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Diesel Generator Not Starting


The dreaded has happened…

Your diesel generator not starting? Maybe you have had a power cut and the generator didn’t start leaving you in the dark?

Or you have gone to push the start button on your generator like you do every other day, and it won’t start up or it just shuts down straight away or after a short period of running.

Either way, your generator not starting is big trouble. A generator failing to start means you can’t run your equipment or you will have no backup during a power cut.

Fancy having a look at this yourself to see if it is something simple or obvious?

First lets go through the symptoms one by one and see if we can help point you in the right direction:

My generator tries to start and the engine is rotating and cranking but it won’t fire and run.

If your engine is cranking (starter motor engaged and trying to start) but it won’t fire up, you will likely have a fuel problem or a problem related to the fuel supply to the pump or injectors.

This can be a number of things such as:

  • Low fuel – Check your generator fuel tank level. This is usually on a small clock face mechanical gauge on the generator base tank under the engine. If you have an external diesel tank this may have gauges too. It is possible that you don’t have mechanical fuel gauges you may have electronic fuel level senders which will show a display on your generator controller (you may need to go to a different screen on the controller to see it)
  • Poor quality or contaminated fuel – How long has your diesel been sat in your tank for? Did you know that diesel only has a storage life now of around 12 months without any maintenance? Old diesel, dirty diesel or diesel contaminated with water can cause your generator to fail to start or run and often causes engine failure. 90% of diesel engine failure is caused by poor or contaminated fuel. If you think it could be a fuel issue, try removing the diesel from the generator and replacing it with new clean diesel. It is also worth noting that red diesel is now illegal in the UK for commercial use since 2021 so if you have red diesel I would suggest replacing it with white. We can help with the removal of your generator diesel and supply of new diesel if you need.
    Get in touch if you wish to discuss generator fuel replacement.
  • Air in your fuel – Diesel engines will not start if there is air in the fuel lines. Air can make its way into your fuel system through loose fittings, damaged hoses and loose filters. These issues will need to be checked and fixed and the fuel system will need to be ‘bled’ to remove all air before it will start. Leave engine fuel line bleeding to a specialist.
    Call us if your generator isn’t starting and see if we can help.
  • Blocked fuel filter – When was the last time that the fuel filter was replaced? This should be done at each service for prime power and at least every 12 months if a standby generator. If the fuel filter is blocked, it will cause a restriction of fuel supply to the fuel pump and this can prevent starting or taking load.

So you have checked the basics….fuel levels ok, fuel filter replaced, all fitting tight and you are confident there is no air in the line.

Electrical fuel control issues

There are some electrical issues that can also cause fuel problems. Here are some below:

Generator Fuel Solenoid

  • On a mechanically governed (speed self regulates) fuel pump the throttle is actuated by a solenoid. This solenoid is typically either 12vdc or 24vdc depending on what battery system you have on your generator. Check that this solenoid is receiving the correct DC power supply when the generator is cranking. If you do not have any voltage at your fuel solenoid, it will not open your throttle and allow starting of the engine.
  • If you don’t have a DC supply to your fuel pump solenoid, this signal is provided from the generator controller and driven through a fuel relay so you may have a fault with these components. You can check if you have a fuel output from your generator controller by checking for DC voltage on the fuel output terminal. On a deep sea electronics generator controller, the fuel output is usually on terminal 4 an this is common on other controllers also. If you think your controller could be faulty please give us a call and we can assist with this.
  • Your fuel pump and governor (controls engine speed) could also be supplied through a speed controller. This is typically used on larger applications above 150kVA or for smaller applications that need to run at isochronous speed and maintain a stable 50Hz supply. This is often the case when supplying sensitive equipment such as electronics. Synchronous generators that parallel with the mains or other gensets will feature external speed controllers. These can be more complicated to diagnose, as there are various components and settings involved with speed control. Contact us for help if you think you may have a generator speed controller fault.

If it is suspected that your generator has an electrical fault and will not start, please give us a call and we will be able to help you to resolve this.

At Solent Power we have a team of in-house generator engineers that can attend your generator and find and fix any faults you have.

My generator won’t try to start at all. Engine not cranking?

If you are trying to start your generator from the generator controller or keyswitch and it wont start you may have an issue with the starting system.

Generator battery failed

Generator battery testing
Generator battery testing

The most common issue with generators not starting is your battery. Your battery voltage should be at least 12vdc to attempt to start the generator. A generator battery voltage should be 12.6vdc when fully charged.

Most standby generators have a battery charger fitted or supplying their generator battery. A static battery charger will usually supply between 13.1 -13.5vdc to charge your battery, so you may see this voltage level on your generator controller, panel gauge or when measuring your battery terminals with a voltmeter.

Sometimes your battery can fail, but this is not always obvious because your generator controller will still be displaying the charger supply voltage which would appear to indicate a healthy battery supply. When this happens the generator controller will not be able to provide low battery alarm as the charger supply prevents the battery voltage from dropping below the threshold necessary to activate the generator controller alarm.

To properly test your generator battery, you need to isolate the battery charger first. You may find that when the charger is off that your controller drops off at the same time. This is a strong indicator that your battery is dead.

Sometimes a battery may seem ok, but there are other tests that need to be done such as CCA (cold cranking amps) test, voltage drop on crank tests and capacity checks. To perform these battery tests you need the correct equipment. Our engineers all have battery testers and will do this as part of our regular inspections and maintenance.

