Cummins/Onan MDJC Generator Set

⚠ Having engine stopped when readjusting avoids accidental shorts.

Tools: Hydrometer, Voltmeter

1. Set slide tap on adjustable resistor R21 in generator air outlet for about 2 amperes charging rate.
2. For frequent-start applications, check battery specific gravity periodically.
3. Increase charging rate slightly if battery will not stay charged; avoid overcharging.
4. With separate automatic demand control, adjust charge rate for maximum 4.5 amperes.
5. If charge winding AC output is low, test charging circuit for opens or grounds.

Tools: Accurate AC voltmeter

1. Set voltage adjust potentiometer R22 on meter panel halfway between minimum and maximum.
2. With unit running at no load, observe generator output voltage.
3. Turn potentiometer R26 on VR21 clockwise to increase output voltage.
4. Turn R26 counterclockwise to decrease output voltage.

Tools: AC voltmeter

1. With generator set running at no load, turn potentiometer R27 until voltage tends to be unstable or hunt.
2. Turn R27 clockwise slowly until voltage first stabilizes.
3. This setting gives stable voltage under all conditions at maximum regulator response time.

Tools: Ohmmeter on RX 100 scale

1. Remove rotor leads F1+ and F2- from rotating rectifier assemblies.
2. Connect ohmmeter leads between F1+ and rotor shaft and between F2- and rotor shaft.
3. Meter should not register; if meter registers, rotor is grounded.
4. Check resistance between F1+ and F2- leads; see Table 2 for proper values.
5. Replace defective rotor with new identical part.

Tools: Ohmmeter, Kelvin Bridge

1. Using ohmmeter set at RX 100, test each stator winding for shorts to laminations.
2. A reading less than one megohm indicates a ground.
3. Test for continuity between coil leads shown in Figure 18; all pairs should have equal resistance.
4. Use accurate instrument such as Kelvin Bridge; values given in Table 3.
5. If windings are shorted, open, or grounded, replace stator assembly.

Tools: Accurate ohmmeter

1. Disconnect all leads from assembly to be tested.
2. Connect one test lead to F1+ stud; connect other lead to CR1, CR2, and CR3 in turn; record resistance.
3. Connect one lead to F2- stud; connect other lead to CR4, CR5, and CR6 in turn; record resistance.
4. Reverse ohmmeter leads and record resistance values again.
5. All three readings should be high in one test and low in the other.
6. Replace defective rectifier assembly with new identical part.

Tools: Hydrometer

1. Check charge condition of battery with a hydrometer.
2. Specific gravity should be between 1.290 and 1.225 when 75 percent charged.
3. If not, recharge the battery; if battery will not recharge, replace it.
4. Check electrolyte level; add approved water to keep electrolyte at proper level.
5. Keep battery connections tight and clean.

Tools: Feeler gauge

1. Locate start-disconnect switch on gear cover above the oil filter.
2. Check the switch contacts for pitting.
3. If necessary, loosen stationary contact and adjust point gap to 0.040 inches.
4. Replace burned or faulty points.

⚠ A series resistor MUST be used to protect the meter. Polarity must be observed.

⚠ Contact terminals just long enough until voltage builds up or damage may occur to exciter-regulator system.

Tools: Six volt dry cell battery, 12 amp DC 300 volt avalanche diode, 10-ohm 10-watt resistor, Clip leads

1. Check generator residual voltage at terminals 1 and 2; should be at least 20 VAC at rated speed.
2. Locate terminals 7(-) and 8(+) on voltage regulator printed circuit board VR21.
3. Connect six volt dry cell, 12 amp 300 volt avalanche diode, and 10-ohm resistor as shown.
4. After starting engine, touch positive battery lead to VR21-8 and negative lead to VR21-7.
5. Contact terminals just long enough until voltage starts to build up.

⚠ Be careful not to let weight of rotor rest on or bend the through-stud.

Tools: Torque wrench, Lead hammer

1. Clean and inspect all mating surfaces.
2. Coat mating area between generator bearing and end bell bearing hole with thin film of Molykote or equal.
3. Install rotor-through-stud in engine crankshaft and install key in crankshaft.
4. Slide rotor over through-stud and onto crankshaft.
5. Install baffle ring, stator through-studs, stator and end bell.
6. Torque nuts on through-studs to 35 to 38 ft-lbs; torque rotor-through-stud nut to 55-60 ft. lb.
7. Tap end bell to align at horizontal and vertical plane; recheck torque.
8. Reconnect leads, install B1 lead on adjustable resistor R21, and install end cover.

⚠ Be careful not to let stator touch or drag on rotor.

⚠ Be careful not to hit bearing or windings.

Tools: Heavy soft faced hammer, Hoist and sling, Screwdriver

1. Disconnect battery to prevent accidental starting of engine.
2. Remove end bell cover to reveal rotor-through-stud nut.
3. Remove B1 lead from tapped adjustable resistor in generator air outlet.
4. Remove leads from control box to ignition system, choke, start disconnect switch.
5. Remove stator-through-stud nuts, end bell, and stator assembly.
6. Turn rotor-through-stud nut to end of stud; strike nut a sharp blow to loosen rotor.
7. Support rotor with hoist and sling to avoid bending rotor-through-stud.

Tools: Screwdriver

1. Stop engine.
2. Disconnect and if necessary label wires 3, 4, 5 or 6, 7, 8, 9, and 10.
3. Remove four screws at corners and remove used PC board.
4. Install new PC board and secure with four screws.
5. Reconnect wires removed in step 2 at the proper terminals.
6. Place jumper W10 at proper terminals for your voltage code and voltage connection.
7. Perform voltage calibration and stability adjustment procedures.