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Cummins/Onan L634T Engine

The Cummins/Onan L634T is a turbocharged engine rated at 1800 rpm. It powers generator sets from Cummins/Onan, including the DL6T. It holds 10.8 L (2.9 gal) of oil and 20.6 L (5.4 gal) of coolant.

Also known as: Onan L634T

1800 rpm

Rated speed

10.8 L

Oil capacity

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

Engine specifications

Oil capacity	10.8 L
Coolant capacity	20.6 L
Aspiration	turbocharged
Rated speed	1800 rpm

Generator sets powered by this engine

Cummins/Onan DL6T

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

Common problems

Engine will not crank
Cause: Indicates possible fault with control or starting system; fault lamp on
Fix: Correct fault and reset control
SWITCH OFF lamp flashes
Cause: Indicates Start/Stop/Remote switch is in the Stop position which will prevent automatic starting if an automatic transfer switch is used
Fix: Move the Start/Stop/Remote switch to the Remote position for automatic starting
PRELO OIL PRES lamp lights. Engine continues to operate
Cause: Indicates engine oil pressure has dropped to 20 psi (138 kPa)
Fix: If engine can be stopped, follow procedures in step 5; if generator is powering critical loads and cannot be shut down, wait until next shutdown period and then follow step 5 procedure
PRE HI ENG 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: If engine can be stopped, follow procedures in step 3; if generator is powering critical loads and cannot be shutdown: check air inlets and outlets and remove any obstructions to airflow; open doors or windows in generator area to increase ventilation; reduce load if possible by turning off non-critical loads
OVERCRANK lamp lights and engine stops cranking after 45 to 75 seconds
Cause: Indicates possible fuel system problem
Fix: When the engine has cooled, make the following checks: check for 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 Onan Dealer or Distributor for service if none of the above
No AC output voltage
Cause: Field breaker is tripped
Fix: Reset breaker; contact an Onan Dealer or Distributor if voltage build up causes breaker to trip
LO OIL PRES lamp lights. Engine shuts down
Cause: Indicates engine oil pressure has dropped to 14 psi (97 kPa)
Fix: Check oil level and replenish if low; reset control and restart; contact an Onan Dealer or Distributor if oil pressure is not in the range of 30 to 55 psi (207 to 380 kPa)
LO ENG TEMP lamp lights. Generator set is in Standby mode or was just started
Cause: Indicates engine block temperature has dropped below 70°F (21°C)
Fix: Lamp will go out when block temperature rises
HI ENG TEMP lamp lights. Engine shuts down
Cause: Indicates engine has overheated (engine temperature has risen above 220°F/104°C) or coolant level is low (sets with coolant level sensor)
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 Onan Dealer or Distributor if none of the above
Green RUN lamp lights following engine start-up
Cause: Indicates all engine systems are normal
Fix: No corrective action required
Fault lamp lights but no fault exists. Engine gauges show oil pressure, engine temperature, and frequency (speed) are within normal limits
Cause: The monitor board or a sensor may be at fault
Fix: Reset control; if control will not reset, contact an Onan Dealer or Distributor for service
Engine starts from generator control panel but won't start automatically or from a remote panel (Note: The Start/Stop/Remote switch must be in the Remote position for automatic or remote starting)
Cause: Remote circuit breaker is tripped
Fix: Reset breaker and restart; contact an Onan Dealer or Distributor if breaker trips after resetting
Engine runs and then shuts down. OVERSPEED lamp lights
Cause: Indicates engine has exceeded normal operating speed
Fix: Contact an Onan Dealer or Distributor for service
Engine does not start after second starting attempt
Cause: No fuel delivery (absence of blue/white exhaust smoke during cranking indicates no fuel delivery)
Fix: Check fuel supply
Engine does not start after 30 seconds of cranking
Fix: Place the Start/Stop/Remote switch in the Stop position; wait two minutes and then repeat steps 1 and 2

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan L634T.

