In short. Generator troubleshooting is a sequence, not a guess: record the symptom and the conditions, read the controller’s alarm and event log before resetting anything, work through the subsystem the evidence points to, and verify the repair under the conditions that produced the fault. Most failures fall into five patterns, from a set that will not crank to one that overheats on load, and each pattern has a short list of likely causes that measurement can separate in minutes.
Preserve the evidence before touching anything
The single most useful habit in generator diagnosis is to photograph the controller display before pressing reset. The alarm text, the hour reading, the last recorded voltage and frequency, and the event log sequence are the only record of what the set saw at the moment of failure, and a reset often clears them. Alongside the photograph, note the time, the ambient conditions, the load that was connected or being started, whether the DISCOM had just failed or just returned, and any work done on the set in the previous weeks. Many of the faults we see on breakdown calls trace back to a recent service visit, a changed setting or a new load.
Five symptom patterns and where to look
| Symptom | Check first | Then | Rarely |
|---|---|---|---|
| Will not crank | Battery voltage under load, terminals, charger output, controller in Auto, emergency stop released | Starter solenoid and motor, crank relay, wiring, low-fuel or other start inhibit | Seized engine |
| Cranks but will not fire | Fuel level, fuel solenoid or ECM enable, air in fuel lines, blocked fuel filters | Lift pump, water in fuel, injector pump or injectors, glow plugs on small sets | Low compression, timing |
| Starts then shuts down | Alarm text: low oil pressure, high coolant temperature, overspeed, underspeed, fail to reach voltage | Sensor and wiring faults mimicking a real condition, fuel starvation under load, loose connections | Governor or ECM fault |
| Runs but no or unstable voltage | Engine speed and frequency, AVR fuse and sensing, output breaker | Loss of residual magnetism, rotating diodes, AVR failure, winding fault | Exciter or main winding damage |
| Trips or overheats on load | Actual load versus rating, single large motor start, radiator fouling, fan belt, coolant level, room ventilation | Thermostat, water pump, air filter restriction, exhaust restriction, injector condition | Head gasket, liner or bearing wear |
The pattern “runs but no voltage” deserves a specific note. If the frequency is correct at 50 Hz, the engine is doing its job and the fault is in the excitation chain: AVR, sensing, exciter, rotating diodes or main windings, in that order of likelihood. If the frequency is wrong, start with the engine and governor, because no AVR can regulate voltage on an engine that cannot hold speed. That one measurement halves the search. The excitation side is covered in detail under AVR and excitation service.
Separate the sensor from the condition
A low oil pressure shutdown means one of two things: oil pressure was genuinely low, or the sensor, its wiring or the controller believed it was. Both need investigating, but the response differs completely. Check the oil level and look for leaks first, then confirm pressure with a mechanical gauge at the gallery while the engine runs briefly if it is safe to do so. A sensor reading that disagrees with the gauge is a sensor fault; a gauge that agrees with the alarm is an engine problem and the set must stay down. The same logic applies to high coolant temperature, which is confirmed with an infrared thermometer on the thermostat housing, and to overspeed, which is confirmed against the frequency reading. Controller event logs with timestamps make this far easier, and controller diagnostics can often read a fault history that the front panel does not display.
Faults that live outside the generator
A significant share of generator complaints are system faults the set is merely reporting. A set that trips every time it takes load may be perfectly healthy and simply too small for a motor that was added last year, or facing a UPS whose rectifier draws a distorted current the alternator cannot regulate. A set that starts and runs but never takes the site is usually an AMF, breaker or interlock problem, not an engine or alternator one. A set that overheats only in April and May may be in a room whose ventilation was adequate at the ambient temperature the installer assumed and is not adequate at 43 °C. Voltage that is unstable on generator supply but fine on the DISCOM can be an alternator fault or a large cyclic load that the smaller source cannot absorb. Each of these is diagnosed by measuring load, not by replacing parts on the set.
Verify the repair under the failing conditions
A set that starts after repair has not been proven. It must be run under the conditions that produced the original fault: the same load, the same transfer, the same duration. If the set tripped during a motor start, the motor is started again with voltage and frequency recorded. If it failed to start automatically, the incoming breaker is opened and the AMF sequence is observed. Only then is the fault closed, and the record should say whether the root cause was found and corrected or whether operation was restored without a confirmed cause, because those are different outcomes with different follow-up. Recurrent faults where the cause remains unclear are the business of root-cause diagnostics, and the readings taken during verification feed back into the maintenance plan.
Safety boundary for site staff
Site staff can safely read the controller, check fluid levels with the set stopped, inspect for leaks and photograph alarms. They should not open live electrical panels, bypass any protection device, work on the fuel system while the engine is hot, or reset a shutdown repeatedly in the hope it clears. A low oil pressure shutdown overridden three times can turn a sensor fault into a seized engine. If the first reset does not clear the condition, stop and call for diagnosis.
Related photographs and videos
Common mistakes
- Resetting the alarm before recording it; the only evidence of what happened is erased and the fault returns at the next outage.
- Replacing the AVR for every voltage complaint; a third of those sets had a speed, sensing or diode problem and the new AVR changes nothing.
- Repeatedly overriding a shutdown to restore supply; a protective trip becomes a destroyed engine.
- Declaring the fault fixed after a no-load start; the set fails again the moment the real load is applied.
- Blaming the generator for a load or system problem; money is spent on the set while the undersized cable or oversized motor remains.
What to send us
Send a photograph of the controller display with the alarm or event log, the engine and alternator nameplate photographs, the hour reading, a description of what the set was doing when it failed, the load connected at the time, the weather, and any service or changes in the previous month. If the fault is intermittent, note the pattern. With that information we can usually tell you by phone whether it is a site-staff fix, a service visit or an urgent rental backup while repair proceeds. Send your fault details for breakdown support.
Questions engineers ask
Why does my generator start in manual but not in automatic mode?
The engine and starting system are fine, so the fault is in the AMF start chain: the controller mode selector, the mains-sensing circuit, the start signal wiring between panel and set, or a start inhibit such as a low-fuel or not-in-auto condition. Test by simulating a mains failure at the incoming breaker and tracing the start signal.
What causes a generator to produce no voltage after it starts?
If frequency is at 50 Hz, the usual causes in order are a blown AVR fuse or failed AVR, a sensing wire fault, loss of residual magnetism after long idleness, or failed rotating diodes. If frequency is low, the engine is not reaching speed and the excitation chain is not the problem.
Why does the generator trip only when a particular machine starts?
The starting current of that machine, often six to eight times its running current for a direct-on-line motor, is producing a voltage or frequency dip beyond the set’s limits or the protection settings. Measure the dip, then consider a soft starter, staggered starting or a larger set rather than adjusting protection.
Is black or white exhaust smoke a diagnosis?
No, it is a clue. Black smoke under load suggests air restriction, overload or injector problems; white smoke on a cold start that clears is normal, while persistent white smoke suggests unburnt fuel from low compression, timing or coolant entering a cylinder; blue smoke indicates oil consumption. Each must be confirmed by measurement.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.