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Cummins/Onan 6C Engine

The Cummins/Onan 6C is a turbocharged engine. It powers generator sets from Cummins/Onan, including the 3100 series.

Also known as: 6C, Cummins/Onan 6C

On this pageSpecs Generator sets Maintenance FAQ Torque & specs Fluids Fault codes Procedures

Engine specifications

Aspiration	Turbocharged

Generator sets powered by this engine

Cummins/Onan 3100 series

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

Common problems

WEAK BATTERY warning (232) 232
Cause: Weak or discharged battery
Fix: Recharge or replace the battery; Specific gravity for a fully charged battery is approximately 1.260 at 80° F (27° C)
LOW DC VOLTAGE warning (230) 230
Cause: Weak or discharged battery
Fix: Recharge or replace the battery; Specific gravity for a fully charged battery is approximately 1.260 at 80° F (27° C)
LOW AC VOLTAGE shutdown (303) message 303
Cause: PMG or field wiring could be faulty
Fix: If output voltage is low and both A37/DS2 and A31/DS10 are on, check and repair the PMG or field wiring
LOW AC VOLTAGE shutdown (303) 303
Cause: PMG or field wiring could be faulty
Fix: If output voltage is low and both A37/DS2 and A31/DS10 are on, check and repair the PMG or field wiring
Engine cranks but does not start — "OVERCRANK" (222) message –and While cranking, Run LED DS12 on the engine interface board (A31) is on. NOTE: This suggest that the PCC has sent a run signal to the fuel solenoid 222
Cause: Restricted fuel supply due to: Closed shutoff valve in supply line; Empty propane supply tank; Low natural gas fuel pressure
Fix: Open any closed shutoff valve in the fuel line supplying the engine; Fill propane supply tank; Check for proper adjustment of pressure regulator. Check with the gas utility
WEAK BATTERY (232) warning message 232
Cause: Weak or discharged battery
Fix: Recharge or replace the battery. Specific gravity for a fully charged battery is approximately 1.260 at 80° F (27° C)
LOW DC VOLTAGE (230) warning message 230
Cause: Weak or discharged battery
Fix: Recharge or replace the battery. Specific gravity for a fully charged battery is approximately 1.260 at 80° F (27° C)
HIGH AC VOLTAGE shutdown (301) message 301
Cause: The problem may be in the PCC, the alternator, or the interconnection between them
Fix: Set the display on the PCC to GEN, VOLTS, L-N; Start the genset and observe the voltage displayed on the digital display; If all phases of voltage are balanced and not zero, but lower than normal, the alternator and the input to the PCC are OK. The problem is in the excitation circuit for the alternator. Go to step 2; If one or more phases of voltage are high or the voltages are unbalanced, the PCC is not measuring all phases of voltage and is responding to the low “average” of the three phases of voltage. Go to step 3
HIGH AC VOLTAGE shutdown (301) 301
Cause: The problem may be in the PCC, the alternator, or the interconnection between them; The PCC may indicate either a low AC Voltage or High AC Voltage shutdown due to the design of the logic in the control—you must first determine if the output voltage the control sees is low or high; Residual voltage is normally 5 to 10% of nominal output voltage
Fix: Set the display on the PCC to GEN, VOLTS, L-N; Start the genset and observe the voltage displayed on the digital display; If all phases of voltage are balanced and not zero, but lower than normal, the alternator and the input to the PCC are OK—the problem is in the excitation circuit for the alternator, go to step 2; If one or more phases of voltage are high or the voltages are unbalanced, the PCC is not measuring all phases of voltage and is responding to the low average of the three phases of voltage—go to step 3
SHORT CIRCUIT shutdown (322) message 322
Cause: Short or Overload
Fix: Check the load and load cables. Repair if necessary
OVERLOAD warning (330) message 330
Cause: Short or Overload
Fix: Check the load and load cables. Repair if necessary
OVERCURRENT warning (320) message 320
Cause: Short or Overload
Fix: Check the load and load cables. Repair if necessary
OVERCURRENT shutdown (321) message 321
Cause: Short or Overload
Fix: Check the load and load cables. Repair if necessary
OVERCRANK — While cranking, Run LED DS12 on the engine interface board (A31) is on and while cranking, Run LED DS2 on the governor output module (A38) is on. These indications suggest that the PCC has sent a run signal to the fuel solenoid 222
Cause: Restricted fuel supply due to: fuel level below pickup tube in tank; closed shutoff valve in supply line; fuel injectors clogged; air in fuel system
Fix: Add fuel if low. Prime the fuel system; open any closed shutoff valve in the fuel line supplying the engine; refer to engine service manual; bleed air from fuel system. Refer to engine service manual
UNDER FREQUENCY shutdown (313) message 313
Cause: Improper setup
Fix: Check Initial Setup (set size) and correct, if necessary

