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Cummins/Onan MDJF (Onan J-series diesel) Engine

The Cummins/Onan MDJF (Onan J-series diesel) is a 4-cylinder naturally aspirated engine rated at 1800 rpm. It powers generator sets from Cummins/Onan, including the 12.0MDJF-53CR.

Also known as: Onan MDJF (Onan J-series diesel)

Cylinders

1800 rpm

Rated speed

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

Engine specifications

Cylinders	4
Aspiration	naturally aspirated
Rated speed	1800 rpm

Generator sets powered by this engine

Cummins/Onan 12.0MDJF-53CR

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

Common problems

Water or foreign material in the fuel can ruin the injection system.
Cause: If daily inspection shows water or dirt in the fuel pump sediment bowl
Fix: Fuel handling and storage facilities should be checked and situation corrected. Fi Iter elements should be replaced following corrections of fuel contamination problems.
This service must be conducted by trained diesel injection equipment personnel with suitable test facilities.
Fix: Omit this service until these conditions can be met.
Slip ring brushes not contacting the slip rings.
Fix: Replace slip ring brush spring which may have come off or broken; or replace brushes which may have become worn down to far to contact the slip rings.
Short circuit in field
Cause: Dampness or deteriorated insulation.
Fix: Bake if damp, repair or replace if insulation is deteriorated.
Overload 10
Open circuit in field
Cause: Rough usage or original short circuit which may have burned a coil or connection.
Fix: Examine field connections and test with ohmmeter. If a coil is open, replace it.
No specific name provided for e008 e008
No generator output or low output.
Cause: Defective shunt field bridge rectifier
Fix: Check AC components as listed above and preceding page. Check bridge rectifier with an ohmmeter. Replace if necessary.
No battery charger output (1 amp normal).
Cause: Diode CR2 bad.
Fix: Open component in charging circuit. Check diode with an ohmmeter and replace if open or shorted.
Insulating film on slip rings.
Fix: Clean slip rings with stone or fine sandpaper and blow out dust.
Incomplete circuit between exciter and slip rings.
Cause: Slip ring brush shunt broken
Fix: Check all slip ring brush shunts with an ohmmeter and replace broken brush shunts.
Faulty load
Cause: Open circuit or short circuit on line.
Fix: Check line and load and the load.
Ammeter indicating ‘O’
Cause: Generator has not built up voltage
Fix: Check the automatic field flash circuit which receives power from the § terminal of the start solenoid. When the start solenoid is energized, power should be applied to the static exciter and revolving field for faster voltage build-up.
AC stator winding shorted
Cause: Insulation or coils broken.
Fix: Rewind or replace stator winding.
AC stator winding open
Cause: Original short circuit may have burned a coil or connection.
Fix: Test with an ohmmeter and if open, rewind or replace with a new stator winding.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan MDJF (Onan J-series diesel).

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter			140-0606		Amazon eBay
Fuel filter			122-0325	600 hr	Amazon eBay $75.00 NAPA $35.18
Oil filter			122-0185	100 hr	Amazon Amazon eBay $19.55 eBay NAPA $12.88
Battery	SAE group 1H, two in series, 12 volt source; 105 amp-hr (spec table lists 120 amp-hr SAE 20-hr nominal)		—
Belt			511-0067		Amazon eBay NAPA $4.76
Coolant	Clean alkali-free water plus approved rust inhibitor; anti-freeze if freezing		—
Coolant filter	Remove bolt, end cap and gasket at front end of heat exchanger; clean water passages		—
Oil	API CD/SE or CD/SD; initial fill CC (formerly DM) oil for first 50-100 hours	6.0 U.S. quarts	—	100 hr

