Cummins/Onan 4MKY Generator Set

Tools: Frequency meter

1. Connect a frequency meter and run the genset until warmed up.
2. Disconnect all loads.
3. Pull the governor linkage toward the front so the throttle lever tang bears against the throttle stop screw.
4. Adjust the stop screw to obtain 54-56 Hz on 60 Hz gensets (44-46 Hz on 50 Hz gensets).

⚠ The choke housing becomes hot during operation and can cause severe burns if touched

1. Allow genset to cool down; housing temperature must equal ambient temperature.
2. Loosen the adjustment screw holding the adjustment plate.
3. Slowly rotate adjustment plate until bimetal choke shaft connection is visible in the sight window (correct at 70 deg F [21 deg C]).
4. For other ambient temperatures, make a mark opposite the center mark; each mark equals 10 deg F (6 deg C) difference.
5. Rotate plate clockwise if warmer, counterclockwise if cooler than 70 deg F (21 deg C); tighten screw.
6. Move choke lever back and forth to check for free movement; verify choke does not bind or stick.

⚠ Voltage/frequency-sensitive equipment may be damaged by power line frequency variations

Tools: Digital frequency/voltmeter (0.3% freq accuracy, 0.5% voltage accuracy), Digital ammeter, Variable load bank with 4 kW capacity

1. Run genset at least 10 minutes at 3/4-rated load; check choke completely open.
2. Check governor linkage for binding or excessive looseness; check motion spring.
3. At no-load, turn speed adjustment screw to obtain 62-63 Hz (60 Hz) or 51.5-52.5 Hz (50 Hz).
4. Check frequency and voltage with load and no load against Table 8-1 limits.
5. Adjust governor sensitivity for closest regulation without hunting.
6. Recheck speed setting and readjust if necessary.
7. Set carburetor throttle stop screw as specified in FUEL SYSTEM.

⚠ Disconnect voltage/frequency-sensitive equipment before adjustments.

Tools: Digital frequency/voltmeter, Digital ammeter, Variable load bank 4 kW

1. Run the genset at least 10 minutes at 50% to 75% of rated load; check choke is fully open.
2. Check governor linkage for binding or looseness; check motion spring for damage.
3. At no-load turn the speed adjustment screw to obtain 62 Hz for a 60 Hz genset (output 123-132 VAC).
4. For a 50 Hz genset adjust to 51.5-52.5 Hz (220-231 VAC if rated 220 VAC).
5. Check frequency and voltage with load then no load; verify within Table 8-1 limits.
6. Adjust governor sensitivity to give closest regulation without hunting; turn CCW to increase.
7. Recheck the speed setting and readjust if necessary.
8. Set the carburetor throttle stop screw as specified in Fuel System.

⚠ Adjust the valve clearance only when the engine is at ambient temperature.

Tools: Feeler gauge, Wrench

1. Remove the head cover and inspect valve stems for alignment with the rocker arms.
2. Advance the engine until both valves are closed (piston at top dead center).
3. Check clearance with a feeler gauge between the top of the valve stem and rocker arm.
4. Loosen the outer locking nut and turn the adjusting nut to obtain the correct clearance.
5. Tighten the locking nut after adjustment.
6. Recheck the clearance and check the rocker arm bolts have not loosened.
7. Reinstall the head cover and torque the bolts to specification.

⚠ Adjust valve clearance only when engine is at ambient temperature

Tools: Feeler gauge, Wrench set

1. Remove head cover; inspect valve stems for alignment with rocker arms.
2. Advance engine until both valves are closed and no pressure on lifters (piston at TDC).
3. Check clearance with feeler gauge between valve stem top and rocker arm.
4. Loosen outer locking nut and turn adjusting nut as needed to obtain correct clearance.
5. Tighten locking nut after adjustment.
6. Recheck valve clearance and check rocker arm bolts have not loosened.
7. Reinstall head cover and torque bolts.

⚠ Attach and remove meter leads only when genset is not operating.

Tools: DC voltmeter, AC voltmeter

1. Test A: Connect DC voltmeter positive to J4 pin 7 and negative to J4 pin 3.
2. Push start switch; approximately 12 VDC indicates field flash voltage is available; proceed to Test B.
3. Test B: Move positive lead to pin 9 and negative to pin 10; start the genset and allow speed to stabilize.
4. Measure field voltage no load then full load; 40 VDC no load to 100 VDC full load is normal.
5. Test C: Disconnect J4 from P4, attach AC voltmeter to L1 and L2, and apply 12 VDC to P4 pins 9 (+) and 10 (-).
6. Start the genset; approximately 40 VAC indicates brushes, rotor and stator are OK.

⚠ Do not substitute an automotive electric fuel pump for the Onan-supplied fuel pump

Tools: Pressure gauge

1. Make sure fuel tank has sufficient fuel.
2. Check genset starting battery voltage; pump will not work properly below 6 VDC.
3. Remove fuel line from carburetor inlet and install a pressure gauge.
4. Press the stop switch and hold for several seconds until pressure stabilizes.
5. Reading should be 2.5-4.00 psi (17-27 kPa); pressure should hold constant or drop off slowly.

⚠ Disconnect battery negative cable to prevent accidental starting

Tools: Leak down tester

1. Start engine and warm up for ten minutes.
2. Disconnect battery negative (-) cable and remove spark plug.
3. Manually rotate engine to top dead center (TDC) on compression stroke.
4. Connect tester to shop air and set calibration; secure fan wheel.
5. Screw air fitting into spark plug hole; attach plug fitting to tester.
6. Read percentage of cylinder leakdown.
7. If leakage greater than 30 percent, listen for air leakage at check points listed in Table 9-1.

Tools: Ohmmeter, Megger

1. Ground Test: touch one prod to the rotor shaft and the other to a slip ring; infinity should be measured.
2. A reading less than one megohm indicates a grounded rotor - replace.
3. Open Test: place prods on the slip rings; continuity of 19-27 ohms should be indicated.
4. Short Test: set ohmmeter to lowest scale; less than 17 ohms at 77° F (25° C) indicates shorted windings - replace.

Tools: Voltmeter

1. Connect a voltmeter between the start relay output terminal (starter motor lead side) and ground.
2. Press the start switch.
3. If approximately 12 VDC is present but the starter does not crank, the starter is defective.
4. If no voltage, check the battery, start solenoid and control wiring.

Tools: Ohmmeter, Digital type ohmmeter

1. Remove both connector plugs from the stator.
2. Ground Test: connect one prod to the housing and touch terminals T1, T2, B1, B2, Q1, Q2; less than one megohm indicates a ground.
3. Winding Resistance: connect prods to the terminals per Table 9-2 and read resistance for open windings.
4. Short Test: use a digital ohmmeter reading within 0.001 ohms; a reading less than Table 9-2 at 77° F indicates a short.