation
Crankcase breather	Clean	200 hr
Crankcase breather tube	Clean	500 hr	500 hour interval coincides with valve lash adjustments
Start-disconnect circuit	Check	500 hr
Valve clearances	Check	500 hr	Note 4: contact an authorized service center for service
Primary fuel filter	Change	600 hr
Torsional washers	Inspect	1500 hr	Note 5: check yearly on non-commercial sets
Generator	Clean	2000 hr
Nozzle spray pattern	Check	2000 hr	Note 4
Valves	Other	2000 hr	Grind valves (if required); Note 4
Secondary fuel filter	Change	3000 hr
General overhaul	Other	5000 hr	If required; Note 4

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	Start-disconnect centrifugal switch point gap	0.020	in. (0.51 mm)
Pressure	Engine oil pressure at operating temperature	35 to 55	psi (241 to 379 kPa)	at operating temperature
Pressure	Exhaust total back pressure of all system components	must not exceed maximum back pressure limits
Pressure	LO OIL PRES shutdown at engine oil pressure	14	psi (97 kPa)
Pressure	PRE LO OIL PRES lamp lights at engine oil pressure	20	psi (138 kPa)
Resistance	Battery electrolyte specific gravity, fully charged (corrected)	1.260	specific gravity	corrected to 80°F (27°C)
Resistance	Charge battery if specific gravity is below	1.215	specific gravity
Speed	Centrifugal switch closes when engine speed reaches	900	rpm
Speed	Engine speed (50/60 Hz)	1500/1800	rpm
Speed	Governor speed variation, no-load to full-load	3	hertz	no-load to full-load operation
Speed	Starter disconnects when engine speed reaches	450 to 570	r/min
Temperature	Coolant temperature	170° to 216°	°F (77° to 102° C)	depending on the load and ambient temperature
Temperature	Exercising: engine oil temperature to be held at	180	°F (82.2° C)	for at least 30 minutes
Temperature	HI ENG TEMP shutdown above	230	°F (110° C)
Temperature	LOW ENG TEMP (LET) lamp lights at coolant temperature of	70	°F (21° C)
Temperature	PRE HI ENGINE TEMP lamp lights at approximately	220	°F (104° C)
Torque	Cylinder head bolt torque	44 - 46	ft-lb (60 - 62 N·m)
Torque	Glow plug torque	10-15	ft-lb (14 - 20 N·m)
Valve clearance	Valve clearance, exhaust	0.016	in. (0.41 mm)	engine at room temperature about 70°F (21°C)
Valve clearance	Valve clearance, intake	0.011	in. (0.28 mm)	engine at room temperature about 70°F (21°C)
Voltage	DC voltmeter reading while genset running	12 to 14	volts	genset running
Voltage	Voltage adjust rheostat range	plus or minus five	percent of 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
Engine oil	SAE 15W40	API Classification CC/CD, CD/SG or CE/SG		6.5 U.S. quarts (6.0 l)	Table 9-2 viscosity/temperature chart: 5W30, 15W40, 10W30/10W40; 15W40 preferred summer. Limited use of 10W-30 for easier starting below 23°F (-5°C)
Fuel	ASTM 2-D (No. 2 Diesel) or ASTM 1-D (No. 1 Diesel)	minimum Cetane number of 45; viscosity above 1.3 cSt; low-sulfur fuel with cloud point at least 10 degrees below lowest expected fuel temperature	below 0°C		Use number 1 diesel fuel when ambient temperatures are below 0°C (32°F) or during long periods of light engine load

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 shutdown or non-shutdown, and as timed or non-timed fault (normally set for timed shutdown).	Shutdown	Nature of fault determined at installation design; correct the fault and reset the control.
FAULT 2	Fault 2 (red lamp)	Indicates same features as Fault 1 (normally set for timed shutdown).	Shutdown	Nature of fault determined at installation design; correct the fault and reset the control.
HI ENG TEMP	High Engine Temperature (red lamp)	Indicates engine has shut down because of critically high engine temperature (above 230°F / 110°C) or low coolant level.	Shutdown	Allow engine to cool; check coolant level and replenish; check for obstructions to cooling airflow and slipping fan belt; reset control and restart.
LO FUEL	Low Fuel (yellow lamp)	Indicates fuel supply is marginally low.	Warning	Check fuel supply and replenish as required.
LO OIL PRES	Low Oil Pressure (red lamp)	Indicates engine has shut down because of critically low oil pressure (14 psi / 97 kPa).	Shutdown	Check oil level, lines and filters; replenish oil if low; reset control and restart; contact service center if oil pressure is 20 psi (138 kPa) or lower.
LOW ENG TEMP	Low Engine Temperature (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, coolant level and heater operation.
OVERCRANK	Overcrank (red lamp)	Indicates engine has failed to start during the cranking period.	Shutdown	Check for loss of engine prime, empty fuel tank, fuel leaks, or plugged fuel lines; check filters; 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-High Engine Temperature (yellow lamp)	Indicates engine temperature is marginally high (approximately 220°F / 104°C) or coolant level is low on optionally wired sets.	Warning	Reduce load if possible; check air inlets and outlets and remove obstructions to airflow.
PRE LO OIL PRES	Pre-Low Oil Pressure (yellow lamp)	Indicates engine oil pressure is marginally low (20 psi / 138 kPa).	Warning	Reduce load or prepare to shut down; follow troubleshooting step 5/6 procedures.
RUN	Run (green lamp)	Comes on when starter circuit opens after set starting.	Info	Indicates all engine systems are normal; no corrective action required.
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

