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Cummins/Onan Cummins DFFB Engine

The Cummins/Onan Cummins DFFB is an engine. It powers generator sets from Cummins/Onan, including the DFFB.

Also known as: Cummins Cummins DFFB

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

Engine specifications

Generator sets powered by this engine

Cummins/Onan DFFB

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

Service intervals

Item	Action	Hours	Months	Notes
Rupture basin leak detect switch	Test		12 mo	Check once a year; in some areas weekly inspections may be required by safety code regulations

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	Angular alignment T.I.R. limit	not more than .001	in per inch of radius	center of shaft to indicator axis
Clearance	Angular alignment maximum T.I.R.	0.001	inch per inch of radius (center of shaft to indicator axis)	measuring deflection to bolt heads
Clearance	Float switch assembly float lift clearance below C-clip stop	1/8	inch (3 mm)	use a feeler gauge
Clearance	Generator-to-engine angular alignment (T.I.R.)	.001 in. per inch of radius (center of shaft to indicator axis), maximum	in	750 kW gensets and larger
Gap	MPU sensor to flywheel ring gear tooth	back it out one quarter turn	turn	sensor threaded in until it just touches gear tooth
Other	AVR Droop setting	5	% at full load, 0 PF
Other	AVR voltage regulation (load regulation)	approximately ±0.5	% RMS
Other	Angular alignment maximum T.I.R.	.001 inches per inch of radius	in/in	center of shaft to indicator axis
Other	Battery specific gravity (high ambient, above 95F/35C)	1.225		recommended to reduce electrolyte loss
Other	Battery specific gravity (recharge threshold)	below 1.260
Other	Battery specific gravity, fully charged	approximately 1.260	specific gravity	at 80 F (27 C)
Other	Fuel lift height limit	6	feet	(1.8 meters) between fuel lift pump and bottom of dip tube
Other	GOV GAIN default value for paralleling applications	70	%
Other	Maximum cranking voltage drop	2	volts	difference between battery and starter terminals while cranking
Other	Over/Under Frequency Sensor Module - Over Hz Set, 50 Hz nominal	55	Hz	reset 53 Hz
Other	Over/Under Frequency Sensor Module - Over Hz Set, 60 Hz nominal	65	Hz	reset 63 Hz
Other	Over/Under Frequency Sensor Module - Under Hz Set, 50 Hz nominal	45	Hz	reset 47 Hz
Other	Over/Under Frequency Sensor Module - Under Hz Set, 60 Hz nominal	55	Hz	reset 57 Hz
Other	Over/Under Voltage Sensor Module time delay	typically 25 percent, approximately 2.5	seconds
Other	Overcrank cranking limit	75	seconds	three 15-second cranking cycles with two 15-second rest periods
Other	Overcurrent shutdown threshold	110% of rated	% of rated	PCC time/current calculation
Other	Overcurrent warning threshold	110% of rated for 60 seconds	% of rated	generator output current
Other	Overload warning threshold	105% of standby or 115% of prime	% of rating	three-phase power output; load dump after five seconds, warning after 60 seconds
Other	REG VHZ (underfrequency roll-off) adjustable range	1 through 50; normal range 7 through 21
Other	Short circuit shutdown threshold	175% of rated	% of rated	generator output current
Other	Start delay adjustment range	0 to 300	seconds
Other	Stop delay adjustment range	0 to 600	seconds
Other	Under Frequency Roll Off (UFRO) knee point, 50 Hz units	48 to 49	Hz
Other	Under Frequency Roll Off (UFRO) knee point, 60 Hz units	58 to 59	Hz
Other	Under-frequency roll-off threshold (UFRO)	typically 59 Hz for 60 Hz applications and 49 Hz for 50 Hz applications	Hz	AVR UFRO pot preset
Other	Weak battery warning threshold	below 60% of nominal for two seconds	% of nominal	during starting
Pressure	Engine oil pressure - K19 normal	50-70	psi (345-482 kPa)	run mode
Pressure	Engine oil pressure - K38 normal	45-65	psi (310-448 kPa)	run mode
Pressure	Engine oil pressure - K50 normal	50-70	psi (345-482 kPa)	run mode
Pressure	Engine oil pressure - L10/M11 normal	30-50	psi (207-345 kPa)	run mode
Pressure	Engine oil pressure - NTA 855 normal	35-50	psi (241-345 kPa)	run mode
Pressure	Engine oil pressure - V28 normal	50-90	psi (345-620 kPa)	run mode
Pressure	Engine oil pressure shutdown limit - L10/M11	20	psi (138 kPa)	run mode value
Pressure	Engine oil pressure shutdown limit - L10/M11	8	psi (55 kPa)	idle mode value
Pressure	Engine oil pressure warning limit - L10/M11	12	psi (83 kPa)	idle mode value
Pressure	Engine oil pressure warning limit - L10/M11	25	psi (172 kPa)	run mode value
Pressure	K19 engine idle oil pressure	20	psi (138 kPa)	idle
Pressure	K19 engine normal oil pressure	50-70	psi (345-482 kPa)	normal run
Pressure	K19 normal oil pressure (run/idle)	50-70 / 20	psi (345-482 / 138 kPa)	NORM run / IDLE
Pressure	K19 oil pressure shutdown limit	10	psi (69 kPa)	idle mode
Pressure	K19 oil pressure shutdown limit	35	psi (241 kPa)	run mode
Pressure	K19 oil pressure shutdown/warning limits	shutdown 35 psi (241 kPa) run; warning 40 psi (276 kPa) run; shutdown 10 psi (69 kPa) idle; warning 15 psi (103 kPa) idle	psi (kPa)
Pressure	K19 oil pressure warning limit	15	psi (103 kPa)	idle mode
Pressure	K19 oil pressure warning limit	40	psi (276 kPa)	run mode
Pressure	K38 engine idle oil pressure	20	psi (138 kPa)	idle
Pressure	K38 engine normal oil pressure	45-65	psi (310-448 kPa)	normal run
Pressure	K38 normal oil pressure (run/idle)	45-65 / 20	psi (310-448 / 138 kPa)	NORM run / IDLE
Pressure	K38 oil pressure shutdown limit	10	psi (69 kPa)	idle mode
Pressure	K38 oil pressure shutdown limit	35	psi (241 kPa)	run mode
Pressure	K38 oil pressure shutdown/warning limits	shutdown 35 psi (241 kPa) run; warning 40 psi (276 kPa) run; shutdown 10 psi (69 kPa) idle; warning 15 psi (103 kPa) idle	psi (kPa)
Pressure	K38 oil pressure warning limit	15	psi (103 kPa)	idle mode
Pressure	K38 oil pressure warning limit	40	psi (276 kPa)	run mode
Pressure	K50 engine idle oil pressure	20	psi (138 kPa)	idle
Pressure	K50 engine normal oil pressure	50-70	psi (345-482 kPa)	normal run
Pressure	K50 normal oil pressure (run/idle)	50-70 / 20	psi (345-482 / 138 kPa)	NORM run / IDLE

