Home›Engines›Cummins/Onan MAJB

Cummins/Onan MAJB Engine

The Cummins/Onan MAJB is a 0.24 L 1-cylinder four-cycle, water-cooled engine rated at 3600 rpm. It powers generator sets from Cummins/Onan, including the 2.5 MAJB-53BR, 2.5 MAJB-53CR and 3.0 MAJB-1R. It holds 0.95 L (0.3 gal) of oil.

Also known as: Onan MAJB

0.244 L

Displacement

Cylinders

3600 rpm

Rated speed

0.95 L

Oil capacity

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

Engine specifications

Oil capacity	0.95 L
Displacement	0.244 L
Cylinders	1
Aspiration	Four-Cycle, Water-Cooled
Rated speed	3600 rpm

Generator sets powered by this engine

Cummins/Onan 2.5 MAJB-53BRCummins/Onan 2.5 MAJB-53CRCummins/Onan 3.0 MAJB-1R

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

Common problems

Run Lamp, Time Meter, or Battery Condition Meter Does Not Operate
Cause: 1. Open circuit between the control and the remote Start/Stop switch. 2. If the battery condition meter works but the time meter does not, the time meter is defective.
Fix: 1. Check for continuity and correct if circuit is open. 2. Replace time meter.
Low Oil Pressure
Cause: Oil viscosity too light or oil is diluted. Low oil level. Low oil pressure switch defective. Faulty oil bypass valve. Excessive engine wear or defective oil pump.
Fix: 1. Drain oil and refill with correct viscosity oil. 2. Add oil as required. 3. Replace oil pressure switch (see Engine Block Assembly section). 4. Inspect oil bypass valve and clean or replace as required (see Engine Block Assembly section). 5. See Engine Block Assembly Section
Low Battery
Cause: 1. Weak or discharged battery. Generator set charger will not recharge a battery that is in a very low state of charge. 2. Load connected to battery while set is turned off. 3. Open in charge circuit due to: a. blown charging fuse or b. open charging diode (CRI).
Fix: 1. Check and replace weak or discharged battery. 2. Disconnect load from battery when generator set is turned off. 3a. Replace blown charging fuse. 3b. Replace open charging diode.
High Oil Consumption
Cause: Oil viscosity too light or oil is diluted. Crankcase breather valve is dirty or defective. Oil leaks. Excessive engine wear.
Fix: 1. Drain oil and refill with correct viscosity oil. 2. Clean crankcase breather and replace if defective. 3. Locate source of leak and repair as required. 4. See Engine Block Assembly section.
Engine Starts But Stops When Start Switch is Released
Cause: 1. Low oil pressure switch not closing due to: a. low oil level, b. open circuit between switch and control, c. defective low oil pressure switch, or d. low oil pressure. 2. Output voltage from generator not being supplied to control due to: a. open circuit in wiring between generator and control, b. defective bridge rectifier, or c. no output voltage from generator.
Fix: 1a. Check oil level and add oil if low. 1b. Check for continuity and correct if circuit is open. 1c. Replace low oil pressure switch. 2a. Check for continuity and correct if circuit is open. 2b,c. Refer to Generator section for test and service procedures.
Engine Starts And Runs; Then Stops. Set Restarts Immediately or Set Restarts After Cooling Down
Cause: 1. Fuel level is below generator set fuel pickup tube or oil level is low. 2. Faulty choke operation due to sticking choke linkage, incorrect choke adjustment, open circuit in wiring between choke heater and generator, or defective choke heater. 3. Vapor lock due to: a. high ambient air temperatures or b. faulty fuel pump. 4. Breaker points sticking. 5. Contaminated fuel.
Fix: 1. Check fuel and oil levels and refill as necessary. 2. Refer to Fuel System section for testing and service procedures.
Engine Overheats
Cause: Restricted water flow due to dirt or debris blocking water inlet passages. Faulty thermostat Incorrect ignition timing. Lean fuel mixture due to: a. incorrectly adjusted fuel mixture screws, b. incorrect float level, or c. dirt in carburetor. Faulty water pump impeller.
Fix: 1a. Clean away any debris from water inlet and flush the engine cooling system. 2. Replace thermostat. 3. Adjust breaker point gap. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. 5. Replace impeller.
Engine Lacks Power
Cause: Faulty ignition due to: a. incorrect ignition timing or b. incorrect spark plug gap. Restricted fuel flow due to: a. plugged fuel filter or b. faulty fuel pump. Incorrect fuel mixture due to: a. incorrectly adjusted fuel mixture screws, b. incorrect float level, or c. dirt in carburetor.
Fix: 1a. Adjust breaker point timing. 1b. Reset spark plug gap. 2a. Clean fuel filter. 2b. Test fuel pump and repair or replace if faulty. 3a. Adjust carburetor main and idle adjustment screws. 3b. Adjust carburetor float level. 3c. Disassemble carburetor and clean all internal passages.
Engine Hunts or Surges
Cause: Sticking or binding governor linkage. Incorrect governor adjustment Faulty governor spring. Incorrect fuel mixture due to: a. incorrectly adjusted fuel mixture screws, b. incorrect float level, or c. dirt in carburetor. Governor mechanism worn excessively.
Fix: 1a. Clean and lubricate governor linkage. 2. Adjust governor speed and sensitivity. 3. Replace governor spring. 4a. Adjust carburetor main and idle adjustment screws. 4b. Adjust carburetor float level. 4c. Disassemble carburetor and clean all internal passages. 5. See Engine Block Assembly section.
Engine Does Not Crank
Cause: A. open circuit in remote control or b. remote start switch faulty.
Fix: 1a. Check for continuity and correct if circuit is open. 1b. Replace remote start control switch.
Engine Cranks But Does Not Start
Cause: 1. Faulty ignition due to worn or fouled spark plugs, magneto malfunction, worn ignition points, incorrect ignition timing, or faulty plug wires. 2. Faulty fuel system due to sticking choke, faulty fuel pump, or carburetor mixture screws incorrectly adjusted.
Fix: 1. Refer to Ignition System section for test and service procedures. 2. Refer to Fuel System section for test and service procedures.
Black Exhaust Smoke
Cause: Rich fuel mixture due to: a. choke sticking, b. incorrectly adjusted fuel mixture screws, or c. dirt in carburetor.
Fix: 1a. Clean choke and choke linkage. 1b. Adjust carburetor idle and main adjustment screws. 1c. Disassemble carburetor and clean all internal passages.