Service intervals

Item	Action	Hours	Months	Notes
Coolant Level	Check	50 hr
Crankcase Oil and Filter	Change	200 hr		First service — Perform after first 50 hours on new sets
Fan Belt	Check	50 hr
Generator Set	Inspect	50 hr		With engine running, visually/audibly check exhaust for leaks
Oil Level	Check	50 hr
Valve Lash Clearance	Check	400 hr		First service — Perform after first 50 hours on new sets; refer to L-Series Engine Service Manual
Air Cleaner	Check	100 hr		Perform more often under extremely dusty/dirty conditions
Exhaust Condensate Trap	Drain	100 hr
Fuel Level	Check	100 hr
Battery Condition	Check	200 hr
Electrolyte Level	Check	200 hr
Generator Assembly	Clean	200 hr
Air Cleaner Element	Change	400 hr
Crankcase Breather Cap (DL6TB)	Clean	400 hr
Fuel Filter	Change	400 hr	12 mo	Whichever comes first
Generator Bearing	Inspect	500 hr		Replace every five 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
Gap	Overspeed switch magnetic air gap	0.005	inch (0.13 mm)	must not be less than
Gap	Overspeed switch magnetic air gap	0.005 inch (0.13 mm) minimum	inch
Other	Battery electrolyte specific gravity	1.260	specific gravity	fully charged battery at 80F (27C)
Other	Battery electrolyte specific gravity (fully charged)	about 1.260	specific gravity	at 80 deg F (27 deg C)
Other	Control breaker rating	7	amperes
Other	Field breaker rating	5	amperes
Other	Low Oil Pressure Delay before shutdown/pre-alarm	7.5 to 12.5	seconds	after Start-Disconnect Relay energized
Other	Low oil pressure delay circuit	7.5 to 12.5	seconds	after start-disconnect relay energized
Other	Maximum allowable exhaust back pressure	41	inches (1041 mm) H2O
Other	Oil pressure gauge range	0 to 100 psi (0 to 700 kPa)	psi
Other	Overcranking timer cranking limit	45 to 75	seconds
Other	Overcranking timer limit	45 to 75	seconds
Other	Remote breaker rating	7	amperes
Other	Switching regulator (standard setting) nominal switching frequency	approximately 125	Hz
Other	Water temperature gauge range	100 to 250F (40 to 121C)	F
Pressure	Air cleaner cleaning air pressure	35	psi (241 kPa)
Pressure	Air cleaner rinse water pressure maximum	40	psi (276 kPa)
Pressure	Low oil pressure shutdown (LO OIL PRES)	14	psi (97 kPa)
Pressure	Low oil pressure switch closes	14	psi (97 kPa)
Pressure	Low oil pressure switch closes	14 psi (97 kPa)	psi
Pressure	Oil pressure gauge normal range	30 to 55	psi (207 to 380 kPa)	operating temperature
Pressure	Oil pressure gauge range	0 to 100	psi (0 to 700 kPa)
Pressure	Pre-low oil pressure switch closes	20	psi (138 kPa)
Pressure	Pre-low oil pressure switch closes	20 psi (138 kPa)	psi
Pressure	Pre-low oil pressure warning	20	psi (138 kPa)
Resistance	Commutating reactor L21 coils 1-2	0.031 to 0.37	ohms	at 77 deg F (25 deg C)
