Quick Answer
When a generator won't start, one observation splits the fault in half: does the engine crank at full speed? No crank or slow crank points at the battery, cables and starter. Full-speed crank with no fire points at air, fuel or the shutdown latch. If the engine runs but the set produces no voltage, the cause sits in excitation: lost residual magnetism, an AVR with no supply, or failed rotating rectifier diodes. Run the five-step excitation test before ordering an AVR. It takes about twenty minutes and rules out the expensive parts first. If the generator won't start at all, work the engine-side table below first.
Generator Won't Start: Is It Air, Fuel, or the Cranking Circuit?
A generator won't start for four broad reasons: the engine cannot be turned, the engine turns but cannot compress, the fuel or gas does not arrive, or a protection circuit is holding the stop device closed. Check them in that order, because the first two cost nothing to test and the fourth costs nothing to reset.
| Symptom | Most likely cause | First check | Typical replacement |
|---|---|---|---|
| Nothing happens, no click | Controller unpowered, emergency stop latched, battery isolated | DC voltage at the controller supply terminals | Fuse, isolator, controller |
| Solenoid clicks, no crank | Starter solenoid contacts or starter motor brushes | Voltage at the starter motor terminal while cranking | Starter motor 1004365597 or 1000728415 (M33 gas) |
| Cranks slowly | Battery state of charge, cable or terminal resistance | Terminal voltage drop during cranking | Battery, cables, earth strap |
| Cranks fast, no fire (diesel) | Air in fuel, blocked filter, stop solenoid held closed | Fuel at the injector inlet pipe, then at the filter outlet | Fuel filter element 330205000731 (12M26) |
| Cranks fast, no fire (gas) | No gas pressure, blocked gas filter, no ignition | Gas pressure at the train inlet, then spark at the plug | Gas filter 1002480574, spark plug 1001392970, coil 1002254991 |
Respect the cranking duty while you test it. The Baudouin M33 gas platform limits a start attempt to 15 seconds with at least 30 seconds of rest between attempts. Ignore that and you will burn the starter you were trying to diagnose. Most starters returned to us were killed during diagnostics on a machine where the generator won't start for an unrelated reason. If the generator won't start after the fuel side checks out, move to the cranking circuit: measure at the starter motor terminal while cranking. As a field rule of thumb, a 12 V system falling below about 9.6 V at the starter during cranking β about 19 V on a 24 V system β has resistance upstream of the starter, in the cables, terminals or battery, and replacing the starter will not fix it.
Generator Runs but No Power Output: Where Does the Voltage Go?
Separate the two cases before you read any further: either the generator won't start, or it starts and makes nothing. This section is the second case. An engine that fires and settles at speed has done its job. Everything after that is excitation, and excitation is where most no-output calls end up. The alternator needs three things: residual magnetism in the exciter, a supply to the AVR, and a path for the AVR output to reach the exciter stator. Break any one and you get zero volts on the terminals with a perfectly healthy main stator. Do not confuse the two cases. A generator won't start for engine reasons; a generator that starts and shows zero volts has an excitation reason.
| Output symptom | Probable cause | Severity |
|---|---|---|
| No output voltage at all | AVR failed, excitation supply lost, residual magnetism lost | Severe β do not keep cranking |
| Output at 70β85% of rated | Rotating rectifier diodes partially shorted | High β test diodes before ordering an AVR |
| Voltage swinging Β±5% or more | AVR potentiometer drift or loose sensing leads | Medium β usually a cheap fix |
| Overvoltage above 110% | AVR runaway or field short circuit | Severe β stop before connecting load |
| High harmonic distortion | Rotating rectifier diode open circuit | Medium β diode set replacement |
| High rotor vibration | Rectifier assembly out of balance | High β mechanical damage follows |
That second row is the one that costs money. A set sitting at 70β85% of rated voltage looks like a dead AVR, and plenty of AVRs get replaced on that evidence alone. Test the diodes first: a partially shorted rotating rectifier produces exactly that symptom and a new AVR will not correct it.
