• Date:
  • Category: Blog

A Komatsu D20 QF-5 dozer experienced a common but frustrating issue: it was accidentally run out of diesel fuel. After refueling, the engine would crank and briefly start, but then stall. Shortly thereafter, the machine stopped cranking altogether and only produced a clicking sound when the key was turned. This sequence of symptoms—initial fuel starvation followed by electrical failure—suggests a layered problem involving both the fuel delivery system and the starter motor circuit.

Understanding the Fuel System Behavior

When a diesel engine runs out of fuel, air enters the fuel lines and injection pump. Unlike gasoline engines, diesel systems rely on high-pressure fuel delivery and cannot self-prime. After refueling, the system must be bled to remove air pockets. If this step is skipped or incomplete, the engine may start briefly on residual fuel, then stall as air reaches the injectors.

Typical steps to bleed a Komatsu D20 fuel system include:

  • Loosening the bleed screw on the fuel filter housing
  • Operating the manual priming pump until fuel flows without bubbles
  • Cracking injector lines at the injectors and cranking the engine until fuel spurts from each line
  • Retightening lines and attempting a full start

However, in this case, the engine stopped cranking before full bleeding could be completed.

Starter Circuit Diagnosis: Clicking Without Cranking

The transition from cranking to only clicking indicates a failure in the starter motor circuit. The most likely causes include:

  • Burned or pitted solenoid contact plate
  • Worn-out starter brushes or armature
  • Weak or corroded battery cables
  • Faulty ignition switch or starter relay

The clicking sound typically comes from the starter solenoid engaging but failing to deliver current to the starter motor. This is often due to high-resistance contacts inside the solenoid.

Solenoid Contact Plate Failure

The solenoid contact plate is a copper or brass disc that bridges the high-current terminals inside the solenoid when the key is turned. Over time, arcing and heat can erode the contact surface, preventing full engagement. This is especially common in older machines or those exposed to moisture and vibration.

To confirm this failure:

  • Remove the starter from the machine
  • Disassemble the solenoid housing
  • Inspect the contact plate and terminals for burn marks or pitting
  • Replace the contact plate or the entire solenoid if damage is found

Starter Removal Challenges

One complication in this case was that the loader arms were in the down position, obstructing access to the starter. This is a common issue in tracked loaders and dozers. Solutions include:

  • Using another machine to lift the loader arms
  • Employing a wrecker or crane to raise the front end
  • Accessing the starter from underneath, if clearance allows

A similar situation occurred with an Allis-Chalmers loader, where the operator had to use a second tractor to lift the bucket before removing the starter.

Preventive Measures and Lessons Learned

To avoid similar issues in the future:

  • Always monitor fuel levels closely—diesel engines are unforgiving when run dry
  • Bleed the fuel system thoroughly after refueling from empty
  • Inspect and clean battery terminals regularly
  • Replace starter solenoids or contact plates as part of preventive maintenance
  • Keep a second machine or lifting device available when working alone in remote areas

Final Thoughts

The Komatsu D20’s failure to crank after a fuel-out incident illustrates how mechanical and electrical systems can compound each other’s faults. What began as a simple fuel starvation event escalated into a starter failure due to solenoid wear. With methodical diagnosis and a bit of mechanical agility, the issue can be resolved—and the dozer returned to productive service. In the field, understanding both fuel and electrical systems is essential for keeping aging equipment running strong.

Tag: none

Add New Comment论