Resistance	Commutating reactor L21 coils 3-4	0.037 to 0.46	ohms	at 77 deg F (25 deg C)
Resistance	Commutating reactor L21/L22 coil 1-2 resistance	.031 to 0.37	ohms	at 77F (25C)
Resistance	Commutating reactor L21/L22 coil 3-4 resistance	0.037 to 0.46	ohms	at 77F (25C)
Resistance	Commutating reactor coil to frame	infinity	ohms
Resistance	Exciter rotor leads T1-T2, T2-T3, T1-T3 resistance	0.6 to 0.7	ohms	at 68F (20C)
Resistance	Exciter rotor winding between lead pairs T1-T2, T2-T3, T1-T3	0.6 to 0.7	ohms	at 68 deg F (20 deg C)
Resistance	Exciter stator / rotor / generator winding insulation to ground	1 megohm (1,000,000 ohms) or greater	ohms	minimum
Resistance	Exciter stator field leads F1-F2 resistance	13.05 to 15.95	ohms	at 77F (25C)
Resistance	Exciter stator winding between F1+ and F2-	13.05 to 15.95	ohms	at 77 deg F (25 deg C)
Resistance	Generator rotor field leads F1-F2 resistance (60 Hz)	2.9	ohms	at 77F (25C)
Resistance	Generator rotor field leads F1-F2 resistance (60 Hz)	3.1	ohms	at 77F (25C)
Resistance	Reference transformer T21 primary coil resistance	333-407	ohms
Resistance	Reference transformer T21 primary coil resistance	350-427	ohms
Resistance	Reference transformer T21 primary coil resistance	531-649	ohms
Resistance	Stator resistance Code 15R (60 Hz)	33.5	milliohms	Test between T1-LO, T2-LO, T3-LO at 77F (25C)
Resistance	Stator resistance Code 15R (60 Hz)	39.4	milliohms	Test between T1-LO, T2-LO, T3-LO at 77F (25C)
Resistance	Stator resistance Code 15R (60 Hz)	9.8	milliohms	Test between T1-LO, T2-LO, T3-LO at 77F (25C)
Resistance	Stator resistance Code 515R (50 Hz)	46.5	milliohms	at 77F (25C)
Resistance	Voltage reference transformer T21 primary winding (H leads)	350-427 / 333-407 / 303-371 / 282-344 / 531-649	ohms
Speed	Engine rated speed (per flow charts)	1800	rpm
Speed	Frequency-to-speed conversion	1	hertz = 30 r/min
Speed	Nominal switching frequency (switching regulator)	125	Hz	approximately
Speed	Overspeed switch contact closure	2200 to 2250	rpm
Speed	Overspeed switch contacts close	3950 to 4050	rpm
Temperature	High engine temperature shutdown	220	F (104 C)
Temperature	High engine temperature switch closes	222	deg F (106 deg C)
Temperature	High engine temperature switch closes	222F (106C)	F
Temperature	Low engine temperature switch closes	70F (21C)	F	coolant drops below
Temperature	Low engine temperature switch closes below	70	deg F (21 deg C)
Temperature	Low engine temperature warning (LO ENG TEMP)	70	F (21 C)	engine block temperature
Temperature	Oil temperature gauge range	150 to 300	deg F (66 to 149 deg C)
Temperature	Pre-high engine temperature switch closes	216	deg F (102 deg C)	nominal
Temperature	Pre-high engine temperature switch closes	216F (102C)	F	nominal
Temperature	Pre-high engine temperature warning	215	F (102 C)