How Do You Test the AVR, Exciter Winding and Rotating Diodes in Order?
Run these five steps in sequence. The order matters because each step can invalidate the next, and step three is the one that stops people buying the wrong part. Nothing here applies until the engine is running: if the generator won't start at all, go back to the engine-side table.
| Step | Measurement | Pass condition | If it fails |
|---|---|---|---|
| 1. Speed | Engine speed with a tachometer or the controller readout | 1,500 rpm Β±2% (50 Hz) or 1,800 rpm Β±2% (60 Hz) | Fix the governor first; no AVR can correct off-speed |
| 2. AVR supply | AC at the AVR input terminals | PMG types 170β220 VAC; shunt types present at the sensing terminals | Chase the supply and wiring, not the AVR |
| 3. Exciter winding | Resistance and insulation to earth at 500 VDC | Resistance within Β±5% of the nameplate value; insulation 1 MΞ© or better | Do not fit a new AVR; a shorted field will destroy it |
| 4. Rotating diodes | Diode test each way across every diode | Forward 0.3β0.7 V, reverse open circuit | Replace the diode assembly |
| 5. AVR output | DC volts at the exciter stator while running | Variable 0β50 VDC that responds to the voltage adjust | Now the AVR is the suspect; order by excitation type |
When you do reach step five, order by excitation type rather than by voltage. A shunt machine takes a Stamford SX440 or SX460 class regulator, an AREP machine takes the Leroy-Somer R438, R449 or R450 family, and a PMG machine such as one running a Stamford MX341 or MX321 needs a regulator that expects 170β220 VAC on its supply. Aftermarket ranges from Basler (the BE2000E line) and Kutai (EA440, ADVR-12) cover the common frames, but only after the three hard checks pass. A generic shunt regulator will not excite a PMG machine at all. The full mapping, including field current bands and sensing voltages, is in how to match an AVR to your alternator's excitation system.
What Does Low or Unstable Output Voltage Tell You?
Low and unstable output belong to the same fault family. A generator won't start building voltage cleanly when the sensing circuit is loose, and it will not hold voltage when the AVR is drifting. A slow drift upward over months is usually the adjust potentiometer or a sensing connection working loose. A sudden drop to 70β85% is electrical, and a set that reads correct at no load then collapses under load is either a speed droop problem or a diode on its way out.
That is why the intervals matter. A generator won't start is a callout; a generator that drifts out of tolerance is a maintenance finding, and the second one is far cheaper to deal with. Most of these failures announce themselves first. Visual inspection of the AVR sits at 500 hours, PMG output voltage check at 1,000 hours, rotating diodes plus exciter winding resistance and insulation at 2,000 hours, and the exciter air gap at 4,000 hours. The air gap on these machines runs 1.5β3.0 mm (0.059β0.118 in). A set that never gets the 2,000-hour check usually meets its diode failure as a no-output call on a night shift instead.
When Does a No-Start Become a Stop-Work Condition?
Some faults let you keep testing. Others do not, and the difference is what separates a repair from a rebuild. When a generator won't start on the first three attempts, the fourth attempt is usually where the starter dies.
- Stop cranking after 15 seconds. Beyond that, or without the 30-second rest, you are cooking the starter and the battery.
- Stop immediately on any fuel smell. On a gas set, stop on any gas smell and ventilate before touching electrical switches.
- Do not fit a replacement AVR if the exciter winding reads below 1 MΞ© to earth at 500 VDC. Find the earth fault first.
- Do not connect load if output is above 110% of rated. Shut down and find the field fault.
- Hit the emergency stop if the engine runs away or overspeeds on start. Never investigate an overspeed from the control panel alone.
One more pattern worth naming: a standby set. ISO 8528 puts emergency standby duty at roughly 200 running hours a year, so a set can hold a five-year-old machine with under 1,000 hours on the clock and still refuse to start, because batteries sulphate, fuel ages and moisture settles in the windings. When a generator won't start after a long idle, suspect time before wear.