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan 6C.

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter			—
Fuel filter			—
Oil filter			—
Battery			—
Belt			—
Coolant			—
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 specifications

Type	Item	Value	Unit	Condition
Injection timing	Ignition timing GGKB/GGKC (preferred)	28 to 32	degrees BTDC
Injection timing	Ignition timing GGKD (preferred)	26 to 30	degrees BTDC
Other	Exhaust O2 (excess oxygen)	5 ±1	%	rated load and rpm
Other	Fuse A31-F1 (control B+)	5	Amp	Engine Interface board
Other	Fuse A31-F3 (auxiliary B+)	5	Amp	Engine Interface board; panel lights, run and start relays
Other	Fuse A38-F1 (Network B+)	10	Amp	Governor Output Module
Other	Fuse A38-F2 (Switched B+)	10	Amp	Governor Output Module
Other	Fuse A38-F3 (Governor B+)	10	Amp	Governor Output Module
Other	Fuse TB-Bat F1 (Customer B+)	20	Amp	Engine Harness Assembly
Other	Setup/calibration menu access code	574
Other	Start delay adjustment range	0 to 300	seconds	remote starting in Auto mode
Other	Stop delay adjustment range	0 to 600	seconds	remote stopping in Auto mode
Other	Voltage regulator PWM duty cycle range	0 - 60	%	normal duty cycle less than 10%
Pressure	Fuel supply pressure to gas regulator (maximum permissible at any load)	20	inches WC
Pressure	Fuel supply pressure to gas regulator (minimum at full load)	10	inches WC	full load
Pressure	Idle oil pressure (6C engine)	10-30	psi (69-207 kPa)	idle
Pressure	Idle oil pressure (normal)	10-30	psi (69-207 kPa)	Engine model 6C, idle
Pressure	Idle oil pressure shutdown limit	8	psi (55 kPa)	Engine model 6C, idle
Pressure	Idle oil pressure shutdown limit (6C engine)	8	psi (55 kPa)	idle
Pressure	Idle oil pressure warning limit	12	psi (83 kPa)	Engine model 6C, idle
Pressure	Idle oil pressure warning limit (6C engine)	12	psi (83 kPa)	idle
Pressure	Intake manifold pressure (maximum)	15	psi	rated load
Pressure	LP gas pressure (full load)	-1.0	inches WC	full load
Pressure	LP gas pressure (no load)	-0.5	inches WC	no load
Pressure	NG gas pressure on mixer side	4 to 6	inches WC	full load
Pressure	Normal oil pressure	40 - 70	psi (276 - 483 kPa)	Engine model 6C
Pressure	Normal oil pressure (6C engine)	40 - 70	psi (276 - 483 kPa)	normal
Pressure	Oil pressure shutdown limit	15	psi (103 kPa)	Engine model 6C
Pressure	Oil pressure shutdown limit (6C engine)	15	psi (103 kPa)	normal
Pressure	Oil pressure warning limit	20	psi (138 kPa)	Engine model 6C
Pressure	Oil pressure warning limit (6C engine)	20	psi (138 kPa)	normal
Resistance	A31 AVR drive continuity (A31 J1-2 to J4-10 and A31 J1-1 to J4-11)	270	ohms
Resistance	Coolant temperature sender resistance	500 to 2k	ohms
Resistance	Exciter stator/rotor winding insulation resistance	greater than 1	megohm	Replace if less than 1 megohm
Resistance	PMG winding resistance	approximately 4.4	ohms	across lead pairs P2-P3, P3-P4 and P4-P2
Resistance	PMG winding resistance (each winding)	approximately 4.4	ohms
Resistance	PT primary resistance	1K-2.5K	ohms
Resistance	PT secondary resistance	140-225	ohms
Resistance	PT/CT module 3-ohm resistors (J8-15 to 14, J8-22 to 23, J8-7 to 8)	3	ohms
Speed	Backup start disconnect initiation	850	RPM	sensed PMG voltage greater than 105 volts RMS
Speed	Backup start disconnect initiation speed (A37)	about 850	RPM	PMG voltage greater than 105 volts RMS
Speed	Engine idle governed speed	800	RPM	Engine idle function enabled
Speed	Idle speed	800 ±100	RPM	idle mode (default 800 RPM)
Speed	Idle speed adjustment range	800 ±100	RPM	Default value 800 RPM
Speed	OVERSPEED shutdown	115% ±1% of nominal
Temperature	EGT in exhaust manifold ahead of turbocharger (maximum)	1375	°F (746°C)	full load
Temperature	HIGH COOLANT TEMP shutdown trip point	223° F (106° C) - standby or 215° F (102° C) - prime
Temperature	HIGH COOLANT TEMP warning limit	215° F (102° C) - standby or 207° F (97° C) - prime
Temperature	High coolant temperature shutdown	212	°F (100°C)	standby or primary
Temperature	High coolant temperature warning	202	°F (94°C)	standby or primary
Temperature	LOW COOLANT TEMP warning	70° F (21° C) or lower
Temperature	Low coolant temperature warning	70 or lower	°F (21°C)
Torque	Adaptor-to-engine bolts	35	ft-lbs (48 Nm)
Torque	Diode pigtail terminals	24	in-lbs (2.7 Nm)
Torque	Drive disc-to-flywheel bolts	50	ft-lbs (67 Nm)
Torque	Drive disc-to-rotor bolts	190	ft-lbs (257 Nm)
Torque	Exciter stator mounting screws	7	ft-lbs (10 Nm)
Torque	Generator end plate mounting bolts	25	ft-lbs (34 Nm)
Torque	Generator mounting bracket bolts (M10)	35	ft-lbs (47 Nm)
Torque	Generator mounting bracket bolts (M12)	65	ft-lbs (88 Nm)

