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Why a Diesel Generator Cranks But Won’t Start?

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Investing in a diesel generator gives you access to reliable prime and standby power, ensuring your home or business remains productive. In most cases, diesel generators offer reliability and longevity, seamlessly integrating into your operations—but what happens when they fail to perform?

One common yet frustrating problem is when a generator cranks vigorously but refuses to start. This guide explores the underlying causes of this issue, from fuel system faults to electrical discrepancies, and offers insight into the troubleshooting steps that can help restore your generator’s functionality.

Let’s get to it.

The most common reasons for a generator cranking but failing to start

Many people invest in a diesel generator for its reliability. Most come with functions that allow them to switch on when you lose access to a mains supply, providing power for appliances and equipment to keep your business operational.

Both prime and standby generators can experience problems that impact their functioning, but there’s usually an easy fix. Here are the most common reasons generators might crank but won’t start.

Generator Needs Service

Battery issues

As with most battery-powered devices, even a slight problem can impact your generator’s ability to function. Diesel generator batteries activate the starter motor, which then powers the engine, but if the battery is damaged, the generator won’t start.

The most common causes of battery failure include:

  • Battery Discharge: Also known as battery flattening, discharge occurs over time. As the battery ages, the generator can no longer start.
  • Corrosion: Battery corrosion is a common problem that usually happens when the battery’s terminals accumulate a substance caused by chemical reactions. Corrosion reduces electricity flow from the battery, so it won’t be able to provide the generator with enough power.
  • Broken Connections: Batteries can suffer damage over time, resulting in loose connections. If the battery’s connections cannot supply enough power to the motor, the generator won’t start.

The fix

Multimeters are beneficial for checking whether your battery is functioning correctly but they only indicate its open circuit voltage and charging levels. To confirm the health and performance of a starting battery you need a battery tester or analyser that performs a range of tests such as:

  • Capacity – how much charge is stored as a percentage of total capacity
  • internal impedance – Older batteries will have higher internal resistances
  • voltage drop on crank – see example of this here –
  • CCA test – can it deliver the correct cold cranking amps?

You should also examine the battery for signs of corrosion and broken connections also.

In most cases, the generator will function properly again once you replace the battery.

Generator Battery Failed

Fuel system faults

Diesel generators rely on their fuel systems, so any faults will inevitably inhibit their function. Here’s what could be causing fuel problems with your generator:

  • Empty Fuel Tank: OK, this is an easy one to fix. While it might seem like an obvious and avoidable error, many people find refilling the tank kicks their generator into action.
  • Fuel Filter Clogs: Fuel filters have important jobs because they prevent debris from entering the tank. When the filters suffer air bubbles or fail to block debris, clogs can restrict fuel flow to the engine.
  • Fuel Tank leaks: When a non bunded fuel tank causes leaks or overflow, it can stop the generator from functioning while also causing environmental threats.
  • Fuel Line and Pipe Issues: Fuel lines and pipes can also suffer blockages or wear and tear due to overuse, which means they can’t supply the engine with fuel.
  • Fuel Pump Damage: Corrosion, cracks and general wear are typical with fuel pumps, but they can also become defective if they suffer damage.
  • Fuel hoses: Older fuel hoses perish and crack and can leak fuel or allow air into your fuel system and prevent starting or taking load

The fix

Damaged fuel pumps and pipes often need replacing, but clogs and blockages are easier to fix. Cleaning the generator’s fuel system and removing debris can ensure optimal fuel flow. Regular maintenance is key to identifying and preventing issues.

Reduced air intake

Diesel engines need optimal air intake to ensure their smooth operation, and air filter blockages can lead to debris accumulation. In most cases, clogged air intake occurs due to a lack of maintenance or poor installation.

The fix

Perform regular checks on your diesel generator, including the air filters. If the air filter elements show tears or damage, it’s best to replace them.

Starter motor malfunctions

The starter motor plays a central role in ensuring the engine turns over, so any minor faults or general wear and tear will impact its ability to function. While starter motor problems are rare, understanding and fixing their root cause can be challenging.

The fix:

  • Perform a visual inspection of the motor and check for any damage, including loose connections or debris.
  • Try to turn the generator on and listen for a clicking noise. If you hear one, the motor is trying to engage but failing to start the generator. Watching the generator and seeing if the motor spins is also a good idea.
  • Assess electrical connections and test the battery’s voltage. If the battery can’t supply adequate power to the motor, it won’t be able to start the generator.
  • Contact a professional generator servicing and maintenance specialist to examine the motor. They’ll be able to fix it if needed.
Cummins Standby Generator

Less common generator problems

If you perform all the above checks and the generator still won’t start, it could indicate a less common issue. These problems are more challenging, but seeking professional support can help you identify and fix them.

Faulty solenoids

The solenoid is a vital generator component, engaging the starter motor. If you notice the diesel generator cranking but doesn’t start, cleaning the solenoid might fix the issue and ensure it can engage the injector pump. However, you’ll probably need a replacement if you notice any damage.

Leaving the generator for long periods

A standby generator might go unused for long periods, which can impact the fuel tank and engine. Red diesel and engine oil can become contaminated over time, causing them to thicken, which means the generator might crank, but it won’t be able to start.

Seeking support from fuel contamination services gives your generator the best chance of recovery. Specialists will remove debris and polish the tank.

Glow plugs

Glow plugs warm up the combustion chamber, ensuring the engine can start. However, cold weather can impact their ability to function, leading to the engine failing to start. A multimeter can help you determine whether the glow plugs are performing. If not, replacing them should fix the issue.

Regular maintenance is critical to your diesel generator’s lifespan

Diesel generators can last for decades, providing your home or business with a reliable power source. Regularly assessing and maintaining the generator can highlight potential problems with your diesel engine, fuel system and other components.

Investing in a diesel generator from a reputable manufacturer gives you long-term peace of mind. Solent Power is an authorised distributor of diesel generators, and our sales team is always available to offer advice.

We also have an installation service and regular maintenance packages that include protection and repairs. Please get in touch today and make power issues a thing of the past.

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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