Centrifugal Switch Adjustment adjustment

⚠ Shut down the set and disconnect the battery cable, negative (-) terminal first, before setting the breaker point gap

Locate the start-disconnect centrifugal switch on the side of the engine, above the oil filter.
If necessary, loosen the stationary contact and adjust the point gap to 0.020 inch (51 mm).
Replace burned or faulty points.

Charge Rate Adjustment adjustment

⚠ Avoid overcharging the battery

⚠ Do not smoke or allow any flame, sparks, or arc-producing devices in the battery area

Locate the adjustable resistor slide tap in the charging circuit, found in the generator air outlet.
If necessary, increase the charging rate slightly by moving the slide tap up.
Make small increments of change until the proper rate has been determined.
Adjust this slide tap only when the engine is stopped.

Generator Voltage Check and Adjustment adjustment

⚠ High voltages can cause injury or death; use extreme caution when working on electrical components

Tools: Screwdriver, Wrench

Move the Run/Stop/Remote switch on the engine control panel to the Run position.
Move the Phase Selector switch to read generator output current and voltage.
Insert a screwdriver into the Voltage Adjust potentiometer and carefully loosen the locking nut with a wrench.
While observing the voltmeter, slowly turn the screwdriver clockwise to increase voltage or counterclockwise to decrease voltage.
Adjust to the rated nameplate voltage.
Tighten the locking nut, being careful not to change the adjustment.
Move the Phase Selector switch to the Off position.

Governor Sensitivity Adjustment adjustment

Turn the sensitivity adjusting ratchet to adjust governor sensitivity (no-load to full-load speed droop).
Radiator cooled sets: turn the ratchet clockwise for more sensitivity (less speed drop at full load).
Radiator cooled sets: turn the ratchet counterclockwise for less sensitivity (more speed drop).
City water cooled sets: turn the ratchet counterclockwise for more sensitivity.
After making the sensitivity adjustment, readjust the speed.

Governor Speed Adjustment adjustment

Tools: Stroboscope, Frequency meter

Turn the governor speed adjusting nut clockwise (more spring tension) to increase rpm.
Turn the nut counterclockwise to reduce governed speed.
Use a stroboscope or a frequency meter to make this adjustment.

Valve Clearance Adjustment adjustment

⚠ Incorrect valve clearance adjustment can lead to irrevocable damage to the generator set; only qualified personnel should perform this adjustment

Tools: Socket wrench, Feeler gauge, Torque wrench

Check the valve clearance when the engine is at room temperature, about 70°F (21°C).
Turn the flywheel until the cylinder to be adjusted is on its compression stroke.
Turn the flywheel an additional 10 to 45 degrees clockwise so the piston is in its power stroke with both valves closed.
Check cylinder head-bolt torque prior to valve clearance adjustment.
Loosen the locknut securing the rocker arm to the cylinder head to increase clearance; tighten it to reduce clearance.
Using a feeler gauge, check the clearance between the rocker arm and the valve until the proper gap is established.

Battery Cleaning and Specific Gravity Check maintenance

⚠ Do not smoke or allow any source of ignition while servicing batteries

⚠ Always remove the negative (-) cable first, and reconnect it last to prevent arcing

Tools: Battery hydrometer

Disconnect the negative ground strap from the battery before working on any part of the electrical system or engine.
Wipe the batteries clean with a damp cloth whenever dirt appears excessive.
If corrosion is present, remove battery cables and wash terminals with an ammonia solution or a solution of 1/4 pound baking soda in 1 quart of water.
Flush the outside of the battery and surrounding areas with clean water.
Coat the terminals with a light application of petroleum jelly or non-conductive grease.
Use a battery hydrometer to check the specific gravity of the electrolyte in each cell; hold the hydrometer vertical.
Correct the reading by adding four gravity points (0.004) for every ten degrees the electrolyte temperature is above 80°F (27°C).

Contractor Model Air Cleaner Cleaning maintenance

⚠ Filters should be handled with care to prevent damage; if the filter becomes damaged, install recommended replacement part

Remove pre-cleaner and wash out dirt; dry and reinstall.
Loosen clamp and remove end cover.
Remove thumbscrew and take out element.
Wash element in detergent and water (use new element after 6 washings); dry and reinstall.
Remove air cleaner baffle from cover, wash out dirt, and reinstall in cover.
Install cover with "TOP" up and tighten clamp.

