Trouble Starting the John Deere 120G: Diagnosis and Solutions

[MikePhua]

Introduction
The John Deere 120G is a powerful and reliable piece of equipment in the world of heavy machinery, used primarily for grading and construction. However, like all mechanical systems, it can experience issues that hinder its performance. One such issue that many operators face is trouble starting the machine. Whether you're working on a construction site, a landscaping project, or any other heavy-duty task, dealing with a machine that won’t start can cause significant delays and frustration. In this article, we explore the common causes of starting issues in the John Deere 120G and provide practical solutions for diagnosing and resolving them.
Common Causes of Starting Issues in the John Deere 120G

1. Weak or Dead Battery
   

A common reason for a machine not starting is a weak or dead battery. The battery provides the necessary power to start the engine, and if it is not providing sufficient voltage, the engine won’t turn over.
Symptoms of a Weak or Dead Battery:

• The engine doesn’t turn over when attempting to start.
  
• Electrical components, such as lights or the horn, may be weak or non-functional.
  
• A clicking sound occurs when attempting to start the engine.
  

Solution:

• Test the Battery: Start by checking the battery’s voltage using a multimeter. A healthy battery should read around 12.6 volts when fully charged. If the reading is much lower, the battery may need to be replaced.
  
• Inspect Battery Terminals: Corroded or loose battery terminals can prevent the flow of power. Clean the terminals with a wire brush and tighten the connections.
  
• Jump-Start the Machine: If the battery is the issue, a jump-start can help temporarily. However, if the problem persists, replacing the battery will be necessary.
  

1. Faulty Starter Motor
   

The starter motor is responsible for turning the engine over when the ignition is engaged. If the starter motor is malfunctioning, the engine may not start at all.
Symptoms of a Faulty Starter Motor:

• No sound when the ignition is turned on.
  
• A single loud click or repeated clicking noises, but the engine does not turn over.
  

Solution:

• Inspect the Starter Motor: First, check if the starter motor is receiving power. If it is, but the engine still doesn’t turn over, the starter motor may be faulty and require replacement.
  
• Test the Solenoid: The starter solenoid controls the flow of electricity to the starter motor. If it fails, the motor won’t function. A multimeter can be used to test the solenoid, and if it's defective, it should be replaced.
  

1. Fuel Supply Issues
   

Fuel issues are another common cause of starting problems in heavy machinery. If the engine is not receiving enough fuel or if there is air in the fuel lines, the engine may fail to start.
Symptoms of Fuel Supply Problems:

• The engine cranks but doesn’t start.
  
• You hear the fuel pump running, but fuel may not be reaching the engine.
  
• The engine starts briefly but stalls after a few seconds.
  

Solution:

• Check Fuel Levels: Ensure that the fuel tank is full. Sometimes, the issue could be as simple as running out of fuel.
  
• Inspect Fuel Lines and Filters: Clogged or damaged fuel lines can prevent fuel from reaching the engine. Inspect the fuel lines for any signs of wear, cracks, or leaks. A clogged fuel filter can also restrict fuel flow, so it should be replaced if necessary.
  
• Prime the Fuel System: If air has entered the fuel system, you may need to prime it. This can usually be done by engaging the fuel priming pump or following the manufacturer’s procedure for bleeding the system.
  

1. Electrical System Malfunctions
   

The electrical system of the John Deere 120G is complex, and a variety of issues can affect its ability to start. From ignition switches to relays and fuses, any part of the electrical system could be malfunctioning.
Symptoms of Electrical System Malfunctions:

• No power to the ignition system.
  
• Intermittent starting, where the machine sometimes starts and other times doesn’t.
  
• Warning lights on the dashboard or error codes on the display.
  

Solution:

• Check Fuses and Relays: Fuses and relays protect the electrical circuits of the machine. If one blows, it can cause the machine to fail to start. Check the fuses and replace any that are blown.
  
• Test the Ignition Switch: If the ignition switch is faulty, it may not be sending the correct signal to start the engine. A simple test with a multimeter can reveal whether the switch is working correctly.
  
• Inspect Wiring and Connectors: Check for any loose, damaged, or corroded wires and connectors in the electrical system. Loose connections can prevent proper power flow to critical components.
  

1. Glow Plugs (for Diesel Engines)
   

If your John Deere 120G is equipped with a diesel engine, faulty glow plugs could be preventing the engine from starting, especially in colder weather. Glow plugs heat the combustion chamber to ensure the fuel ignites correctly during startup.
Symptoms of Faulty Glow Plugs:

• Difficulty starting, particularly in cold weather.
  
• Engine cranks but fails to start, or starts only after repeated attempts.
  

Solution:

• Test the Glow Plugs: Glow plugs can be tested using a multimeter. If any of the glow plugs are faulty, replace them with new ones.
  
• Check the Relay and Fuse: The glow plug system is controlled by a relay and fuse, both of which should be checked if the glow plugs themselves appear to be in working order.
  

1. Safety Features or Sensors
   

Modern heavy machinery like the John Deere 120G comes equipped with various safety features and sensors that prevent the engine from starting under certain conditions. These can include features like seat sensors, neutral safety switches, and pressure sensors. If these sensors malfunction, they may prevent the machine from starting.
Symptoms of Sensor Issues:

• The machine does not start, but there are no apparent issues with the fuel, battery, or electrical system.
  
• The machine may not start even though it’s in the proper gear or seat.
  

Solution:

• Check Safety Switches: Ensure the machine is in the correct gear (typically neutral) and that the operator is seated properly. Some models will not start unless the operator is seated.
  
• Inspect the Sensors: If the machine still won’t start, inspect the sensors associated with the safety features. These sensors may need to be reset or replaced.
  

Conclusion
Trouble starting a John Deere 120G can be a frustrating issue, but with a methodical approach to diagnosis, it can usually be resolved without too much difficulty. The most common causes of starting issues include weak or dead batteries, faulty starter motors, fuel supply problems, electrical system malfunctions, and glow plug failure. By checking each of these systems and performing routine maintenance, operators can keep their equipment in good working condition and avoid costly downtime. Regular inspections and following the manufacturer’s maintenance guidelines are key to preventing these issues from arising in the first place.