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
Fuel	Natural Gas (NG) or HD-5 Propane (LP)				Gas supply pressure max 20 inches WC and min 10 inches WC at full load; NG mixer side 4 to 6 inches WC at full load.

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
101	IDLE MODE	Indicates that the engine is operating in idle mode; generator build-up is inhibited and engine governed at 800 RPM.	Warning	Remove ground from the engine idle input to return to normal speed and voltage.
102	EMERGENCY STOP	Indicates local or remote Emergency Stop.	Shutdown	Pull the Emergency Stop button out; move Run/Off/Auto switch to Off; press Reset; select Run or Auto as required.
200	LOW OIL PRESSURE	Indicates engine oil pressure has dropped to an unacceptable level.	Warning	If powering critical loads, wait until next shutdown period and follow 201-SHUTDOWN procedure; check oil pressure via menu prior to clearing fault.
201	LOW OIL PRESSURE	Indicates engine oil pressure has dropped below the shutdown trip point.	Shutdown	Check oil level, lines and filters; replenish if low; reset control and restart. Oil pressure limits in Table 4-3.
204	OIL PRES SENDER	The control has sensed that the engine oil pressure sender is out of its working range.	Warning	Check that the engine oil pressure sender is properly connected.
210	LOW COOLANT TEMP	Warning occurs when engine coolant temperature is 70° F (21° C) or lower.	Warning	Check coolant heater power connection, low coolant level, and heater element.
211	HIGH COOLANT TEMP	Engine coolant temperature is getting close to the recommended maximum limit (215° F/102° C standby or 207° F/97° C prime).	Warning	Reduce load if possible; check air inlets/outlets; follow 212-SHUTDOWN procedure if engine can be stopped.
212	HIGH COOLANT TEMP	Engine has overheated (223° F/106° C standby or 215° F/102° C prime).	Shutdown	Allow engine to cool; check cooling airflow, fan belt, coolant mixture, blower fan/circulation pumps; reset control and restart.
213	COOLANT SENDER	Resistance of the coolant temperature sender is out of range.	Warning	Check the resistance of the sender; should be 500 to 2k ohms.
214	LOW COOLANT LVL	Engine coolant level has fallen below the trip point.	Warning	Check coolant level in radiator and coolant recovery bottle; look for leakage; reset control and restart.
215	LOW COOLANT LVL	Engine coolant level has fallen below the trip point (shutdown).	Shutdown	Check coolant level in radiator and bottle; check coolant bottle hose or radiator cap; reset control and restart.
220	MAG PICKUP	Mag pickup speed indication is not being sensed or does not match generator set output frequency.	Shutdown	Restart and check RPM on the digital display; inspect/adjust MPU.
221	FAIL TO CRANK	Possible fault with control or starting system; engine will not crank.	Shutdown	Check fuse F3 on the Engine Interface board; check battery cable connections; recharge or replace the battery.
222	OVERCRANK	Possible fuel system problem; engine stops cranking.	Shutdown	Check for empty fuel tank, fuel leaks, or plugged fuel lines; check/replace dirty fuel filter and air filter; reset and restart.
223	OVERSPEED	Engine has exceeded normal operating speed (115% ±1% of nominal).	Shutdown	Check mechanical fuel linkage; monitor PCC duty cycle to isolate A32 or A38.
230	LOW DC VOLTAGE	Battery voltage is below 10 VDC.	Warning	Recharge or replace battery; clean/tighten connections; check engine DC alternator; check battery charger float level.
