Cummins/Onan MDKC Generator Set

Tools: Voltmeter

1. With starter motor operating, check voltage drop from battery negative post to cylinder block.
2. Check voltage drop from battery positive post to battery terminal stud on solenoid.
3. Each should normally be less than 0.3 volts.
4. Clean all connections in any part of the circuit showing excessive voltage drop.

⚠ Battery electrolyte can cause severe eye damage and burns; wear goggles, rubber gloves and a protective apron.

Tools: Hydrometer

1. Check charge condition of the battery with a hydrometer.
2. Electrolyte specific gravity should be about 1.260 fully charged at 80°F (27°C).
3. Add distilled water to keep electrolyte at proper level and recharge.
4. If battery will not recharge, replace it.

Tools: Voltmeter

1. Apply battery positive (B+) to the terminal marked S.
2. Connect a ground wire to the solenoid mounting bracket.
3. Solenoid should activate.
4. Battery voltage should be read between terminal 1 and ground if contacts good.
5. Voltage drop across contacts should never exceed one volt.

Tools: Wheatstone bridge or digital ohmmeter, Megger or insulation resistance meter (500 VDC or more)

1. Disconnect F1+ and F2- exciter field leads from terminal block in generator end bell.
2. Measure resistance between field leads with a Wheatstone bridge or digital ohmmeter.
3. Resistance should be 11.16 to 13.64 ohms at 77°F (25°C).
4. Apply 500 VDC or more between either field lead and the exciter stator lamination.
5. Insulation reading should be 1,000,000 ohms or greater.
6. A shorted stator must be replaced.

Tools: Megger or insulation resistance meter (500 VDC or more), Digital ohmmeter

1. Remove rotor leads F1+ and F2- from the rotating rectifier assemblies.
2. Connect test leads between F1+ and rotor shaft, then between F2- and rotor shaft.
3. Meter should register 1,000,000 ohms or greater.
4. Use a digital ohmmeter to check resistance between F1 and F2 leads.
5. Replace a grounded rotor with a new identical part.

⚠ Some generators have ground connections to the frame; check wiring diagram.

Tools: Megger or insulation resistance meter, Wheatstone bridge

1. Isolate all stator leads for testing.
2. Apply 500 VDC or more and test each stator winding for short to laminations.
3. Reading less than 1,000,000 ohms indicates a questionable stator; oven dry and retest.
4. Test continuity between coil leads; all pairs should have equal resistance.
5. If a winding is shorted, open or grounded, replace the stator assembly.

Tools: Ohmmeter

1. Disconnect all leads from the rectifier assembly to be tested.
2. Connect one ohmmeter lead to F1+ stud and to CR1, CR2, CR3 in turn; record values.
3. Connect one lead to F2- stud and to CR4, CR5, CR6 in turn; record values.
4. Reverse ohmmeter leads from steps above and record resistance of each rectifier.
5. Readings must be high in one test and low in the other; otherwise replace assembly.
6. Replace defective rectifier assembly with a new identical part.

⚠ Fuel vapor fire/explosion hazard

⚠ Heavy set can cause injury if dropped

1. Disconnect generator set negative (-) battery cable at the battery terminal.
2. Disconnect generator set positive (+) battery cable from the wire harness.
3. Disconnect the remote control plug wire from the generator set, if applicable.
4. Disconnect generator load wires and tag for identification when reconnecting.
5. Disconnect exhaust system and support brackets or hangers to allow set removal.
6. Disconnect fuel line at the generator set housing and plug the fuel line end.
7. Verify the generator set is adequately supported before loosening mounting bolts.

⚠ Incorrect flashing procedure can damage the voltage regulator.

Tools: 12-volt storage battery, 10-amp fuse, Momentary-on switch, 12-amp 300 V diode

1. Assemble 12-volt battery, 10-amp fuse, momentary-on switch and diode.
2. Start the generator set and operate at normal speed.
3. Connect positive lead to F1(+) exciter stator lead and negative to F2(-).
4. Close the switch just long enough for output voltage to build up.
5. Shut down set; restart at no load; output must build up without flashing.

⚠ Generator components are heavy and can cause severe personal injury if dropped during service.

Tools: Hoist, Stator handling tongs, nylon lifting strap or chain and lift hooks

1. Disconnect the negative (-) battery cable.
2. Remove control box cover; disconnect all stator leads and mark if needed.
3. Remove end bell cover and remove field leads F1 and F2.
4. Remove load wires and flexible conduit from control box.
5. Remove leads from the DC voltage regulator plug.
6. Remove capscrews securing control box saddle to stator; lift box and saddle free.
7. Remove end bell stud nuts; slide off end bell and exciter stator.
8. Support stator with a hoist; remove stator assembly without touching the rotor.

Tools: Hoist and sling, Lead hammer, Gear puller

1. Slide exciter rotor over shaft and woodruff key; install exciter nut and torque.
2. Connect generator field leads to F1+ and F2- terminals; torque values shown.
3. Press the end bearing onto the rotor shaft.
4. Assemble rotor fan and drive disc to engine flywheel; chamfer toward flywheel.
5. Install air baffle.
6. Move stator into position over rotor with leads in top position.
7. Apply thin film of Molykote grease to end bearing and end bell mating surfaces.
8. Install end bell assembly; torque end bell stud nuts.

⚠ Fuel leakage presents fire or explosion hazard; plug all fuel line openings and keep ignition sources away.

1. Disconnect the generator set negative (-) battery cable.
2. Disconnect the positive (+) battery cable at the battery terminal.
3. Disconnect the remote control plug wire, if applicable.
4. Disconnect the generator load wires and tag for identification.
5. Disconnect the exhaust system and support brackets or hangers.
6. Disconnect the fuel line at the generator set housing; plug the end securely.
7. Verify the generator set is adequately supported before loosening mounting bolts.