Which Spare Parts Should You Stock Before the Next No-Start?
If your generator won't start twice in a season, the shelf is usually the problem rather than the machine. The parts that end a no-start call fastest are the ones that fail without warning and have no repair option on site. Build the shelf around those, sized to your fleet and your rating class.
| Part | Platform | Part number | Note |
|---|---|---|---|
| Starter motor | M33 gas, 12M26 diesel | 1004365597, 1000728415, 1000728416 | Confirm flywheel tooth count and mounting flange |
| Charging alternator | M33 gas, Weichai WP series | 612600090762, 612600090206D | Check pulley and regulator plug before ordering |
| Speed sensor | 12M26, 6M12 | 330205000408, 612630030007 | A dead speed sensor mimics a dead engine |
| Fuel filter element | 12M26, 6M11 | 330205000731, 1001044158 | Stock per engine: 6M11 takes two per machine |
| Spark plug | 6M33NG, 12M33NG, 16M33NG | 1001392970 | Gap 0.25β0.30 mm (0.010β0.012 in) with a feeler gauge |
| Ignition coil | 16M33NG | 1002254991 | Clean terminals and inspect the boot every six months |
| Gas filter | 12M33NG, 16M33NG | 1002480574 | Shorten the cleaning interval on poor-quality gas |
For the gas platforms listed above, cross-check against which M33 gas genset parts fit the 6M33NG, 12M33NG and 16M33NG before committing to a quantity. If your generator won't start on a known-good battery and a confirmed fuel supply, the table above covers most of what is left, and replacement parts for diesel and gas generator sets is where to check availability. Use the engine series to pull the correct part number when the plate is hard to read. Ignition-side spares for CNG and LNG machines are covered in the CNG and LNG generator ignition parts range, and ignition coil testing on 16M33 gas engines walks through the coil-side checks step by step.
Frequently Asked Questions
My generator won't start, but the starter spins it fine. What is most likely?
On a diesel set, air in the fuel or a blocked filter. Crack the injector inlet pipe and look for fuel while cranking. On a gas set, check gas pressure at the train inlet, then confirm spark at the plug.
The generator won't start after sitting unused for a year. Where do I begin?
With the battery and the fuel, not the engine. A set on standby duty sees roughly 200 hours a year under ISO 8528, so idle time does more damage than running time. Load-test the battery, then check fuel condition.
How do I know if my AVR is bad?
Only after steps one to four pass. Confirm speed, confirm the AVR supply, confirm the exciter winding resistance and insulation, confirm the rotating diodes. If all four pass and the AVR output does not vary 0β50 VDC at the exciter stator, the AVR is the fault. If the generator won't start at all, none of these apply.
Generator runs but output is only about 80% of rated voltage. AVR or diodes?
Test the diodes first. Output at 70β85% of rated is the classic signature of partially shorted rotating rectifier diodes, and a new AVR will not raise it. A generator won't start producing rated voltage again until the diode assembly is replaced.
Can I replace an MX341 with a standard SX440?
No. MX341 and MX321 run on PMG excitation with a 170β220 VAC supply, while SX440 is a shunt regulator. A shunt unit will not excite a PMG machine.
What should I send to get the right part the first time?
Name the symptom precisely first: no crank, slow crank, cranks but no fire, or runs with no output. When a generator won't start, that single line saves a round of questions. Then send the engine model and serial, the alternator model, and a photo of both plates. For ignition items add the fuel gas type and your annual running hours.
Tell us the symptom and engine model, and we match the parts.
If your generator won't start or has lost output, send three things: the symptom and what you have already tested, the engine model and alternator model, and the part quantity per engine. We will cross-reference the numbers and tell you what is on the shelf.
Every hour a generator won't start costs more than the part that would have prevented it. Send the symptom, engine model and quantity and we will return a matched parts list. All parts referenced are compatible with Baudouin and Weichai generator platforms; Hanchi Auto Parts is not affiliated with or endorsed by those OEMs.