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

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter			—
Fuel filter			—
Oil filter			—
Battery			—
Belt			—
Coolant			—
Coolant filter			—
Oil			—
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
Engine oil level	Check	8 hr		Check more frequently on a new or reconditioned engine
Cylinder head	Adjust	25 hr		First service — Retorque before the engine has run a total of 25 hours
Crankcase breather ball valve	Clean	100 hr		Wash ball valve in cleaning solvent
Cooling system	Other		12 mo	Flush the system at least once a year

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	Alternator brush wear limit	5/16	in.	replace if worn to or below
Clearance	Camshaft bearing clearance	0.0012-0.0030	in. (0.038-0.076 mm)
Clearance	Camshaft journal diameter	1.3740-1.3745	in. (34.900-34.912 mm)
Clearance	Connecting rod bearing clearance	0.0015-0.0025	in. (0.038-0.064 mm)
Clearance	Crankshaft end play	0.0080-0.0120	in. (0.203-0.305 mm)
Clearance	Crankshaft main bearing clearance	0.0015-0.0043	in. (0.0381-0.109 mm)
Clearance	Crankshaft main bearing journal diameter	1.6857-1.6865	in. (42.817-42.837 mm)
Clearance	Crankshaft rod journal diameter	1.3742-1.3759	in. (34.905-34.925 mm)
Clearance	Cylinder bore (std. size honed)	2.7520-2.7530	in. (69.900-69.926 mm)
Clearance	Piston clearance in cylinder	0.0040-0.0060	in. (0.102-0.152 mm)
Clearance	Piston pin diameter	0.6250-0.6252	in. (15.875-15.88 mm)
Clearance	Piston ring #1 (top) groove width	0.0955-0.0965	in. (2.426-2.451 mm)
Clearance	Piston ring #1 (top) ring gap	0.0060-0.0180	in. (0.152-0.457 mm)
Clearance	Piston ring #1 (top) side clearance	0.0020-0.0080	in. (0.051-0.203 mm)
Clearance	Piston ring #2 groove width	0.0950-0.0960	in. (2.413-2.438 mm)
Clearance	Piston ring #3 groove width	0.1565-0.1575	in. (3.975-4.000 mm)
Clearance	Starter brush wear limit	1/4	in. (6.4 mm)	replace if worn below
Clearance	Valve guide diameter	0.3110-0.3120	in. (7.889-7.929 mm)
Clearance	Valve seat width	0.0310-0.0470	in. (0.787-1.194 mm)
Clearance	Valve spring free length (approx)	1.600	in. (40.64 mm)
Clearance	Valve stem clearance (exhaust)	0.0030-0.0035	in. (0.076-0.089 mm)
Clearance	Valve stem clearance (intake)	0.0010-0.0025	in. (0.025-0.064 mm)
Clearance	Valve stem diameter (exhaust)	0.3080-0.3085	in. (7.823-7.836 mm)
Clearance	Valve stem diameter (intake)	0.3095-0.3100	in. (7.861-7.874 mm)
Clearance	Valve tappet to block clearance	0.0030-0.0040	in. (0.076-0.102 mm)
Gap	Magneto air gap	0.010-0.015	in. (0.254-0.381 mm)
Gap	Spark plug gap	0.025	in. (0.64 mm)
Other	Battery charge current	0.1-1.0	A
Other	Cooling water flow minimum	1.8	GPM (6.8 litres/min)
Other	Ignition timing	25	deg BTC
Pressure	Fuel pump outlet pressure	4-1/2 to 5	psi (31 to 34.5 kPa)
Pressure	Low oil pressure switch setpoint	9	psi (62 kPa)