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	soft water / E.G. / DCA mixture				Cooling system to be filled with soft water/E.G./DCA mixture (on-site review checklist).
Fuel	Diesel				Day tank fuel transfer pump maintains reservoir of diesel fuel in the day tank.
Grease	Lithium based grease	Mobilux No. 2 or Shell Alvania R3		2.74 oz (81 ml)	Generator end bearing. Temperature range -22F to +248F (-30C to +120C). Grease should be equally divided between the bearing, bearing cap cavity, and bearing cartridge cavity.
Grease	Lithium based grease, Mobilux No. 2 or Shell Alvania R3		-30°C to 120°C	2.74 oz (81 ml)	End bearing lubricant; grease equally divided between bearing, bearing cap cavity and bearing cartridge cavity.

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 (governed at 800 RPM, generator build-up inhibited).	Warning	Remove ground from engine idle input to return to normal speed and voltage.
102	EMERGENCY STOP	Indicates local or remote Emergency Stop.	Shutdown	Turn the switch clockwise and allow it to pop out (local only). Move the Run/Off/Auto switch to Off. Press the Reset switch. Select Run or Auto, as required.
200	LOW OIL PRESSURE	Indicates engine oil pressure has dropped to an unacceptable level.	Warning	If generator is powering critical loads and cannot be shut down, wait until next shutdown period and then follow 201-SHUTDOWN procedure. Access the Oil Pressure menu prior to clearing the fault.
201	LOW OIL PRESSURE	Indicates engine oil pressure has dropped below the shutdown trip point.	Shutdown	Check oil level, lines and filters. If oil system is OK but oil level is low, replenish. Reset control and restart. Oil pressure limits are listed in Table 4-3.
204	OIL PRES SENDER	Indicates that 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	Set is not operating. Warning occurs when engine coolant temperature is 70° F (21° C) or lower.	Warning	Indicates engine coolant heater is not operating or is not circulating coolant. Check coolant heater power supply, low coolant level, and heater element current draw.
211	HIGH COOLANT TEMP	Indicates the engine coolant temperature is getting close to the recommended maximum temperature limit: 215° F (102° C) − standby or 207° F (97° C) − primary.	Warning	Reduce load if possible by turning off non-critical loads. Check air inlets and outlets and remove any obstructions to airflow. If engine can be stopped, follow 212-SHUTDOWN procedure.
212	HIGH COOLANT TEMP	Indicates engine has overheated (coolant temperature above 215° F (102° C) − standby or 207° F (97° C) − primary).	Shutdown	Allow engine to cool down completely. Check for obstructions to cooling airflow; check fan belt; check coolant mixture; check blower fan and circulation pumps on remote radiator installations. Reset control and restart.
213	COOLANT SENDER	Indicates that the resistance of the coolant temperature sender is out of range.	Warning	Check the resistance of the sender. Resistance should be 500 to 2k ohms.
214	LOW COOLANT LVL	Indicates engine coolant level has fallen below the trip point.	Warning	Allow engine to cool down completely. Check coolant level in both radiator and coolant recovery bottle and replenish if low. Look for coolant leakage points and repair if necessary.
215	LOW COOLANT LVL	Indicates engine coolant level has fallen below the trip point.	Shutdown	Allow engine to cool down completely. Check coolant level in both radiator and coolant recovery bottle and replenish if low. Reset control and restart after correcting problem. Shutdown will not occur if genset is in Idle mode (low coolant warning only).
220	MAG PICKUP	Indicates 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.
221	FAIL TO CRANK	Indicates possible fault with control or starting system.	Shutdown	Check fuse F3 on the Engine Interface board. Check for poor battery cable connections; clean battery cable terminals and tighten. Recharge or replace a discharged or defective battery.
222	OVERCRANK	Indicates possible fuel system problem.	Shutdown	Check for empty fuel tank, fuel leaks, or plugged fuel lines. Check for dirty fuel filter and replace if necessary. Check for dirty or plugged air filter and replace if necessary. Reset the control and restart after correcting the problem.