231	HIGH DC VOLTAGE	Battery voltage exceeds 32 VDC.	Warning	Check battery charger voltage float level; check/replace engine DC alternator.
232	WEAK BATTERY	Battery voltage drops below 60% of nominal for two seconds during starting.	Warning	Recharge or replace the battery; see 230 LOW DC VOLTAGE.
240	LOW FUEL - DAY	Day tank fuel supply is running low.	Warning	Check fuel supply and replenish as required.
241	LOW FUEL	Fuel supply is running low.	Warning	Check fuel supply and replenish as required.
250	EEPROM ERROR	PCC memory error; data corruption of critical operating parameters.	Shutdown	Perform the initial start setup procedure; if shutdown repeats, replace the digital board (A32).
251	EEPROM ERROR	PCC memory error; data corruption of noncritical operating parameters.	Warning	Perform adjustment procedures and save; perform calibration procedures; if error repeats, replace digital board (A32).
252	EEPROM ERROR	PCC memory error; data corruption of noncritical operating parameters.	Warning	Verify selected values and repeat the save operation; if error repeats, replace digital board (A32).
260	CUSTOMER FAULT 1	Customer defined input closed to ground; message editable.	Warning	Isolate the source of the false signal; check external wiring; replace A34 or A32 as isolated.
261	GROUND FAULT	Customer defined input (Ground Fault) closed to ground; message editable.	Warning	Isolate the source of the false signal; check external wiring; replace A34 or A32 as isolated.
262	RUPTURE BASIN	Customer defined input (Rupture Basin) closed to ground; message editable.	Warning	Isolate the source of the false signal; check external wiring; replace A34 or A32 as isolated.
263	HIGH GEN TEMP	Customer defined input (High Gen Temp) closed to ground; message editable.	Warning	Isolate the source of the false signal; check external wiring; replace A34 or A32 as isolated.
301	HIGH AC VOLTAGE	One or more phase voltages has exceeded 130% of nominal, or has exceeded 110% of nominal for 10 seconds.	Shutdown	Determine whether output voltage the control sees is low or high; isolate to regulator module (A37), A31, A32, PT/CT module (A36), A34, or A33.
303	LOW AC VOLTAGE	One or more of the phase voltages has dropped below 85% of nominal for 10 seconds.	Shutdown	Check PMG or field wiring, rotating rectifier diodes, load, connectors; isolate regulator module (A37), A31, A32, A36, A33 or A34.
313	UNDER FREQUENCY	Engine speed has dropped below 90% of nominal for 10 seconds; five seconds before shutdown a Load Dump signal is initiated.	Shutdown	Check fuel supply, intake air supply and load; isolate governor output module (A38), A32 or A31.
320	OVERCURRENT	Generator output current has exceeded 110% of rated for 60 seconds.	Warning	Check load and load lead connections.
321	OVERCURRENT	Generator output current has exceeded 110% of rated and a PCC time/current calculation has initiated an overcurrent shutdown.	Shutdown	Check load and load lead connections.
322	SHORT CIRCUIT	Generator output current has exceeded 175% of rated.	Shutdown	Check load and load lead connections.
330	OVERLOAD	Three-phase power output exceeds 105% of standby (or 115% of prime) rating.	Warning	Check load and load lead connections.
335	REVERSE POWER	Improper CT or PT phasing (non-parallel units only).	Shutdown	Check CT / PT wiring; refer to CT Installation in Section 5.