Pressure	Normal oil pressure	30 or higher	psi (207 kPa)	normal operating temperature
Pressure	Oil bypass valve opening	30	psi (207 kPa)
Resistance	Rotor winding resistance	25.6 to 28.4	ohms	room temperature
Speed	No-load frequency setting	62	Hz (52 Hz on 50 Hz units)	warmed-up unit at no load
Temperature	High engine temperature switch setpoint	200	deg F (93.33 deg C)
Temperature	High exhaust temperature switch setpoint	190	deg F (88 deg C)
Temperature	Thermostat fully open	165	deg F (73.88 deg C)
Temperature	Thermostat starts to open	145	deg F (62.78 deg C)
Torque	Carburetor mounting nuts	12-15	ft-lbs (16-20 N.m)
Torque	Connecting rod	10-12	ft-lbs (14-16 N.m)
Torque	Cylinder head (cold)	24-26	ft-lbs (33-35 N.m)	cold
Torque	Flywheel mounting screw	35-40	ft-lbs (48-54 N.m)
Torque	Fuel pump	10-15	ft-lbs (14-20 N.m)
Torque	Gearcase cover	15-20	ft-lbs (21-27 N.m)
Torque	Oil base	25-30	ft-lbs (34-40 N.m)
Torque	Oil pump	7-9	ft-lbs (10-12 N.m)
Torque	Rear bearing plate	4-8	ft-lbs (6-10 N.m)
Torque	Rotor through-bolt	17-21	ft-lbs (23-28 N.m)
Torque	Spark plug	25-30	ft-lbs (34-40 N.m)
Torque	Starter brush screws	3 to 3-1/2	ft-lbs (4 to 5 N.m)
Torque	Starter lug terminal nut	4 to 5	ft-lbs (5 to 7 N.m)
Torque	Starter mounting screws	30 to 33	ft-lbs (41 to 45 N.m)
Torque	Starter pinion gear stop nut	20 to 25	ft-lbs (27 to 34 N.m)
Torque	Stator through-bolts	15-20	ft-lbs (21-27 N.m)
Torque	Valve cover nut	8-12	ft-lbs (11-16 N.m)
Valve clearance	Valve clearance, exhaust	0.011	in. (0.279 mm)	ambient temperature
Valve clearance	Valve clearance, intake	0.011	in. (0.279 mm)	ambient temperature
Voltage	AC voltage build-up	approximately 128	V	at governed speed

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	Heavy-duty detergent oil, API service designation SF (SE or SE/CC if SF not available)	MS Sequence Tests and MIL-L-2104B		2.0 QT	Capacity 2.0 QT (0.95 Litre) with filter; keep level about halfway up threads on inside of oil-plug hole
Fuel	Regular or Unleaded Gasoline

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

Procedures

Breaker Points Adjustment adjustment

⚠ Disconnect battery ground cable before servicing

Tools: Ohmmeter or continuity tester, Hex-head socket, Socket wrench

Disconnect the negative (-) battery cable at the battery terminal.
Remove the breaker box cover.
Disconnect the breaker box control and magneto leads.
Connect an ohmmeter or continuity tester to the hot and ground sides of the breaker points.
Remove the spark plugs and turn the rotor through-bolt clockwise until the points begin to open.
Adjust the points to open when the flywheel timing mark aligns with the 25 degree mark on the gear case cover.
Replace the point box cover, spark plugs, and spark plug leads.
Connect the negative (-) battery cable to the negative battery terminal.