223	OVERSPEED	Engine has exceeded normal operating speed (115% ±1% of nominal).	Shutdown	Check mechanical fuel linkage and actuator; check governor output module (A38) and digital board (A32).
224	FAIL TO SYNCHRONIZE	Generator set has not synchronized to the system bus within the allowable time frame.	Warning	Check governor system stability; adjust governing and synchronizer parameters; check fuel system.
226	FAIL TO CLOSE	Paralleling breaker has been given a signal to close, but has not closed properly.	Shutdown	Verify breaker charging mechanism; check close signal; verify auxiliary contact signals.
230	LOW DC VOLTAGE	Indicates battery voltage is below 10 VDC.	Warning	Recharge or replace the battery; check the battery charger fuse. Clean and tighten battery cable connections. Check engine DC alternator; replace if normal charging voltage is not obtained. Check float level if applicable.
231	HIGH DC VOLTAGE	Indicates battery voltage exceeds 32 VDC.	Warning	Check float level on battery charger if applicable (lower float level). Check engine DC alternator; replace if normal battery charging voltage is not obtained.
232	WEAK BATTERY	Indicates battery voltage drops below 60% of nominal for two seconds, during starting.	Warning	Discharged or defective battery. See Warning message 230, LOW DC VOLTAGE.
240	LOW FUEL−DAY	Indicates day tank fuel supply is running low.	Warning	Check fuel supply and replenish as required.
241	LOW FUEL	Indicates day tank 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 initial start setup procedure; replace digital board (A32) if fault repeats.
251	EEPROM ERROR	PCC memory error; data corruption of noncritical operating parameters.	Warning	Perform adjustment and calibration procedures; replace digital board (A32) if fault repeats.
252	EEPROM ERROR	PCC memory error; data corruption of noncritical operating parameters.	Warning	Verify selected values; repeat save operation; replace digital board (A32) if fault repeats.
260	CUSTOMER FAULT 1	When a customer defined input is closed to ground, the corresponding fault message is displayed. Default message; editable for customer site requirements.	Warning	Fault functions can be programmed to initiate a shutdown or a warning.
261	GROUND FAULT	When a customer defined input is closed to ground, the corresponding fault message is displayed. Default message; editable for customer site requirements.	Warning	Fault functions can be programmed to initiate a shutdown or a warning.
262	RUPTURE BASIN	When a customer defined input is closed to ground, the corresponding fault message is displayed. Default message; editable for customer site requirements.	Warning	Fault functions can be programmed to initiate a shutdown or a warning.
263	HIGH GEN TEMP	When a customer defined input is closed to ground, the corresponding fault message is displayed. Default message; editable for customer site requirements.	Warning	Fault functions can be programmed to initiate a shutdown or a warning.
270	PHASE ROTATION	The phase relationship between the generator set and the system bus is not matched.	Shutdown	Verify genset phase rotation matches bus; verify control wiring to bus PT module.
272	FIRST START	PowerCommand control is not receiving a proper signal from the system master first start sensor.	Warning	Verify Master First Start Sensor and interconnection wiring.
301	HIGH AC VOLTAGE	One or more phase voltages exceeded 130% of nominal, or exceeded 110% of nominal for 10 seconds.	Shutdown	Check PCC, alternator, excitation circuit, and PT/CT module wiring.
303	LOW AC VOLTAGE	One or more phase voltages dropped below 85% of nominal for 10 seconds.	Shutdown	Check PMG/field wiring, rotating rectifier, load and connectors.
313	UNDER FREQUENCY	Indicates that 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.
320	OVERCURRENT	Indicates that generator output current has exceeded 110% of rated for 60 seconds.	Warning	Check load and load lead connections.
321	OVERCURRENT	Indicates that generator output current has exceeded 110% of rated, and that a PCC time/current calculation has initiated an overcurrent shutdown.	Shutdown	Check load and load lead connections.