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	Notes
Coolant	Ethylene glycol anti-freeze (permanent type) with rust/corrosion inhibitor, no stop-leak additive		Use at least 50/50 mixture of anti-freeze and clean, soft water
Engine oil	15W-40, 30W, 20, 20W-20, 10W (API CD/SF, CD/SE, or MIL-L-2104C); synthetic 5W-20 or 5W-30 (MIL-L-46167, MIL-L-2104C, MIL-L-46152B)	API CD/SF, CD/SE, MIL-L-2104C	Mineral base 5W-20/5W-30 not recommended under any conditions; select grade for lowest expected temperature
Fuel	ASTM 2-D (No. 2 Diesel) or ASTM 1-D (No. 1 Diesel), Cetane 45 minimum	ASTM 2-D / ASTM 1-D	Use No. 1 diesel below 0C (32F) and during long light-load periods; cloud point at least 10 degrees below lowest expected fuel temperature
Grease	MolyKote grease or equivalent		Apply thin film to mating surfaces of end bearing and end bell bearing hole
Grease	Molykote grease or equivalent		Apply a thin film to the mating surfaces of the end bearing and the end bell bearing hole.
Other	Approved water for battery electrolyte		Add approved water to keep battery electrolyte at proper level.

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
HI ENG TEMP	High Engine Temperature	Engine overheated above 220F (104C) or low coolant	Shutdown	Check coolant level, airflow, fan belt
LO ENG TEMP	Low Engine Temperature	Engine block temperature below 70F (21C)	Info	Wait for block temperature to rise
LO OIL PRES	Low Oil Pressure	Oil pressure dropped to 14 psi (97 kPa)	Shutdown	Check oil level, reset control
OVERCRANK	Overcrank	Engine failed to start after 45-75 seconds cranking	Shutdown	Check fuel system
OVERSPEED	Overspeed	Engine exceeded normal operating speed	Shutdown	Contact Onan Dealer or Distributor
PRE HI ENG TEMP	Pre-High Engine Temperature	Engine temperature approximately 215F (102C)	Warning	Check ventilation, reduce load
PRE LO OIL PRES	Pre-Low Oil Pressure	Oil pressure dropped to 20 psi (138 kPa)	Warning	Check oil level
SWITCH OFF	Switch Off	Start/Stop/Remote switch in Stop position	Info	Move switch to Remote 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

Changing Sensitivity Reference adjustment

⚠ Do not mark on printed circuit boards with a pencil; graphite lines are conductive.

Tools: soldering iron

Unsolder W1 on VR21 at hole F for frequency sensitive.
Resolder W1 to hole S for semi-frequency sensitive.
Resolder W1 to hole N for non-frequency sensitive.

Fan Belt Adjustment adjustment

Tools: belt adjustment tool

Loosen alternator bolt through elongated slot in mounting bracket
Slide alternator until fan belt tension of 60 lbs (267 N) is obtained
Tighten alternator mounting bolt to lock in place
Check tension and repeat if necessary

Governor Control Adjustment adjustment

⚠ Hold the throttle linkage so that the set will not overspeed.

Tools: frequency meter

Place Start/Stop/Remote switch in STOP.
Attach frequency meter to generator output leads if none on panel.
Locate governor control in control box.
Adjust GAIN potentiometer fully counterclockwise.
Hold throttle linkage to prevent overspeed; start the set.
Adjust SPEED potentiometer to correct frequency (50 or 60 hertz).
Adjust GAIN clockwise until governor hunts, then counterclockwise until stable.
Test by tapping actuator lever; set should return to speed without hunting.
Stop and restart set to verify it does not overspeed.

Magnetic Pickup Sensor Adjustment adjustment

Place Start/Stop/Remote switch in the STOP position.
Loosen stop-nut on the magnetic sensor on the flywheel housing.
Turn sensor IN until it just contacts flywheel ring gear.
Back sensor OUT 1/2 turn and tighten stop-nut.

Reconnection (Test Q) adjustment

Refer to the generator reconnection possibilities when changing voltage.
Reconnect lead W12 (inside control box) to the proper H terminal (H2, H3, H4, H5, or H6) depending on voltage selected.
Change jumper W12 from TB2-2 to the proper TB2 terminal.

Voltage Adjustment (Standard Regulator) adjustment

Center the panel mounted voltage adjustment knob with pointer vertical.
Start the set and allow it to stabilize at correct frequency and speed.
Turn R18 counterclockwise to decrease voltage or clockwise to increase.
Set voltage with no load connected (e.g., 246 volts for 120/240).
Turn R26 counterclockwise until unstable, then clockwise until voltage stabilizes.

Voltage Adjustment (Test P) adjustment

Tools: voltmeter

Center the panel-mounted voltage adjustment knob vertically.
Start the set and allow it to stabilize at the correct frequency and speed.
Standard regulator: turn R18 counterclockwise to decrease voltage, clockwise to increase; set no-load voltage (e.g. 246 volts for 120/240 connection).
Standard regulator: turn R26 counterclockwise until voltage hunts, then clockwise until stable.
Switching regulator: turn R36 counterclockwise to increase voltage, clockwise to decrease; set no-load voltage.
Switching regulator: turn R5 counterclockwise until voltage hunts, then clockwise until stable.

Alternator Checkout diagnostic

Tools: voltmeter

With engine running, check battery charge rate DC ammeter.
Ammeter should show slight charge tapering to zero as battery becomes charged.
If constant discharge or high charge, stop set and check drive belt, terminals, leads.
Measure alternator output voltage: positive lead to output terminal, negative to ground.
Nominal output voltage should be 13.8 to 14.8 volts.
If over 15 volts, check voltage regulator leads; replace regulator if shorted.

