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Cummins/Onan MDJA Engine

The Cummins/Onan MDJA is a 0.5 L 1-cylinder natural engine rated at 1800 rpm, producing up to 5.2 kW. It powers generator sets from Cummins/Onan, including the 3.0MDJA. It holds 2.8 L (0.7 gal) of oil and 4.3 L (1.1 gal) of coolant.

Also known as: Onan MDJA

0.5 L

Displacement

Cylinders

1800 rpm

Rated speed

5 kW

Gross power

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

Engine specifications

Oil capacity	2.8 L
Coolant capacity	4.3 L
Displacement	0.5 L
Cylinders	1
Aspiration	natural
Rated speed	1800 rpm
Gross power	5 kW

Generator sets powered by this engine

Cummins/Onan 3.0MDJA Cummins/Onan MDJA

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

Troubleshooting

Fault codes

Code	Meaning
e008	No specific name provided for e008
10	Overload

Symptoms & remedies

Symptom	Remedy
Do not work on energized equipment	Do not work on energized equipment. Unauthorized personnel must not be permitted near energized equipment. Induced voltage remains even after equipment is disconnected from the power source. Plan maintenance with authorized personnel so equipment can be de-energized and safely grounded.

Fault codes and symptom→remedy steps extracted from this engine’s service/owner documentation; verify against the official manual before servicing.

Common problems

Overload 10
No specific name provided for e008 e008
Do not work on energized equipment
Fix: Do not work on energized equipment. Unauthorized personnel must not be permitted near energized equipment. Induced voltage remains even after equipment is disconnected from the power source. Plan maintenance with authorized personnel so equipment can be de-energized and safely grounded.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan MDJA.

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter	Engine air intake filter		—
Fuel filter	Secondary fuel filter		—	3000 hr
Oil filter	Full Flow		—	200 hr
Battery	Check condition/specific gravity at least every two weeks; recharge if specific gravity below 1.265		—
Belt	flexible seamless tubing		—
Coolant	Optional heat-exchanger (captive water) cooling system capacity; 50/50 ethylene glycol/water mix recommended	2.25 US quart	—
Oil	Crankcase oil, API CD/SE (CC/SE for first 50-hour fill only); use SAE grade per ambient-temperature chart	100.0 US quart	—	100 hr

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

Repair guides

Step-by-step procedures compiled from the Cummins/Onan service manual for this engine.

Service intervals

Item	Action	Hours	Months	Notes
Cylinder head bolts (J series water-cooled)	Adjust	2 hr		First service — Must be retorqued after 1/2 hour to 2 hours of operation (overhauled units only).
Polyurethane precleaner	Clean	25 hr		Removed, cleaned and oiled every 25 hours.
Crankcase oil	Change	50 hr		First service — after first 50 hours of operation; drain while engine is still hot
Cylinder head bolts (J series air-cooled)	Adjust	50 hr		First service — Must be retorqued after 50 hours (back off 1/2 turn, then retighten).
Cylinder head bolts and valve clearance	Adjust	50 hr		First service — after first 50 hours on an overhauled engine
Air cleaner element	Inspect	100 hr		Check and clean at least every 100 hours; more often in dusty conditions.
Breaker points (BF-BG top adjust)	Inspect	100 hr		Check every 100 hours, replace if necessary.
Breaker points (NHC-NHCV)	Inspect	100 hr		Check every 100 hours and replace if necessary.
Breaker points (NHP-NHPV)	Replace	100 hr
CCK and NH home standby fuel filter	Clean	100 hr
Governor Linkage	Clean	100 hr		Every 100 hours; clean plastic joint only, steel joint with graphite
Onan electric fuel pump filters	Clean	100 hr		Every 100 operating hours or sooner.
Water filter	Check	100 hr		check after every 100 hours; change sooner in dirty water
Water filter (keel cooled models)	Change	100 hr		After every 100 hours; sooner in dirty water.
Breaker points (BF-BG engines)	Replace	200 hr		Side adjust points.
Breaker points (NHA, NHB, NH Power Drawer)	Replace	200 hr
Governor ball joint	Lubricate	200 hr		Check linkage for freedom of movement.
Oil filter	Replace	200 hr		every 200 hours of normal operation
Spark plug	Replace	200 hr		Install new spark plugs every 200 hours.
Fuel pump filter element	Clean	400 hr		Drain fuel pump and check filter element every 400 hours.
Air cleaner element	Replace	500 hr		Replace at least every 500 operating hours.
Carburetor bowl	Clean	500 hr		Remove bowl, clean filter screen in solvent.
Crankcase breather pipe	Clean	500 hr		MDJC, MDJF Begin Spec T
Primary fuel filter	Replace	600 hr		Diesel; drain water periodically.
Secondary fuel filter	Replace	3000 hr		Diesel; regular service period 3000 hours.
Valve stem tip-to-cap clearance	Check	5000 hr		or whenever parts exposed or removed
Anti-freeze	Change		12 mo	Complete change every year.
Cooling system	Clean		12 mo	clean and flush at least once a year
Heat exchanger sea water side	Clean		12 mo	yearly; remove end caps and flush tubes
Pressure cap	Replace		24 mo	Replace every two years or whenever they malfunction.
Zinc pencil (heat exchanger)	Inspect		2 mo	Replace if deteriorated to less than 1/2 original size.
Zinc pencils	Inspect		2 mo	replace if less than half original size