Carburetor Mixture Adjustment adjustment

⚠ Forcing mixture adjustment screws tight will damage the needle and seat

Tools: Carburetor adjustment wrench

Apply a full load to the engine.
Turn in the main fuel adjustment until engine speed drops, then turn out the needle until speed returns to normal.
Remove the load from the engine.
Turn the idle fuel adjustment out until speed drops slightly, then turn the needle in until speed returns to normal.

Governor Adjustment adjustment

⚠ Stay clear of rotating components; ensure guards are in place

Tools: Voltmeter, Frequency meter

Adjust carburetor fuel mixture screws before governor adjustments.
Adjust governor linkage length so the throttle shaft stop almost touches the carburetor stop with engine stopped and tension on the governor spring.
With warmed-up unit at no load, adjust the speed adjustment nut to obtain 62 hertz (60 Hz unit) or 52 hertz (50 Hz unit).
Check frequency and voltage with load and without load; frequency should not vary more than 5 percent of nominal.
Adjust governor sensitivity for closest regulation without hunting; turn sensitivity screw counterclockwise to increase sensitivity.
Recheck the speed setting and readjust if necessary.
Set the carburetor throttle stop screw as specified under Carburetor Adjustments.

Valve Tappet Adjustment adjustment

Tools: Socket wrench, Feeler gauge

Remove all parts necessary to gain access to the valve tappets.
Remove the spark plugs to make turning the engine easier.
Rotate the crankshaft clockwise until the intake valve opens and closes, then until the flywheel mark aligns with the TC mark on the gear cover.
Verify the intake and exhaust valves are closed and there is no pressure on the valve lifters.
Measure the valve clearance and turn the adjusting screw as needed to obtain correct clearance.
Reinstall all parts, tighten all screws securely, and torque the manifold bolts.

Electric Fuel Pump Pressure Test diagnostic

⚠ Do not substitute automotive type electric fuel pumps

Tools: Pressure gauge

Remove the fuel line from the pump outlet and install a pressure gauge.
Press the START switch and hold it for several seconds until the pressure reading is constant.
The pressure reading should be 4-1/2 to 5 psi (31 to 34.5 kPa); if retention is good the pressure should stay constant or drop off very slowly.

Breaker Points and Condenser Replacement repair

⚠ Make adjustments with engine static and cold

Tools: Screwdriver, Ohmmeter or continuity tester

Disconnect the negative (-) battery cable at the battery terminal.
Remove the breaker box cover clip and lift off the cover.
Remove the condenser mounting screw, disconnect the condenser and ignition lead wires, and lift out the condenser.
Remove the breaker points mounting screws and lift out the breaker point assembly.
Replace the condenser and point assembly.
Adjust the points as described in the adjustment procedure.
Replace the breaker box cover, spark plugs, and spark plug leads.
Connect the negative (-) battery cable to the negative battery terminal.

Cylinder Head Removal and Installation repair

⚠ Wait until the engine has cooled before removing the head

Tools: Socket wrench, Torque wrench

Use a socket wrench to remove the cylinder head nuts and lift off the head.
Clean out all carbon deposits; do not damage the outer sealing edge where the gasket fits.
Use a new head gasket and clean the head and cylinder block where the gasket rests.
Place the head in position and tighten nuts in sequence to 5 ft-lbs (7 N.m), then 10 ft-lbs (14 N.m), etc., up to 24 to 26 ft-lbs (33 to 35 N.m).
Retorque before the engine has run a total of 25 hours.

Electric Fuel Pump Service repair

⚠ Do not tamper with the seal at the center of the mounting bracket; electrical system is not serviceable

Tools: 5/8-inch wrench, Thin nose pliers

Using a 5/8-inch wrench, loosen the pump cover, then remove the cover by hand.
Remove the filter, magnet and cover gasket.
Remove the retainer spring from the plunger tube; remove washer, O-ring seal, cup valve, return spring, and plunger.
Wash all parts (except gasket and seal) in parts-cleaning solvent and blow out with low pressure compressed air.
Insert the plunger in the tube spring end first and check for free movement and a click.
Install the plunger spring, cup valve, O-ring seal and washer; compress the spring and install the retainer.
Check the cover gasket and replace if deteriorated, then install gasket, magnet, filter and cover assembly.