Alternator Checkout (C) diagnostic

Tools: DC ammeter, voltmeter

With engine running, check the battery charge rate DC ammeter; it should show a slight charge tapering to zero.
If constant discharge or constant high charge, stop set and check drive belt, terminal connections, and lead wires.
Use a voltmeter between output terminal and ground; nominal output should be 13.8 to 14.8 volts.
If output high (over 15 volts), check for loose or corroded voltage regulator leads; replace regulator if shorted.
If output low, problem could be worn/broken brushes, open regulator, or open field diode.

Battery Cable Checkout (B) diagnostic

Tools: voltmeter

With the starter motor operating, check voltage drop from battery negative post to cylinder block.
Check voltage drop from battery positive post to battery terminal stud on the solenoid.
Normally each should be less than 0.3 volt.
Clean all connections in any part of the circuit showing excessive voltage drop.

Fuel Solenoid Checkout diagnostic

Listen for a click in the injection pump when start switch is placed in run position.
If no click is heard and there is power to solenoid, replace the fuel solenoid.

Fuel Solenoid Checkout (F) diagnostic

To check energized-to-run operation, listen for a click in injection pump when start switch is placed in run position.
If no click and there is power to solenoid, the fuel solenoid must be replaced.
To check energized-to-stop operation, listen for a click when switch is placed in stop position.
If no click and there is no power to solenoid, the fuel solenoid must be replaced.

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
		Connect loose wire W12 from TB2-2 to TB2 (H terminal)	H2, H3, H4, H5 or H6 depending on voltage selected	W12 reconnection lead inside control box; must be reconnected to proper H terminal when reconnecting for a different voltage
		X1 (red) to VR21-9, X2 (red) to VR21-10	X1, X2	Reference transformer T21 secondary connections
110/190	3			Figure 26 generator voltage connections
110/220	1	Connect lead W12 to H6 (per Figure 26 reconnection table)		50 Hz. Figure 26 generator voltage connections. OCR partially garbled.
115/200	3			Figure 26 generator voltage connections
115/230	1	Connect lead W12 to H6 (per Figure 26 reconnection table)		50 Hz. Figure 26 generator voltage connections. OCR partially garbled.
120/208	3			Figure 26 generator voltage connections
120/240	1	Connect lead W12 to H5 (per Figure 26 reconnection table)		60 Hz. Figure 26 generator voltage connections. OCR partially garbled.
120/240	1	Reconnectible generator (code 15/515)	W12 (TB2) to H3/H4/H5/H6 on voltage reference transformer	12-lead generator reconnection
127/220	3			Figure 26 generator voltage connections
130/400	3			Figure 26 generator voltage connections
139/240	3			Figure 26 generator voltage connections
220/380	3			Figure 26 generator voltage connections
220/380	3	Non-reconnectible (code 7)	H3
240/416	3			Figure 26 generator voltage connections
254/440	3			Figure 26 generator voltage connections
277/480	3			Figure 26 generator voltage connections
347/600	3	Non-reconnectible (code 9X)	H5
Various		Reconnect lead W12 (inside control box) from TB2-2 to the proper H terminal (H2, H3, H4, H5, or H6) depending on voltage selected	W12, H2, H3, H4, H5, H6	Figure 25. Single phase generators use leads 1 and 4 (lead 2 not used); three phase generators use leads 4, 7, 8, 9 and 10 for control input and metering.

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 L634T?

The Cummins/Onan L634T engine has an oil capacity of 10.8 liters.

Which generators use the Cummins/Onan L634T engine?

The Cummins/Onan L634T powers 1 generator set(s), including Cummins/Onan DL6T.

What is a common problem with the Cummins/Onan L634T engine?

A commonly reported issue on generators using the Cummins/Onan L634T is: Engine will not crank. Cause: Indicates possible fault with control or starting system; fault lamp on. Fix: Correct fault and reset control.