Cummins/Onan YB series Generator Set

Tools: screwdriver

1. Center the voltage adjust knob so pointer is vertical.
2. Open meter panel doors. Start unit.
3. Using a screwdriver, turn R18 potentiometer on printed circuit board VR21.
4. Observe voltmeter on meter panel while adjusting. Set voltage with no load connected. Example: for 120/240 volt connection, set at approximately 246 volts.
5. If voltage is unstable or tends to hunt, turn R26 potentiometer to increase voltage dampening.

Tools: Wheatstone bridge ohmmeter, Simpson 260 ohmmeter

1. Measure L1 resistance across coil terminals; should be 0.0693- to 0.0847-ohms at 20°C.
2. Set ohmmeter to R x 10K; measure between either coil terminal and frame. Less than infinity indicates low insulation; replace reactor.
3. Measure L2 resistance across terminals 1 and 4; should be 308.8- to 377.4-ohms at 20°C, R x 1 scale.
4. With R x 10K, reading less than infinity between any coil terminal and frame indicates low insulation; replace reactor.
5. Measure continuity between terminal 1 or 4 and terminals 2 and 3 to verify taps are not broken.

Tools: Kelvin bridge, ohmmeter, wrench

1. Use a Kelvin bridge instrument for this test.
2. Remove rotating diodes CR1, CR2, CR3 from heat sinks.
3. Measure resistance of rotor windings. Should be within 0.1463- to 0.1788-ohms at 20°C (68°F).
4. Replace rotating diodes on heat sinks. Hold body with wrench while torquing to 15 lb-in (1.7 N·m).
5. Using ohmmeter on R x 10K, measure between any rotating diode lead and generator frame. Less than infinity indicates low insulation.

⚠ Excessive dust or dirt on diodes causes overheating and eventual failure. Keep assemblies clean.

Tools: ohmmeter

1. Remove one diode at a time from heat sink. Test and reinstall before proceeding.
2. Connect one ohmmeter lead to top of diode and other to diode stud. Observe reading.
3. Reverse leads and again observe reading.
4. A good diode has higher resistance in one direction than the other.
5. If both readings are high or both low, diode is defective and must be replaced with identical part.

Tools: ohmmeter

1. Testing for grounds: using ohmmeter on R x 10K, measure insulation resistance between either lead F1 or F2 and generator frame. Less than infinity indicates ground.
2. Testing winding resistance: measure coil resistance between leads F1 and F2 with ohmmeter on R x 1.
3. Resistance should be 14.85- to 18.15-ohms at 20°C (68°F).

Tools: ohmmeter, Wheatstone or Kelvin bridge

1. Remove F+ and F- rotor leads from diode heat sink assemblies.
2. Ground test: connect ohmmeter on R x 10K between either rotor lead and rotor shaft. Less than infinity indicates a ground.
3. Winding resistance: use Wheatstone or Kelvin bridge. Connect meter leads between F+ and F-.
4. Resistance should be 1.646- to 2.695-ohms at 20°C (68°F).

Tools: ohmmeter, Wheatstone or Kelvin bridge

1. Disconnect stator leads from bus bar assembly; observe wire identification.
2. Connect all stator output leads (T1-T12) together.
3. Use ohmmeter on R x 10K to measure insulation resistance between windings and stator frame. Less than infinity indicates ground.
4. Testing for shorts: connect ohmmeter between each individual winding and other windings connected together. Repeat for all six coils.
5. Measure resistance of windings using Wheatstone or Kelvin bridge. Readings should be 0.00322- to 0.00394-ohms at 20°C (68°F).
6. If any windings are shorted, open, or grounded, replace stator assembly. Check leads for broken wires or damaged insulation.

Tools: ohmmeter

1. L21 mounts inside control box, below VR21 voltage regulator.
2. Coils 1-2 and 3-4 are wound on the same core.
3. Resistance between 1-2 and 3-4 should be equal.
4. Resistance between coils (e.g., 1-4) or from any terminal to frame should be infinity.

Tools: ohmmeter, 6 volt dry cell battery, DC voltmeter

1. Rectifier bridge located in control cabinet, below voltage regulator.
2. Contains 3 diodes CR1, CR2, CR3 and two SCRs CR4, CR5 encapsulated in hermetically sealed box.
3. Failure of any diode or SCR means entire unit must be replaced.
4. Disconnect wires from rectifier unit prior to testing.
5. Test unit in order shown in Table 2.
6. Refer to Figure 24 for CR4 and CR5 test circuit.
7. When test is complete and satisfactory, reconnect unit observing correct wiring hook-up.

Tools: ohmmeter

1. Remove voltage regulator VR1 cover to gain access to SCRs.
2. SCRs on VR1 are identified CR1 and CR2, positioned closest to terminal board.
3. Remove gate and cathode wires from SCR.
4. Set ohmmeter to R x 10K scale.
5. Measure resistance across anode and cathode; should be 1 megohm or greater.
6. Reverse leads; reading should again be 1 megohm or greater.
7. Set ohmmeter to R x 1 scale.
8. Short gate to anode; resistance should drop to 25-ohms or less.
9. Remove short between gate and anode; resistance should remain unchanged.
10. Replace SCR if results differ.

⚠ High voltage will be present on voltage regulator terminals if output builds up.

⚠ Do not keep field flashing circuitry connected longer than 5 seconds or damage may occur.

Tools: six-volt supply, resistor, diode

1. Assemble six-volt supply, resistor, and diode as shown in Figure 3.
2. Remove frontal grille to access voltage regulator.
3. Disconnect lead F1 at VR1-3.
4. Start unit.
5. Touch positive lead of supply to F1 and negative to VR1-2.
6. Hold leads until voltage starts building.
7. Shut down unit and reconnect wires.

⚠ Do not keep excitation circuitry connected longer than 5-seconds, or damage may occur to the exciter regulator.

Tools: six volt supply, resistor, diode

1. Assemble six volt supply, resistor, and diode as shown in Figure 21.
2. If six volt supply not available, a 12-volt can be used; substitute a 20-ohm resistor for the 10-ohm.
3. Start the generator set.
4. Touch positive lead to + on rectifier bridge, and negative lead to - terminal.
5. Hold leads on terminals just long enough for voltage to build up.