322	SHORT CIRCUIT	Indicates that generator output current has exceeded 175% of rated.	Shutdown	Check load and load lead connections.
330	OVERLOAD	Indicates that three-phase power output exceeds 105% of standby (or 115% of prime) rating. After five seconds, the Load Dump output is activated. After 60 seconds, the OVERLOAD warning is activated.	Warning	Check load and load lead connections.
335	REVERSE POWER	Indicates improper CT or PT phasing. (Non-parallel units only.)	Shutdown	Check CT / PT wiring. Refer to CT Installation in Section 5.
337	LOSS OF EXCITATION	Alternator excitation system is improperly adjusted or has failed.	Shutdown	Check output voltage calibration, load sharing lines, and leading power factor loads.
FAULT 1	Fault 1 (Red)	Engine shut down because of a customer-connected system fault; part of a 10 second time delay shutdown circuit.	Shutdown	Service as required by the customer-supplied fault indicating switches.
FAULT 2	Fault 2 (Red)	Engine shut down because of a customer-connected system fault; part of a non-time delay shutdown circuit.	Shutdown	Service as required by the customer-supplied fault indicating switches.
GF	Ground Fault Alarm (Optional)	Ground fault relay monitors neutral to ground connection and line to neutral, activating a fault alarm when the connection is broken or a ground fault is sensed.	Warning	Clear fault condition by pushing the Run/Stop (Reset)/Remote switch to Reset (auto reset link set at factory).
HET	High Engine Temperature (Red)	Engine shut down because of excessively high engine coolant temperature (greater than 230 deg F).	Shutdown	Check coolant level and leaks; service cooling system; reset engine control.
HI ENG TEMP	High Engine Temperature	Engine shut down due to critically high coolant temperature (above 215F/102C).	Shutdown
LET	Low Engine Temperature (Yellow)	Engine temperature is less than 70 deg F and the engine might not start.	Warning	Plug in, repair or install engine coolant and oil heaters.
LO FUEL	Low Fuel	Fuel supply is marginally low, if equipped (Detector-12).	Warning
LO OIL PRES	Low Oil Pressure	Engine shut down due to critically low oil pressure (below 14 psi/97 kPa).	Shutdown
LOP	Low Oil Pressure (Red)	Engine shut down because of excessively low engine oil pressure (less than 14 psi).	Shutdown	Check oil level and oil leaks; service lubricating system; reset engine control.
LOW ENG TEMP	Low Engine Temperature	Engine water jacket temperature is 70F/21C or lower; indicates possible inoperative coolant heater (Detector-12).	Warning
LOW FUEL	Low Fuel (Yellow)	Fuel level in the supply tank has dropped below the reserve necessary to run the set at full load for the prescribed number of hours.	Warning	Fill the main fuel supply tank with the appropriate grade of fuel.
OC	Overcrank (Red)	Engine shut down because it did not start during the timed cranking period (approximately 75 seconds, including two rest periods).	Shutdown	Service the set as necessary; reset engine control.
OS	Overspeed (Red)	Engine shut down because of overspeed.	Shutdown	Adjust the Electronic Overspeed Module; check governor for binding; reset engine control.
OVERCRANK	Overcrank	Starter locked out because of excessive cranking time (limited to 75 seconds).	Shutdown
OVERSPEED	Overspeed	Engine shut down because of excessive speed.	Shutdown
PRE HI ENG TEMP	Pre-High Engine Temperature	Coolant temperature is marginally high (205F/97C or higher).	Warning	Attend to when generator set is shut down.
PRE LO OIL PRES	Pre-Low Oil Pressure	Engine oil pressure is marginally low (20 psi/138 kPa or less).	Warning	Attend to when generator set is shut down.
PRE-HET	Pre High Engine Temperature (Yellow)	Engine coolant temperature is abnormally high (greater than 220 deg F).	Warning	Shut down if possible or disconnect non-critical loads; service cooling system.
PRE-LOP	Pre Low Oil Pressure (Yellow)	Engine oil pressure is abnormally low (less than 20 psi).	Warning	Shut down if possible or disconnect non-critical loads; service lubricating system.
RUN	Run	Green lamp comes on when both primary and secondary starter circuits open after unit starts.	Info
SWITCH OFF	Switch-off (Flashing Red)	The Run/Stop/Remote switch is in the Stop position, preventing remote automatic operation.	Info	Push the Run-Stop-Remote switch to Remote.

