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UNDERFREQUENCY – FAULT CODE 15
WARNING
Some genset service procedures present hazards that can result in severe personal injury
or death. Only trained and experienced persons with knowledge of fuels, electricity, and machinery
hazards should perform genset service. See Safety Precautions.
Logic: Carburetor Models – Frequency less than 54Hz for 30 continuous seconds
EFI Models – Frequency less than 54Hz for 8 continuous seconds
Possible causes: Genset loads, Inverter/Charger, engine governor function, fuel supply, air fuel mixture, ex-
haust system, choke, demand regulator, carburetor, generator drive system, generator windings, ignition, wire
connections
Diagnosis & Repair:
1. Verify that Last Fault is FC 15: Yes, continue diagnosis; No, troubleshoot actual last fault.
2. Measure AC current while running genset with vehicle loads: identify faulty or short cycling loads.
3. Determine battery charge rate (typical default is 80 percent): if adjustable reduce to 30 percent.
4. Measure genset load capability with shop load bank (derate for altitude and temperature as necessary).
5. Measure genset frequency and droop while running.
6. Measure AC frequency while changing engine rpm to determine if frequency response matches engine
rpm response: repair damaged generator drive system as necessary.
7. Disconnect genset control P1 connector, verify P1 pins are fully inserted and inspect pin condition per
PSB–676: insert, repair or replace pins as necessary.
8. Reconnect P1 connector and test run genset for fault occurrence: repair or replace pins in connector as
necessary.
9. Check air filter cleanliness; replace air filter as necessary.
10. Check for blocked or damaged exhaust system: repair or replace exhaust components as necessary.
11. HGJAD only – Run genset in Engine Run Only and externally excite field with 12 VDC or 24 VDC and
measure main and quadrature winding AC output.
12. Non EFI only – If the genset will not stay running measure genset frequency, quadrature winding and
main winding voltages using Break-Out Tool 420–0603 and Instruction Sheet R1098.
13. All Gasoline Models
A. Vapor lock may occur in high ambient temperatures: operate in ambients at less than 120°F; verify
proper installation to prevent air recirculation; correct as necessary.
B. Measure steady DC voltage at fuel pump while genset is cranking: repair wiring as necessary.
C. Try to start genset on shop fuel supply: verify tank level and fuel line condition in vehicle.
14. All Gasoline Carburetor Models
A. Verify carburetor altitude setting: readjust as necessary.
B. Verify choke operation, setting, linkage, vacuum pull off assembly and sustain valve: readjust, repair
or replace as necessary.
C. Measure fuel pump pressure/flow: replace fuel filter or fuel pump as necessary.
D. Inspect carburetor butterfly for binding: replace carburetor as necessary.
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