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
Cylinder head bolts and valve clearance	Adjust	2 hr		First service — Tighten head bolts and adjust valve clearance after first 2 hours on a new or overhauled engine
Exhaust system	Inspect	8 hr		With plant running, visually and audibly check for leaks
Fuel sediment bowl	Drain	8 hr
Fuel supply	Check	8 hr
Oil level	Check	8 hr		Should be at full mark on oil indicator
Unit	Inspect	8 hr		Operator maintenance schedule
Water level (heat exchanger models)	Check	8 hr
Crankcase oil	Change	100 hr		Dirty conditions - change every 50 operating hours
Crankcase oil	Drain	50 hr		First service — Drain initial oil after 50 hours while engine is still hot
Governor linkage	Clean	100 hr		Perform more often in extremely dirty conditions; lubricate old style linkage
Oil filter	Replace	100 hr
Battery	Check	200 hr		Keep electrolyte level above plates
Brushes	Check	200 hr		Replace collector ring brushes when worn to 5/16 in; commutator brushes when worn to 5/8 in
Commutator	Check	200 hr
Fuel system condensation traps	Drain	200 hr
Slip rings	Check	200 hr		Critical maintenance schedule
Generator	Clean	500 hr
Rocker box oil line holes	Clean	500 hr
Valve clearance	Check	500 hr
Water pump (leaks and wear)	Inspect	500 hr		Replace if necessary
Primary fuel filter	Change	600 hr
Secondary fuel filter	Change	600 hr		Replace with every 5th change of the primary filter
Nozzle opening pressure and spray pattern	Test	1000 hr		Must be conducted by trained diesel injection equipment personnel
Valves	Adjust	1000 hr		Grind valves if required
Breather pulsation dampener element	Replace	2000 hr
Fuel filters	Change	2000 hr
Engine	Other	5000 hr		General overhaul if required
Zinc pencil (heat exchanger)	Inspect		2 mo	Replace if deteriorated to less than 1/2 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 bearing clearance limit	.0012-.0037	in
Clearance	Camshaft end play (in back position)	.007-.039	in
Clearance	Clearance, large bearing to crankshaft	.001-.0033	in
Clearance	Collector ring run-out	0.002	in	compared to generator bearing
Clearance	Connecting rod large bearing diameter (I.D.)	2.0615-2.0630	in
Clearance	Connecting rod small bushing diameter (I.D.)	0.9903-0.9906	in
Clearance	Crankshaft connecting rod journal diameter	2.0597-2.0605	in
Clearance	Crankshaft end play	.010-.015	in
Clearance	Crankshaft main bearing clearance (original equipment)	.0030-.0043	in
Clearance	Crankshaft main bearing journal diameter	2.2427-2.2435	in
Clearance	Cylinder diameter limits	3.4995-3.5005	in
Clearance	Exhaust valve clearance in guide	.0030-.0050	in
Clearance	Exhaust valve stem diameter	.3405-.3415	in
Clearance	Intake valve clearance in guide	.0015-.0030	in
Clearance	Intake valve stem diameter	.3405-.3410	in
Clearance	Piston clearance to cylinder wall below oil ring groove	.0055-.0075	in	measure 90 degrees to pin
Clearance	Piston pin clearance	Thumb Push Fit
Clearance	Starting motor armature end play	.005-.030	in
Clearance	Starting motor pinion clearance to pinion stop	.070-.120	in	solenoid plunger bottomed
Clearance	Starting motor pinion rest position (pinion housing mounting face to outer edge of pinion)	1-9/32 to 1-15/32	in
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)
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	Centrifugal switch breaker points gap	.040	in
Gap	Centrifugal switch gap	0.020 (0.51 mm)	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 pump timing (DJA)	17	degrees BTC
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	Bilge blower time delay	5	minutes
Other	Cooling water flow	6	gallons per minute	after thermostat opens
Other	Cranking limiter time	45 to 90	seconds
Other	Cylinder bore	3-1/2	in
Other	Emergency relay K14 operation time	15	seconds
Other	Emergency time delay low oil shutdown	45	seconds
Other	Exhaust valve face seat angle	45	degrees
Other	Intake valve face seat angle	42	degrees
Other	Maximum operation angle MCCK, MDJA, MDJE	20	degrees	any direction
Other	Maximum operation angle MDJC, MDJF	30	degrees	any direction
Other	Starting motor brush spring tension	32-40	ounces	with new brushes
Other	Starting motor no-load test	10 volts - 80 amps, 5000 rpm Min.
Other	Starting motor stall torque	4 volts - 420 amps, 7.8 lbs. ft. Min.
Pressure	Delivery valve line pressure	300-600	psi (2070-4140 kPa)	engine running
Pressure	Engine oil pressure	20	psi	minimum, pressure-relief valve is not adjustable
Pressure	Fuel pump pressure (MDJA)	5-6	psi (34.5-41.4 kPa)	at 1800 rpm
Pressure	Fuel pump pressure (MDJC, MDJE, MDJF)	12-14	psi (83-97 kPa)	at 1800 rpm
Pressure	Fuel transfer pump vacuum	15-18	inches (25.4-43.4 mm)	at sea level
Pressure	Injection line pressure	1900	psi (13,110 kPa)
Pressure	Nozzle opening pressure	1900-1950	psi (13,110-13,455 kPa)
Pressure	Nozzle opening pressure (Kiki/Yanmar)	2133-2204	psi (14707-15196 kPa)
Pressure	Nozzle opening pressure after several hundred hours	1750	psi (12,075 kPa)