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

Procedures

Aligning Generator with Engine (Angular Alignment) adjustment

⚠ Accidental starting of the generator set during this procedure presents the hazard of severe personal injury or death.

Tools: Dial indicator, Shims

Disconnect the negative (-) battery cable(s) before beginning.
Fasten a dial indicator to the generator shaft or cooling fan with the sensing point on the capscrew head or flat surface of the drive disc at the bolt circle diameter.
Bar the engine over in a clockwise rotation as viewed from engine flywheel; do not allow it to roll back on compression.
Determine the total indicator reading (T.I.R.) as the sum of the maximum positive and negative dial indicator readings over one revolution.
Measure the distance from the center of the generator shaft to the indicator measuring point.
Measure the distance from the generator side of the flex discs to the center of the generator mounting bolt.
Compare the distances; multiply this ratio times one half the T.I.R. to approximate the amount of shims to remove or add.

Angular Alignment Procedure adjustment

⚠ Accidental starting of the generator set during this procedure presents the hazard of severe personal injury or death. Make sure to disconnect the negative (-) battery cable(s) before beginning.

Tools: Dial indicator, Wooden shims

Disconnect the negative (-) battery cable(s) before beginning.
Fasten a dial indicator to the generator shaft or cooling fan with sensing point on capscrew head or flat surface of drive disc at bolt circle diameter.
Bar the engine over in a clockwise rotation as viewed from engine flywheel; do not allow it to roll back on compression.
Record maximum positive and negative dial indicator readings as the engine completes one revolution.
T.I.R. equals the sum of the maximum positive and negative readings.
Maximum T.I.R. should not be more than .001 inch for each inch of radius.
Remove equal shims from under both generator mounting feet to lower the generator.

Angular Alignment Procedure (Generator to Engine) adjustment

⚠ Accidental starting during this procedure presents the hazard of severe personal injury or death.

⚠ Do not bar engine over by prying on fan blade.

