Cummins/Onan MDL3 Generator Set

⚠ Accidental starting of the generator set can cause severe personal injury or death; disconnect starting battery cables (negative first) before service.

Tools: belt tension tool

1. Remove battery cables (negative cable first) and remove belt guard.
2. Inspect belt for ply separation, fraying and glazing; replace if any condition exists.
3. Measure belt tension with a belt tension tool; should be 60 pounds (267 N).
4. Loosen bolt through elongated slot in sea water pump bracket and slide pump until proper tension is secured.
5. Reinstall belt guard; do not operate genset without belt guard in place.

Tools: Accurate tachometer or strobotach

1. Correct the speed range by turning the adjusting screw, thereby changing the magnetic air gap (Figure 5-3).
2. Check the overspeed trip point with an accurate tachometer or strobotach after adjustment.

⚠ Do not adjust the other potentiometer (Volts/Hz Adjust); it is factory set using special calibration equipment.

1. With the generator set running, verify the set has stabilized and is running at correct speed (frequency).
2. Set the Voltage Adjust potentiometer on the regulator board assembly for correct voltage (Figure 4-12).

⚠ Accidental starting of the set can cause severe personal injury or death. Disconnect the battery cables when repairs are made to the engine, controls, or generator.

1. Stop the generator set.
2. Change tap connections on the transformer T21 to adjust output voltage.
3. When replacement is necessary, install leads to the same terminals of the new transformer.
4. Refer to the model wiring diagram or the generator reconnection diagrams under Test (Q).

1. Watch for solenoid actuation when B+ is applied (start switch in start or run position).
2. If there is no actuation when B+ is applied, the fuel solenoid must be replaced.
3. When B+ is removed, the solenoid must de-activate.

Tools: Voltmeter

1. Connect relay coil voltage across relay coil terminals; relay should activate if coil is okay.
2. Connect voltage source to one side of relay contacts.
3. Connect a voltmeter to the other side of the relay contact and the voltage source.
4. If voltage appears when relay energizes, contact is okay; reading appears in reverse order when checking normally closed (NC) contacts.

Tools: Voltmeter

1. Apply battery positive (B+) to the terminal marked S.
2. Connect a ground wire to the solenoid mounting bracket; solenoid should activate.
3. If contacts are good, battery voltage should be read between terminal I and ground.
4. The voltage drop measured across the contacts should never exceed one volt in circuit application.

Tools: Ohmmeter

1. Remove battery B+ cable.
2. Place ohmmeter leads across switch.
3. Open and close switch while observing the ohmmeter; a normally open (NO) switch should have infinite resistance when open and continuity when closed; a normally closed (NC) switch should have continui
4. Replace switch if defective.

Tools: Accurate ohmmeter

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

Tools: Ohmmeter

1. Inspect all wires for breaks, loose connections, and reversed connections; refer to applicable wiring diagram.
2. Remove wires from terminals at each end and check each wire end to end for continuity or opens.
3. Check each wire against each of the other wires and to ground for possible shorts or insulation breaks.
4. Reconnect or replace wires according to applicable wiring diagram.

Tools: Voltmeter

1. With the starter motor operating, check the voltage drop from the battery negative post (not the cable clamp) to the cylinder block.
2. Check the voltage drop from the battery positive post to the battery terminal stud on the solenoid.
3. Each voltage drop should normally be less than 0.3 volt.
4. Thoroughly clean all connections in any part of the circuit showing excessive voltage drop.

Tools: Hydrometer

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