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
Clearance	Brush wear limit (replace)	5/8	in
Clearance	Camshaft end play	0.007-0.039	in	room temperature
Clearance	Center main bearing clearance (4 cylinder)	0.0024-0.0052	in	original
Clearance	Collector ring run-out	0.002	in	compared to generator bearing
Clearance	Connecting rod bearing clearance	0.001-0.003	in
Clearance	Crankshaft end play	0.010-0.015	in	room temperature
Clearance	Main bearing clearance (1 & 2 cylinder)	0.0014-0.0052	in	replacement
Clearance	Main bearing clearance (1 & 2 cylinder)	0.002-0.0033	in	original
Clearance	Piston clearance in cylinder	0.005-0.007	in	90 degrees to pin, below oil ring
Clearance	Ring gap	0.010-0.020	in
Clearance	Shim thickness (injection pump timing)	0.006 to 0.052	in	0.15 to 1.32 mm
Clearance	Stator to flywheel coil post gap	0.010 to 0.015	in	0.25 to 0.38 mm
Clearance	Valve clearance	.020 (0.51), .018 (0.33), .011 (0.28), .016 (0.41), .017 (0.43), .017 (0.43)	inch (mm)	engine at room temperature about 70°F (21°C)
Clearance	Valve guide clearance exhaust	0.0030-0.0050	in
Clearance	Valve guide clearance intake	0.0015-0.003	in
Clearance	Valve stem tip-to-cap clearance	0.001-0.005	in
Gap	Breaker point gap (AK, AJ, LK)	0.020	in	at full separation
Gap	Breaker point gap (BF engines)	0.025	in
Gap	Breaker point gap (BF-BG side adjust)	0.020 to 0.023	in	BG engines
Gap	Breaker point gap (MAJ)	0.022	inch (0.56 mm)	full separation
Gap	Breaker point gap (MCCK)	0.020	inch (0.51 mm)
Gap	Breaker point gap (NB)	0.020	in	at full separation
Gap	Breaker point gap (NHC-NHCV)	0.016	in
Gap	Breaker point gap (NHP-NHPV)	0.019	in
Gap	Centrifugal switch gap	0.020	in
Gap	Centrifugal switch gap	0.020 (0.51 mm)	in
Gap	Ignition point gap (AK, AJ)	0.022	in	at full separation
Gap	NB breaker module pin gap	0.008	in
Gap	Spark plug gap (4-cylinder)	0.035	in	gasoline or gaseous fuel
Gap	Spark plug gap (gaseous fuel, 1- and 2-cylinder)	0.018	in
Gap	Spark plug gap (gasoline, 1- and 2-cylinder)	0.025	in
Injection timing	Ignition timing (gasoline)	19	degrees BTC	1800 rpm or slower
Injection timing	Ignition timing (gasoline)	25	degrees BTC	3600 rpm or faster
Injection timing	Injection timing	17	degrees BTC
Injection timing	Injection timing	18	degrees BTC
Injection timing	Injection timing	19	degrees BTC	Later models after Spec P
Injection timing	Injection timing	21	degrees BTC	Before Spec P
Injection timing	PLB injection pump port closing (DJA)	17	degrees BTC
Injection timing	PLB pump timing (DJA)	17	degrees BTC
Injection timing	PSU injection pump port closing	19	degrees BTC	begin Spec P
Injection timing	PSU injection pump port closing	21	degrees BTC	prior to Spec P
Other	Battery charging high rate	6 to 8	amps
Other	Battery charging low rate	2 to 3	amps
Other	Battery charging rate solid state combined	5.26	amps
Other	Battery charging rate solid state high	3.7	amps
Other	Battery charging rate solid state low	1.56	amps
Other	Battery specific gravity	1.265	(specific gravity)	fully charged at 80 F (26.7 C)
Other	Bilge blower time delay	5	minutes
Other	Cranking limiter time	45 to 90	seconds
Other	Emergency relay K14 operation time	15	seconds
Other	Emergency time delay low oil shutdown	45	seconds
Other	Maximum operation angle MCCK, MDJA, MDJE	20	degrees	any direction
Other	Maximum operation angle MDJC, MDJF	30	degrees	any direction
Pressure	Compression (sea level)	300-350	psi	Spec A to N
Pressure	Compression (sea level)	350-400	psi	Begin Spec P, warm engine at about 300 rpm
Pressure	Compression pressure	300-350	psi	cranking at about 300 rpm, prior to Spec P
Pressure	Compression pressure	350-400	psi	cranking at about 300 rpm, Spec P and later
Pressure	Delivery valve line pressure	300-600	psi	engine running
Pressure	Diesel transfer pump pressure	12 and 14	psi	governed speed, gauge 16 inches above pump
Pressure	Expansion tank pressure cap	12-15	psi