Tools: dial indicator

Disconnect the negative (-) battery cable(s) before beginning.
Fasten a dial indicator to the generator shaft or cooling fan with sensing point resting on the drive disc capscrew head/flat surface at the bolt circle diameter.
Bar the engine over in a clockwise rotation as viewed from the engine flywheel; do not allow it to roll back on compression at the end of travel of each reading.
Record the total indicator reading (T.I.R.) as the sum of the maximum positive and negative dial indicator readings over one engine revolution.
Compare the measured T.I.R. to the maximum allowed (.001 in. per inch of radius from center of shaft to indicator axis); add or remove shims under the generator mounting feet to correct.

Axial Alignment Procedure (Generator to Engine) adjustment

Tools: dial indicator

Fasten a dial indicator holding device to the skid base, engine block, or generator shell with the sensor point resting on the generator shaft hub.
Bar the engine over in a clockwise rotation through a couple of rotations, recording indicator readings at eight equally spaced points around the shaft diameter to obtain axial T.I.R.
Reduce the measured T.I.R. runout by one half if excessive vibration remains after angular alignment.

Governor Calibration Checks adjustment

Tools: Frequency meter or tachometer, Jumper wire

Start the generator set.
Adjust engine no-load speed to the desired allowable speed droop using the Run Speed potentiometer.
With no load, turn GAIN clockwise slowly until engine speed oscillates.
Reduce GAIN slowly counterclockwise until engine speed is stable.
Upset the governor by momentarily shorting terminals 7 and 10 on the governor control.
Apply and remove loads to check generator set response.

HC4-7 Axial Alignment Procedure adjustment

Tools: Dial indicator with magnetic base or clamp

Check only when an objectionable vibration is present.
Fasten dial indicator holding device to skid base, engine block, or generator shell with magnetic base or clamp.
Position sensor point on the generator shaft hub.
Bar the engine over in a clockwise rotation as viewed from engine flywheel through a couple of rotations.
Record indicator readings in eight equally spaced points around the shaft diameter.
Remove the capscrews connecting drive discs and flywheel and mark discs and flywheel.
Rotate engine or generator so drive disc holes are repositioned 180 degrees from original location.
Put capscrews back in and retorque. Recheck shaft alignment.
If runout unchanged, rotate discs to 30, 60 or 90 degrees from original location.
If T.I.R. does not improve, inspect flywheel pilot and drive disc runouts.

P7 Axial Alignment Procedure adjustment

Check only when an objectionable vibration is present.
Remove the capscrews connecting drive discs and flywheel.
Mark the drive discs and flywheel with respect to each other.
Rotate engine or generator so drive discs holes are repositioned two bolt holes from original location.
Put capscrews back in and retorque. Recheck/record vibration levels.
Repeat this test until drive disc holes are repositioned 180 degrees from original location.
Review vibration data and position drive disc in lowest recorded level location.

UFRO Adjustments adjustment

⚠ Use an insulated handle screw driver to prevent electrical shock.

Tools: Insulated handle screwdriver, Frequency meter or tachometer

Make a note of the no-load frequency.
Determine roll-off frequency by turning the governor speed pot until the LED lights; if okay go to step 5.
Adjust the governor speed pot to the proper roll-off frequency.
Turn the UFRO pot until LED goes off, then slowly turn it back until the LED just goes on.
Adjust the governor speed pot back to the initial frequency setting.

Voltage and Voltage Stability Adjustments adjustment

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

Tools: Insulated handle screwdriver, AC voltmeter

Use the control panel mounted voltage trimmer, if provided, for small voltage adjustments.
Measure generator output voltage while running without load at nominal frequency.
If trimmer is insufficient, lock it at midpoint.
Turn the VOLTS pot fully counterclockwise and the STABILITY pot to its midpoint.
If the red LED lights, refer to Jumper Reconnections and UFRO Adjustments.
Turn the VOLTS pot clockwise until rated voltage is obtained.
If voltage becomes unstable with a large load, turn the STABILITY pot clockwise until stable.
Check and readjust the VOLTS pot each time the STABILITY pot is readjusted.