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	Clean, alkali-free water	Add an approved rust inhibitor; anti-freeze if freezing			Closed cooling (heat exchanger) system
Coolant	antifreeze				Replace annually in captive system
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	CC (formerly DM)	Passing MS Sequence Tests			First fill only (50 to 100 hours) for new or rebuilt J series diesel
Engine oil	SAE 10W-30, 5W-20, or 10W, 5W-30	API CD/SE or CD/SD			0 deg F to 32 deg F
Engine oil	SAE 30	API CD/SE or CD/SD (Series 3 Test; MS Sequences IIA and IIIA)		6.0 U.S. quarts	Above 32 deg F; do not mix brands or grades; plus 1/2 quart for new filter
Engine oil	SAE 5W-20 or 5W-30	API CD/SE or CD/SD			Third temperature row (label not legible in source)
Engine oil	SAE 15W-40 or 20W-40		-10°C to 32°C		J-series water-cooled diesel engines
Fuel	ASTM 2-D or 1-D	min Cetane 45			Diesel; use 1-D below 32°F (0°C)
Fuel	No. 1 diesel	Low sulfur content; pour point at least 10 deg F below lowest expected temperature			When ambient temperature is below 32 deg F, or at all temperatures during long periods of light engine load
Fuel	No. 2 diesel				Best economy
Fuel	nonleaded gasoline				Recommended for gasoline engines
Grease	graphite lubricant				Steel governor ball joints

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.

Checking Centrifugal Breaker Points adjustment

Tools: Thickness gauge, Socket wrench, File or fine stone

Rotate engine crankshaft a few degrees counterclockwise before adjusting points (use socket wrench on flywheel retaining screw).
Measure gap with a thickness gauge; set gap at .040 in.
Loosen and move the stationary contact to correct gap.
Dress slightly burned points smooth with a file or fine stone; replace burned or faulty points.

Checking and Adjusting Valve Clearance adjustment

Tools: Feeler gauge, Socket wrench

Turn flywheel until cylinder to be adjusted is on compression stroke (socket wrench on flywheel screw hex head).
Turn flywheel clockwise an additional 10 to 45 degrees so piston is on power stroke with both valves closed.
Loosen the rocker arm locknut to increase clearance; tighten to reduce clearance.
Using a feeler gauge, check clearance between rocker arm and valve; set to .017 in intake and .017 in exhaust.
Always adjust valve clearances in the firing order (1-2-4-3) sequence, rotating flywheel 180 degrees between cylinders.

Governor Sensitivity Adjustment adjustment

⚠ If the governor is too sensitive, a rapid hunting condition occurs.

Turn the sensitivity adjusting ratchet counterclockwise for more sensitivity (less speed drop at full load).
Turn clockwise for less sensitivity (more speed drop).
Adjust for maximum sensitivity without hunting.
Readjust speed after sensitivity adjustment.
Secure the lock nut.

Governor Speed Adjustment adjustment

⚠ Excessive droop may be caused by engine misfiring; correct this before adjusting the governor.

Tools: Tachometer or frequency meter

Change spring tension by turning the governor spring nut.
Turn nut clockwise (more spring tension) to increase RPM.
Turn nut counterclockwise to reduce governed speed.
Hold a tachometer against the flywheel cap screw or use a frequency meter.
After adjusting, secure the lock nut.

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.

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

Which generators use the Cummins/Onan MDJF (Onan J-series diesel) engine?

The Cummins/Onan MDJF (Onan J-series diesel) powers 1 generator set(s), including Cummins/Onan 12.0MDJF-53CR.

How often should you change the oil in the Cummins/Onan MDJF (Onan J-series diesel)?

The Cummins/Onan MDJF (Onan J-series diesel) oil change interval is every 100 hours.

What is a common problem with the Cummins/Onan MDJF (Onan J-series diesel) engine?

A commonly reported issue on generators using the Cummins/Onan MDJF (Onan J-series diesel) is: Water or foreign material in the fuel can ruin the injection system.. Cause: If daily inspection shows water or dirt in the fuel pump sediment bowl. Fix: Fuel handling and storage facilities should be checked and situation corrected. Fi Iter elements should be replaced following corrections of fuel contamination problems..