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	Notes
Coolant	50/50 mixture antifreeze and clean water			increase antifreeze percentage in extremely low temperatures (-25F or lower)
Coolant	antifreeze			Replace annually in captive system
Coolant	ethylene glycol anti-freeze and water	minimum 50-50 mix		Complete change every year in closed water systems.
Engine oil	5W-30	API CC/SE	below 0°C	recommended for service in ambient temperatures of 32F and colder
Engine oil	API CD/SE			Diesel engines; use CC/SE for first fill only (50 hours)
Engine oil	API SE or SE/CC			Gasoline engines
Engine oil	SAE30			Used on air cleaner precleaner; high temperature, low ash crankcase oil recommended for LPG engines.
Engine oil	SAE 15W-40 or 20W-40	API CD/SE or CC/SE	-10°C to 32°C	SAE 15W-40 or 20W-40 in 15F (-10C) through 90F (32C); multi-grade 5W-30 below 32F (0C)
Fuel	ASTM 2-D or 1-D	minimum Cetane number 45		use 1-D below 32F, long periods of light load or no load; low sulfur; pour point at least 10F below lowest expected
Fuel	HD-5 grade propane (95 percent propane)	NFPA Pamphlet 58		Natural or manufactured gases at least 1000 BTU content.
Fuel	nonleaded gasoline			Recommended for gasoline engines
Grease	graphite lubricant			Steel governor ball joints
Grease	lubricating graphite			For governor ball joint.
Other	baking soda and water (lead acid) / vinegar and water (alkaline)			Battery top cleaning to neutralize electrolyte residues.

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

Procedures

Adjust Carburetor adjustment

⚠ Do not force needles against seats.

Open both needles 1 to 1-1/2 turns off seats.
Apply full load; turn main fuel screw in until speed drops, then out to normal.
Remove load; turn idle screw out until speed drops, then in to normal.

Adjust Governor Linkage (Diesel) adjustment

Loosen locknuts on governor linkage, remove link at one ball joint.
Position governor arm and fuel control arm fully forward.
Adjust linkage so both ball joints fit without moving arms.
Tighten locknuts.
Release fuel solenoid plunger.

Adjust Valve Clearance (Diesel) adjustment

Tools: feeler gauge, socket wrench

Turn flywheel to compression stroke for cylinder to adjust.
Turn flywheel clockwise additional 10 to 45 degrees.
Loosen locknut to increase clearance, tighten to reduce.
Use feeler gauge to set proper gap.
Adjust in firing order 1-2-4-3.