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

Procedures

Adjusting Carburetor Power Valve adjustment

Tools: 0-30 psi gauge, Pyrometer, Oxygen meter, Timing light

Install a 0-30 psi gauge on the intake manifold.
Install a pyrometer in the 1/8 inch pipe tapped hole of the exhaust manifold ahead of the turbocharger.
Install an oxygen meter exhaust gas pickup in a 1/8 inch pipe tapped hole of the exhaust elbow.
With the genset at full load, adjust the power valve for the lowest possible exhaust temperature while observing O2.
Turn the power valve closed (toward L) until the engine starts losing RPM.
Then turn the power valve open (toward R) until the exhaust temperature reaches the lowest reading.
LP vapor only: if O2 cannot be set with the power valve, set power valve to mid-point and use the course adjustment valve.

Adjusting Engine Timing adjustment

⚠ DO NOT switch from position 7 to 0, or 0 to 7 while the engine is running.

Tools: Timing light

Connect the timing light high tension lead to the No. 1 spark plug (front cylinder).
Start the genset.
Point the adjustable timing light toward the pointer and TDC mark on the crank pulley.
Remove the white plastic cap and turn the timing switch adjustment screw (10 degree range) of the CD1 Ignition Module.
Switch position 7 gives the most advanced timing; timing retards 1.4 degrees per switch position toward 0 (full retard).
Allow a short delay between each timing position change.
Perform the Carburetor Power Valve Adjustment procedure.

Adjusting Gas Regulator For NG Pressure adjustment

⚠ High gas supply pressure can cause gas leaks which can lead to fire and severe personal injury or death.

Tools: Manometers

Install the manometers into the two test ports of the gas regulator.
Remove the regulator dust cap.
Start the genset and run at full load.
Turn the adjusting screw until the meter shows the appropriate gas pressure for NG on the mixer side.
Turn the screw clockwise to increase pressure.

Servicing the PMG - Testing diagnostic

⚠ HAZARDOUS VOLTAGE. Touching uninsulated high voltage parts inside the control and power output boxes can result in severe personal injury or death.

Tools: Wheatstone bridge or digital ohmmeter

Disconnect leads P2, P3 and P4 from the voltage regulator.
Start the engine at the set and let the speed stabilize.
Measure voltage across lead pairs P2-P3, P3-P4 and P4-P2 (at least 150 VAC for 50 Hz, at least 180 VAC for 60 Hz).
If voltages are low or uneven, check all leads and connections before disassembling the PMG.
Stop the set and measure electrical resistance across P2-P3, P3-P4 and P4-P2 (approximately 4.4 ohms each winding).