Generator Disassembly repair

⚠ The brushes must be removed for inspection before removal

Tools: Screwdriver, Soft-faced hammer (brass or lead), Allen wrench

Remove the control cover and screws securing the end bell cover; remove additional wiring as needed.
Remove the end bell cover.
Disconnect the stator leads.
Remove the four long capscrews, lockwashers and nuts fastening the end bell, stator and generator adapter; pull the end bell and stator straight out.
Remove the internal allen screw, sleeve and rotor through-stud; tap the rotor gently to loosen it from the crankshaft.
Remove the three capscrews, nuts, and lockwashers securing the generator adapter to the engine and remove the adapter.
Assemble in reverse of disassembly following the torque values in Section 4.

Sea Water Pump Assembly repair

⚠ Never operate a neoprene impeller pump dry

Tools: Arbor press, Screwdrivers

Press the bearing over the pulley end of the shaft far enough to clear the shaft retaining ring groove.
Install the new water seal as shown; lubricate surfaces; seal surface must be just below the edge of the impeller body.
Install the plastic washer and O-ring over the impeller end of the shaft.
Press the shaft/bearing assembly into the pump until it bottoms against the pump body.
Install the bearing and shaft retainer rings and the plastic end cap/seal.
Install the wear plate in the impeller chamber with the notch over the locator pin.
Install the cam and cam set screw with one copper washer under the screw head.
Lubricate the impeller with vaseline, fit it in the direction of rotation, install gasket, end plate and screws, then pulley, belt and belt guard.

Sea Water Pump Disassembly repair

⚠ Disconnect starting battery cables when repairs are made

Tools: Screwdrivers, Pliers, Set screw wrench

Remove the belt guard, belt and pulley.
Remove the 2 capscrews securing the pump to its bracket.
Remove the pump end plate screws, end plate, and gasket.
Pull out the neoprene impeller using 2 screwdrivers or pliers.
Loosen the set screw on the pump body, tap it lightly, and lift out the cam and wear plate.
Remove the plastic end cap/seal from the pump body (pulley end of shaft).
Remove the bearing and shaft retaining rings at the pulley end of the pump body.
Push the shaft and bearing assembly out the pulley end of the pump body.

Starter Assembly repair

⚠ Do not immerse bearings in cleaning fluid

Tools: Torque wrench, Clean cloth

Wipe dirt and oil from starter components with a clean cloth or filtered low-pressure compressed air.
Assemble the brushes in the brush holder so the chamfered edge is away from the brush springs.
Tighten the brush screws to 3 to 3-1/2 ft-lbs (4 to 5 N.m) torque.
Tighten the lug terminal nut to 4 to 5 ft-lbs (5 to 7 N.m) torque.
Apply a thin film of silicone-based grease to armature shaft bearing contact areas.
Install the pinion gear stop nut and tighten to 20 to 25 ft-lbs (27 to 34 N.m) torque.
Place the armature end cap on the housing, insert the armature, and install the through-bolts.
Mount the starter on the stator housing and tighten mounting screws to 30 to 33 ft-lbs (41 to 45 N.m).

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	Connection	Leads	Notes
110/220		T1-T2, T3-T4	2.5 MAJB-53BR, 50 hertz, two windings
120	single winding	T1-T2	3.0 MAJB-1R, 60 hertz
120/240		T1-T2, T3-T4	2.5 MAJB-53CR, 50 hertz, two windings

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 MAJB?

The Cummins/Onan MAJB engine has an oil capacity of 0.95 liters.

What is the displacement of the Cummins/Onan MAJB?

The Cummins/Onan MAJB has a displacement of 0.244 liters with 1 cylinders.

Which generators use the Cummins/Onan MAJB engine?

The Cummins/Onan MAJB powers 3 generator set(s), including Cummins/Onan 2.5 MAJB-53BR, Cummins/Onan 2.5 MAJB-53CR, Cummins/Onan 3.0 MAJB-1R.

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

A commonly reported issue on generators using the Cummins/Onan MAJB is: Run Lamp, Time Meter, or Battery Condition Meter Does Not Operate. Cause: 1. Open circuit between the control and the remote Start/Stop switch. 2. If the battery condition meter works but the time meter does not, the time meter is defective.. Fix: 1. Check for continuity and correct if circuit is open. 2. Replace time meter..