Crankcase Breather Cleaning maintenance

Remove hose clamp, breather hose, breather cap clamp and insulator halves to release breather cap and valve assembly.
Wash cap, valve assembly and baffle in suitable solvent.
Dry and reinstall.

Oil Change maintenance

⚠ Hot crankcase oil can cause burns if spilled or splashed on skin; keep fingers and hands clear when removing the oil drain plug and wear protective clothing

Tools: Drain pan

Stop the engine and drain the crankcase oil while the engine is still hot.
Place a pan under the drain outlet and remove the oil drain plug or open the oil drain valve.
After the oil is completely drained, replace the drain plug or close the drain valve.
Replace oil filter if necessary before adding oil.
Refill with oil of the correct API designation and SAE viscosity grade for the temperature conditions.

Oil Filter Change maintenance

⚠ Hot crankcase oil can cause burns if spilled or splashed on skin

Tools: Container

Place a container under the oil filter and remove the filter by turning it counterclockwise.
Clean the filter mounting area.
Coat the oil filter gasket with clean oil.
Install the new filter by turning it clockwise until the gasket touches the mounting base.
Tighten the filter 1/2 turn.

Out-of-Service Protection maintenance

Exercise the generator set until the engine is up to operating temperature.
Shut down the engine; disconnect battery and store in a cool, dry place. Connect battery to a charger every 30 days.
Drain the oil base while it is still warm; refill and attach a warning tag indicating the viscosity of oil used.
Service the air cleaner.
Clean throttle and governor linkage and protect by wrapping with a clean cloth.
Plug the intake and exhaust outlets to prevent entrance of moisture, bugs, dirt, etc.
Clean and wipe the entire unit; coat parts susceptible to rust with a light coat of grease or oil.
Provide a suitable cover for the entire unit after the unit has cooled down.

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

Voltage reconnection

Voltage	Phase	Connection	Leads	Notes
100	1	V4-V1	4 lead reconnectable	50 Hz
100/200	1	V4-V1	12 lead reconnectable	50 Hz
110	1	V4-V1	4 lead reconnectable	50 Hz
110/190	3	V4-V1	12 lead reconnectable	50 Hz
110/220	1	V4-V1	12 lead reconnectable	50 Hz
115	1	V4-V2	4 lead reconnectable	50 Hz
115/220	3	V4-V2	12 lead reconnectable	50 Hz
115/230	1	V4-V2	12 lead reconnectable	50 Hz
120	1	V4-V1	4 lead reconnectable	60 Hz
120	1	V4-V3	4 lead reconnectable	50 Hz
120/208	3	V4-V1	12 lead reconnectable	60 Hz; jumper wire connection on voltage regulator VR21, per representative reconnection diagram (YD generators)
120/208	3	V4-V3	12 lead reconnectable	50 Hz
120/240	1	V4-V1	12 lead reconnectable	60 Hz
120/240	1	V4-V3	12 lead reconnectable	50 Hz
127/220	3	V4-V2	12 lead reconnectable	60 Hz
139/240	3	V4-V3	12 lead reconnectable	60 Hz
220/380	3	V4-V1	12 lead reconnectable	50 Hz
230/400	3	V4-V2	12 lead reconnectable	50 Hz
240/416	3	V4-V1	12 lead reconnectable	60 Hz
240/416	3	V4-V3	12 lead reconnectable	50 Hz
254/440	3	V4-V2	12 lead reconnectable	60 Hz
254/440	3	V4-V4	12 lead reconnectable	50 Hz
277/480	3	V4-V3	12 lead reconnectable	60 Hz
347/600	3	V4-V1	12 lead reconnectable	60 Hz

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

Common questions

What is the oil capacity of the Cummins/Onan Onan Diesel 4-cyl?

The Cummins/Onan Onan Diesel 4-cyl engine has an oil capacity of 5.7 liters.

What is the displacement of the Cummins/Onan Onan Diesel 4-cyl?

The Cummins/Onan Onan Diesel 4-cyl has a displacement of 1.966 liters with 4 cylinders.

Which generators use the Cummins/Onan Onan Diesel 4-cyl engine?

The Cummins/Onan Onan Diesel 4-cyl powers 2 generator set(s), including Cummins/Onan 12.5 RDJC, Cummins/Onan 15.0 RDJC.

What is a common problem with the Cummins/Onan Onan Diesel 4-cyl engine?

A commonly reported issue on generators using the Cummins/Onan Onan Diesel 4-cyl is: Engine does not start (little or no exhaust smoke; engine is not getting fuel). Cause: Fuel system not primed. Fix: Prime fuel system; observe fuel flow from return line.