Generator starter motor fault

Sometimes starter motors fail but it is more common that the fault is in the starting circuit. To determine if your starter motor has failed you need to be comfortable making electrical checks.

A starter motor has a fixed 12vdc feed directly from your battery , or through a battery isolator (make sure you check this is on!)

When your generator tries to start, the generator controller will supply an output voltage to a crank/ starter relay and this feeds your starter motor solenoid.

If you are competent, you can initiate the start button and hopefully you will hear a ‘clicking’ sound which indicates that the start output is active. You can then check if you have a DC supply voltage to your starter solenoid. If this voltage is present then you may have a failed starter motor.

Once again…..testing your battery properly is paramount, because if your generator battery is no good, your generator may try to crank and supply the starter motor, but the battery voltage will quickly drop when the starter is connected and prevent the starter motor from operating.

I have lost count of how many times the non-starting is due to the battery. If you are not completely confident that your battery is good I would suggest replacing your generator battery with a new battery as a first move.

Generator controller fault or starting circuit fault

As mentioned above, your generator starter motor relies on a control supply which is provided from your generator controller and through a starter relay in most cases. If your generator controller is not providing a start signal you may have other issues preventing the generator from starting or the generator controller could be faulty.

Sometimes there are alarms, shutdowns and other faults that will prevent your generator from trying to start.

If you suspect that you may have a generator electrical problem please contact us and we can get this quickly resolved for you.

Generator circuit breaker tripping fault
Generator circuit breaker tripping fault

I have alarms on my generator controller or generator shut downs active

Sometimes there are shutdowns and alarms that will prevent your generator from starting. The following shutdowns will prevent your generator from starting:

  • Emergency stop button active –  There will be at least one, and maybe more emergency stop buttons on your generator. If any of these are engaged you will need to reset them and also possibly reset and clear the alarms on the controller before trying to start the generator.
  • Low fuel – Some generators have low fuel switches that will prevent the engine from running and starving the engine of fuel.
  • Fire valve (if fitted) – On standby generators located in plantrooms, they usually have fire stop valves that cut off your fuel supply in the event of an engine fire and this also may have a switch that prevents running.
  • Low coolant level – Your radiator tank or header tank may have a small sensor or float switch fitted that detects low coolant and prevents your generator from starting.
  • Bund leakage switch – Your base fuel tank is surrounded by a bunded area to capture any fuel leaks and prevent fuel from spilling into the environment. Some generators have float switches fitted in this bund to alert the user, and this may prevent starting. This switch is more common on mobile or rental generators.
  • Breaker tripped – If your breaker has any signal wiring for its open, closed or tripped position into the controller it may prevent the generator controller from starting the generator. Make sure you reset your breaker if it is tripped. You could also have an earth leakage relay fitted on the generator, which could be causing a similar fault condition and this may need to be reset.
  • Panel door switches – Most commonly fitted on rental generators, mobile generators or trailer mounted generators they can have a small switch on the cover that protect the cable connections. If this cover is not closed and the switch activated, the generator may not run or will shut down.

There are other conditions that may prevent your generator from starting, and sometimes only an experienced professional engineer will be able to diagnose your fault.

Generator starts and runs for a short while before shutting down

If your generator runs and then shuts down straight away or shuts down after a few seconds, you will likely have issues with protections and similar shutdowns as above.

When generators start, there is often a short period of time where the engine and electrical protections are not active to allow the engine to reach its nominal speed and the alternator to reach correct voltage and frequency.

If your generator starts up but shuts down if may be some of the below issues :

  • Oil pressure – Your generator will be fitted with a low oil pressure switch at a minimum. Check your generator oil level using the engine dipstick to ensure there is enough oil in the sump. If it is low then top this up and hopefully this will resolve the oil pressure issue. Do not run your generator with low oil, as this will prevent oil from being pumped around to key wearable parts and result in engine wear on critical components. If you have topped up the oil and still getting oil pressure shutdowns you may have a faulty switch or oil pressure sensor or wiring.
  • Incorrect voltage – Your generator controller will shut your engine down on under voltage, usually around 10% below the normal operating voltage. Check your voltage gauge or the display when your generator runs to check if it is reaching the correct voltage output. For a three phase generator this should be between 390v-420v but ideally at 400v which is the UK standard. For single phase you should be generating at 230vac but your generator may be producing between 220-245v depending on the engine speed and alternator AVR settings. If you suspect that your generator is not producing the correct voltage or not producing any voltage please contact us to help diagnose the issue.
  • Overspeed or underspeed – If your generator engine speed is incorrect or your generator is not reaching the correct speed after starting withing a set time period, the generator controller may shut the generator down. The first thing to check in this case would be your fuel supply (see earlier steps on this guide) as a restricted fuel supply or air in fuel will cause under or over speed or frequency faults.

If your generator is shutting down please feel free to give us a call and we can go through the symptoms with you and try to advise what the cause may be. Failing this we can provide an engineer to assist if necessary.

If you have tried looking through this guide and still cannot fix your generator fault or your generator is not starting or generator is shutting down please get in touch!

Solent Power has an experienced in house team with mobile engineers that can help with generator repairs, generator servicing and anything else that you may need.


Paul Phelps Solar Power Owner
Paul Phelps
Paul is the MD of Solent Power and specialises in emergency power solutions, helping to protect your facilities from power loss. Get in touch with us today to discuss the different power brands that we can offer and how they can suit your needs.
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