Surge Suppresser Testing and Replacement diagnostic

Tools: Ohmmeter

Remove the suppresser.
Replace the suppresser if it appears to have overheated.
Replace the suppresser if ohmmeter readings indicate less than infinite resistance in both directions.
Torque the terminals to 24 in-lbs (2.7 Nm) when reassembling.

Testing Diodes diagnostic

⚠ Layers of dust can cause diodes to overheat and fail.

Tools: Ohmmeter

Disconnect the diode pigtails from the terminal posts.
Measure electrical resistance between each diode pigtail and the plate on which the diode is mounted.
Reverse the meter test probes and repeat the tests.
Replace the diode if resistance is high or low in both directions.

Testing the Exciter Stator diagnostic

Tools: Wheatstone bridge or digital ohmmeter

Measure winding resistance with a Wheatstone bridge or digital ohmmeter.
Replace the stator if winding resistance is not as specified in Table 6-1.
Disconnect the exciter stator leads from terminals X and XX.
Measure resistance between either lead and the stator laminations.
Replace the stator if insulation resistance is less than 1 megohm (1,000,000 ohms).

Testing the Float Switch Assembly diagnostic

Tools: Ohmmeter, Feeler gauge

Remove the fuel pump control cover, disconnect the wiring jack and unscrew the assembly from the top of the day tank.
With an ohmmeter, test for continuity (switch closed) between each pair of colored leads while holding the assembly vertical; replace if any switch is open.
Lift each float in turn to 1/8 inch (3 mm) below the C-clip stop above it and test for continuity; replace if any switch does not open.
Use pipe thread sealant when replacing the assembly.

Testing the Generator Stator and Rotor Windings diagnostic

⚠ Disconnect the starting battery cables (negative [-] first) to make sure the engine will not start while performing these tests.

Tools: Wheatstone bridge or digital ohmmeter (0.001 ohm precision), Megger or insulation resistance meter

Disconnect battery cables (negative first) so the engine will not start.
Measure winding resistance with a Wheatstone bridge or digital ohmmeter and compare to Table 6-1.
Measure winding insulation resistance between leads and laminations; replace if less than 1 megohm.
For the main stator, measure across each pair of stator leads (U1-U2, U5-U6, V1-V2, V5-V6, W1-W2, W5-W6).

Testing the Rotating Rectifier Assembly Diodes diagnostic

⚠ Layers of dust can cause diodes to overheat and fail. Brush dust off regularly.

Tools: Ohmmeter

Disconnect the diode pigtails from the terminal posts.
Measure electrical resistance between each diode pigtail and the plate on which the diode is mounted.
Reverse the meter test probes and repeat the tests.
Replace the diode if resistance is high or low in both directions.

Current Transformer (CT) Installation installation

⚠ Improper installation of CTs will cause a 335 Reverse Power shutdown error (non-parallel units only).

Orient the CT dot toward the generator (conventional current flowing into the dot); dot also indicates pin 1.
CT21 for U load leads (A phase).
CT22 for V load leads (B phase).
CT23 for W load leads (C phase).
6 lead generator sets: route generator output leads through the CTs.
12 lead generator sets: route load wires through the CTs.
For 12-lead (dual secondary) CTs connect secondary wire 1 to pin 1; wire 2/3 to pin 2 (high voltage) or pin 3 (low voltage).

Magnetic Speed Pickup Unit (MPU) Installation installation

⚠ Do not use fan blade to bar over engine.

Bar the engine until a gear tooth on the flywheel lines up in the center of the mounting hole.
Thread the sensor in gently by hand until it just touches the gear tooth.
Back it out one quarter turn and set the locknut.

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 6C engine?

The Cummins/Onan 6C powers 1 generator set(s), including Cummins/Onan 3100 series.

What is a common problem with the Cummins/Onan 6C engine?

A commonly reported issue on generators using the Cummins/Onan 6C is: WEAK BATTERY warning (232). Cause: Weak or discharged battery. Fix: Recharge or replace the battery; Specific gravity for a fully charged battery is approximately 1.260 at 80° F (27° C).