Adjust Valve Tappet (Gasoline) adjustment

Tools: feeler gauge, wrench

Remove valve covers.
Crank engine until intake valve opens and closes.
Continue 1/4 turn until timing marks align.
Turn adjusting screw to obtain correct clearance.
Use flat feeler gauge between valve and valve rotator.

Battery Charge Rate Adjustment adjustment

⚠ Avoid overcharging.

Tools: DC ammeter

Set slide tap on resistor R4 to about 2 amperes charging rate.
For frequent starts, increase charge rate slightly until battery stays charged.
Stop engine when readjusting to avoid shorts.

Carburetor Final Adjustment (gasoline) adjustment

⚠ Valves may be damaged by turning them in too far.

Tools: Screwdriver

Turn main fuel valve in until engine misses (lean), then out past smooth until uneven (rich); set to mid-point.
Hold engine at idle and set low speed adjustment screw until a fast idle of 1200 rpm is obtained.
Turn idle adjustment valve in (lean) and out (rich) until engine idles smoothly.
Reset low speed adjustment screw so engine idles at 1200 rpm.
Release throttle; engine should accelerate without hesitation, readjust main fuel valve if not.

Flowing the Pump (PLB timing) adjustment

Install pump with .006-inch (0.15 mm) shims between pump and pad.
Loosen delivery valve holder to relieve pressure on spring.
Rotate flywheel to about 15 degrees before port closing point.
Operate transfer pump hand primer lever and rotate flywheel slowly clockwise until fuel stops coming out of pump outlet.
Measure distance from port closing point to PC mark and add shims per table.
Install pump.

Fuel Injection Pump Timing (MDJA Method 2) adjustment

Install pump with 0.006-inch (0.15 mm) shims.
Loosen delivery valve holder to relieve spring pressure.
Rotate flywheel to about 15 degrees before port closing point.
Blow in pump inlet and rotate flywheel slowly clockwise until air stops.
Measure distance from port closing point to PC mark.
Select shims from table and install pump.

Set Breaker Points (MAJ) adjustment

Remove flywheel and examine breaker points.
Adjust gap to .022 inch (0.56 mm) at full separation.
Loosen screw to shift point set, retighten securely.

Time Ignition (MCCK) adjustment

Tools: continuity light, thickness gauge

Crank engine to fully open breaker points.
Loosen locking screws and turn cam to adjust.
Set point gap to .020 inch (0.51 mm).
Shift breaker point box to align timing mark.

Time PLB Pump (DJA) adjustment

Tools: depth micrometer

Install pump tappet in recess, position flywheel on port closing mark.
Measure distance from mounting pad to tappet center.
Subtract from 1.670-inch port closing dimension.
Select correct shims and install pump.

Timing the PLB Pump - Depth Micrometer Method adjustment

⚠ Injection pump must be timed on the compression stroke, not the exhaust stroke.

Tools: Depth micrometer

Install pump tappet in its recess and position flywheel port closing mark at timing pointer on compression stroke.
Measure distance from pump mounting pad to tappet center with a depth micrometer.
Subtract the depth from the 4.670-inch (42.4 mm) port closing dimension to get shim thickness.
Select correct shims and install pump.

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
		12-lead broad range reconnectible alternator	12 lead	User connects for required voltage and amperage
		X1 to VR21-5	X1	for 50 Hertz
		X1 to VR21-6	X1	for 60 Hertz
120	1	2-wire		Reconnectible
120/240	1	3-wire		Standard connection for MCCK/MDJE/MDJC/MDJF
240	1	2-wire		Reconnectible

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 MDJA?

The Cummins/Onan MDJA engine has an oil capacity of 2.8 liters.

What is the displacement of the Cummins/Onan MDJA?

The Cummins/Onan MDJA has a displacement of 0.5 liters with 1 cylinders.

Which generators use the Cummins/Onan MDJA engine?

The Cummins/Onan MDJA powers 2 generator set(s), including Cummins/Onan 3.0MDJA, Cummins/Onan MDJA.

How often should you change the oil in the Cummins/Onan MDJA?

The Cummins/Onan MDJA oil change interval is every 100 hours.

What is a common problem with the Cummins/Onan MDJA engine?

A commonly reported issue on generators using the Cummins/Onan MDJA is: Overload.