Float Switch Assembly Testing diagnostic

Tools: Ohmmeter, Feeler gauge

Remove fuel pump control cover, disconnect wiring jack, and unscrew assembly from top of day tank.
Hold assembly vertical and use ohmmeter to check continuity between each pair of colored leads; all readings should be zero.
Lift each float in turn to 1/8 inch (3 mm) below the C-clip stop above it and test continuity; all readings should be infinity.
Replace assembly if any switch is open in step 2 or does not open in step 3.
Use pipe thread sealant when replacing assembly.

Generator/PCC Control Isolation Procedure diagnostic

⚠ HIGH VOLTAGE. Touching uninsulated high voltage parts inside the control box can result in severe personal injury or death.

Tools: Voltmeter, Jumpers, 12 volt battery

Throw the line circuit breaker OFF and shut down the set.
Remove accessory box cover; remove plug P10 from A37 J10.
Prepare to measure output voltage across generator terminals.
Connect 12V battery jumpers to X (P10-6 red) and XX (P10-3 brown).
Check polarity; incorrect polarity makes the test inconclusive.
Start set and connect battery negative to the XX lead.
Rated or higher balanced voltage means generator circuitry is OK.
Unbalanced voltage: troubleshoot main stator; low voltage: exciter/field.

Insulation Resistance (Megger) & Polarization Index (PI) Testing diagnostic

⚠ The windings of medium-voltage (601 through 15,000 volts) generator sets must be dry before the generator is operated.

Tools: Megger or insulation resistance meter, Grounding cluster, High voltage gloves

Perform the Generator Set Grounding Procedure.
Disconnect plug J10 from A37 and AC control leads 4,5,6,7,8.
If RTD option installed, ground all six RTD temperature leads (18 leads).
Perform Insulation Resistance Test and PI Test for desired windings.
Megger test: apply voltage for up to one minute.
PI test: apply voltage for ten minutes; record 1-minute and 10-minute readings.
PI ratio of 2 or greater is good for new and in-service sets.
Investigate and correct low readings before returning set to service.

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 generator sets	Load wires are routed through the CTs	Reconnectable gensets (12 leads) have dual secondary CTs (3 pins); CT wire 2/3 to pin 2 (high voltage) or pin 3 (low voltage).
		6 lead generator sets	Generator output leads are routed through the CTs	Non-reconnectable gensets (6 leads) have single secondary CTs (2 pins).
		Cummins PowerCommand generic AC Reconnect Wiring Diagram 625-2755 (Rev. D) and 625-3100 (Rev. F)		Applies to models DFAA, DFAB, DFAC, DFBF, DFCB, DFCC, DFEB, DFEC, DFFB, DFGA, DFGB, DFJA, DFJB, DFJC, DFJD, DFLB, DFLC, DFLD, DFMB (625-2755) and models DFAA-DFMB plus DFCE, DFED, DQAA, DQAB, DQBA, DQBB (625-3100).
	A	CT21 – U load leads	A phase	CT dot must face the generator; CT terminal 1 connects to metering circuit; terminal 2/3 connects to ground.
	A (U)	Bus PT module TB1 A	bus A phase	Bus PT module A39.
	A (U)	CT21	U load leads (A phase)	CT dot must face the generator; CT terminal 1 connects to metering circuit, terminal 2/3 to ground.
	B	CT22 – V load leads	B phase	CT dot must face the generator; CT terminal 1 connects to metering circuit; terminal 2/3 connects to ground.
	B (V)	Bus PT module TB1 B	bus B phase	Bus PT module A39.
	B (V)	CT22	V load leads (B phase)	CT dot must face the generator; terminal 1 to metering circuit, terminal 2/3 to ground.
	C	CT23 – W load leads	C phase	CT dot must face the generator; CT terminal 1 connects to metering circuit; terminal 2/3 connects to ground.
	C (W)	Bus PT module TB1 C	bus C phase	Bus PT module A39.
	C (W)	CT23	W load leads (C phase)	CT dot must face the generator; terminal 1 to metering circuit, terminal 2/3 to ground.
	N	Bus PT module TB1 N	bus neutral	Bus PT module A39.
high voltage genset		12-lead CT secondary wire marked 2/3 to pin 2	2/3	For high voltage gensets.
low voltage genset		12-lead CT secondary wire marked 2/3 to pin 3	2/3	For low voltage gensets.

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 Cummins DFFB engine?

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