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Cummins/Onan L634T Engine

Name: Cummins/Onan L634T Engine
Brand: Cummins/Onan

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

PRE HI ENG TEMP lamp lights. Engine continues to operate.
Fix: Indicates engine has begun to overheat and engine temperature has risen to approximately 215°F (102°C). If engine can be stopped, follow procedures in step 3. Use the following procedures if generator is powering critical loads and cannot be shutdown. a. Check air inlets and outlets and remove any obstructions to airflow. b. Open doors or windows in generator area to increase ventilation. c. Reduce load if possible by turning off non-essential equipment.
Green RUN lamp lights following engine start-up.
Fix: Indicates all engine systems are normal. No corrective action required.

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
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 circuit	7.5 to 12.5	seconds	after start-disconnect relay energized
Other	Maximum allowable exhaust back pressure	41	inches (1041 mm) H2O
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
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	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 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 coil to frame	infinity	ohms
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 winding between F1+ and F2-	13.05 to 15.95	ohms	at 77 deg F (25 deg C)
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	Overspeed switch contact closure	2200 to 2250	rpm
Temperature	High engine temperature shutdown	220	F (104 C)
Temperature	High engine temperature switch closes	222	deg F (106 deg C)
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 warning	215	F (102 C)
Temperature	Water temperature gauge normal range	165 to 195	F (74 to 91 C)	depending on load and ambient
Temperature	Water temperature gauge range	100 to 250	deg F (40 to 121 deg C)
Torque	Capscrews holding drive disc to engine flywheel (8)	39	ft. lbs. (52 Nm)
Torque	Drive disc placement bolts (8)	70 to 80	ft. lbs. (95 to 108 Nm)
Torque	End bell stud nuts	20	ft. lbs. (27 Nm)
Torque	Exciter rotor lock nut	130 to 150	ft. lbs. (176 to 203 Nm)
Torque	Fan belt tension	60	lbs (267 N)
Torque	Fuel injection line connecting nut torque	18	ft-lbs (24 N-m)
Torque	Generator field leads F1+ and F2- terminals on exciter assembly	23 to 26	inch lbs. (2.6 to 2.9 Nm)
Torque	Rotating rectifier nuts F1+, F2-, CR1, CR2, CR3, CR4, CR5, CR6	24	lbs-in. (2.7 Nm)
Torque	Stator attachment ring capscrews (12) on DL4 and DL6	39	ft. lbs. (52 Nm)
Voltage	Battery cable voltage drop (negative post to block; positive post to solenoid stud)	less than 0.3	volt
Voltage	Battery charge rate DC ammeter range	+/- 30	amperes
Voltage	Battery charging alternator output voltage	13.8 to 14.8	volts	engine running, stabilized
Voltage	Charge battery if specific gravity below	1.215
Voltage	Exciter field voltage across TB1-5 and 6 (or TB1-11)	5	VDC	or more
Voltage	Maximum charge rate for set-mounted battery charging alternator	35	amperes
Voltage	Residual AC voltage across TB2-2 and H1	5 to 10	VAC
Voltage	Solenoid contact voltage drop across contacts	1	volt	maximum
Voltage	Specific gravity, fully charged battery	1.260		80F (27C)
Voltage	Standard regulator calibration (set V1 to 208 VAC)	90-110	VDC	V2 output
Voltage	Standard regulator range test A (V1=203 VAC)	> 180	VDC
Voltage	Standard regulator range test A (V1=211 VAC)	< 60	VDC
Voltage	Standard regulator range test B (V1=205 VAC)	> 140	VDC
Voltage	Standard regulator range test B (V1=213 VAC)	< 10	VDC
Voltage	Voltage at K1 start solenoid terminal S (S10 in Start)	at least 7	VDC

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 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

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

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 (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 (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 (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.

Relay Checkout (E) diagnostic

Tools: voltmeter, 12-volt source

Connect 12 volts across relay coil terminals; relay should activate if coil is okay.
Connect a 12-volt source to one side of relay contacts.
Connect a voltmeter to other side of relay contact and 12-volt source; 12 volts appear when relay energized indicates contact is okay.

Solenoid Checkout (D) diagnostic

Tools: voltmeter, jumper wire

Apply battery positive (B+) to the terminal marked S.
Jumper a ground wire to the solenoid mounting bracket; solenoid should activate.
If contacts are good, 12 volts should be read between terminal I and ground.
Voltage drop across the contacts should never exceed one volt in circuit application.

Switch Checkout (G) diagnostic

Tools: ohmmeter

Remove battery B+ cable.
Place ohmmeter leads across switch.
Open and close switch while observing ohmmeter.
A normally open (NO) switch should show infinite resistance open and continuity closed; NC is opposite.
Replace switch if defective.

Testing Commutating Reactors (L21 and L22) diagnostic

Tools: ohmmeter

Remove reactor leads from terminal board for testing.
Measure resistance between coils 1-2 and 3-4; should be 0.031-0.37 and 0.037-0.46 ohms at 77 deg F (25 deg C).
Measure resistance between coils and from any terminal to frame; should be infinity.

Testing Exciter Rotor (Test J) diagnostic

Tools: Wheatstone Bridge, ohmmeter

Use a Wheatstone Bridge for this test.
Disconnect main rotor field leads at F1+ and F2- and disconnect leads from diodes CR1-CR6.
Test between exciter lead pairs T1-T2, T2-T3, and T1-T3; resistance should be 0.6 to 0.7 ohms at 68 deg F (20 deg C).
Connect ohmmeter between each CR lead and rotor laminations; less than one megohm indicates defective ground insulation.

Testing Exciter Stator (Test H) diagnostic

Tools: ohmmeter

Disconnect F1+ and F2- exciter field leads from terminal block in generator end bell.
Measure resistance between field leads; should be 13.05 to 15.95 ohms at 77 deg F (25 deg C).
Connect ohmmeter between either field lead and exciter stator laminations on highest range; resistance must be one megohm or greater.

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
110/220	1	Connect lead W12 to H6 (per Figure 26 reconnection table)		50 Hz. Figure 26 generator voltage connections. OCR partially garbled.
115/230	1	Connect lead W12 to H6 (per Figure 26 reconnection table)		50 Hz. Figure 26 generator voltage connections. OCR partially garbled.
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
220/380	3	Non-reconnectible (code 7)	H3
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: PRE HI ENG TEMP lamp lights. Engine continues to operate.. Fix: Indicates engine has begun to overheat and engine temperature has risen to approximately 215°F (102°C). If engine can be stopped, follow procedures in step 3. Use the following procedures if generator is powering critical loads and cannot be shutdown. a. Check air inlets and outlets and remove any obstructions to airflow. b. Open doors or windows in generator area to increase ventilation. c. Reduce load if possible by turning off non-essential equipment..