Troubleshooting Throttle Issues on the John Deere 120: A Comprehensive Guide

[MikePhua]

The John Deere 120 is a compact excavator commonly used in construction, landscaping, and excavation projects. Its reliability and performance are key reasons why it has earned a reputation among heavy equipment operators. However, like all machinery, the John Deere 120 can experience occasional mechanical issues. One common problem faced by operators is throttle-related issues, which can affect the machine’s overall performance. This article provides a detailed overview of common throttle problems on the John Deere 120, their potential causes, and possible solutions.
Understanding the Throttle System in the John Deere 120
The throttle system in the John Deere 120 regulates the engine's speed by controlling the fuel intake. It can be manually adjusted through a throttle lever or electronically in newer models. The throttle controls the speed at which the engine operates, which is vital for performance during tasks like digging, lifting, and driving.
When throttle problems occur, the machine may exhibit symptoms such as erratic engine speeds, difficulty maintaining a consistent RPM, or failure to start. Diagnosing and fixing these issues is essential for ensuring the smooth operation of the machine.
Common Throttle Problems on the John Deere 120

1. Throttle Not Responding or Sticking
   

One of the most common issues with the throttle system on the John Deere 120 is when the throttle lever becomes stuck or unresponsive. This can result in the engine running at a low speed or not responding to adjustments made by the operator.
Symptoms:

• The throttle lever is hard to move or sticks in place.
  
• The engine does not increase in speed when the throttle is pushed forward.
  
• The machine operates at an unusually low or high RPM.
  

Possible Causes:

• Dirt or debris buildup around the throttle linkage or cable.
  
• A broken or frayed throttle cable.
  
• Corrosion in the throttle control assembly.
  
• The throttle lever mechanism may be worn out or damaged.
  

Solutions:

• Inspect the throttle linkage and cable for any signs of wear or obstruction. Clean any dirt or debris that may be impeding the movement.
  
• Lubricate the throttle cable and control mechanism to ensure smooth operation.
  
• If the throttle cable is frayed or broken, replace it with a new one.
  
• Inspect the throttle lever assembly for any damage and replace any worn parts.
  

1. Erratic Engine Speeds
   

Another issue that can occur is erratic engine speeds, where the engine revs up and down unpredictably. This can cause difficulty in controlling the machine’s performance, especially during delicate operations like grading or digging.
Symptoms:

• The engine RPM fluctuates without any input from the throttle lever.
  
• The machine may surge in speed or slow down unexpectedly.
  
• Inconsistent power delivery, especially when operating under load.
  

Possible Causes:

• Issues with the throttle position sensor (TPS), which helps control the engine's speed.
  
• Air or fuel delivery problems that interfere with the engine’s performance.
  
• Vacuum leaks in the intake manifold, which can affect engine speed.
  
• A malfunctioning governor system that regulates the engine’s RPM.
  

Solutions:

• Test the throttle position sensor using a multimeter to ensure it’s working correctly. Replace it if necessary.
  
• Inspect the air intake system for any blockages or leaks that may affect engine performance.
  
• Check the fuel delivery system, including the fuel filter and injectors, for any clogs or restrictions.
  
• Verify the condition of the governor system and replace any faulty components.
  

1. Throttle Surging During Idling
   

Surging during idling is another issue that can affect the John Deere 120, where the engine speed constantly increases and decreases while the machine is idling. This can be especially frustrating when operators are trying to keep the machine in a stable idle state.
Symptoms:

• The engine speed fluctuates up and down while idling.
  
• The machine feels unstable at low engine speeds.
  
• The idle speed is not consistent, even after adjusting the throttle.
  

Possible Causes:

• Dirty or clogged idle jets in the carburetor (for older models with carburetors).
  
• A dirty air filter restricting airflow to the engine.
  
• Faulty idle control solenoid or air control valve.
  
• Engine components like the idle circuit in the carburetor may be worn out or clogged.
  

Solutions:

• Clean or replace the air filter to ensure proper airflow to the engine.
  
• If the machine uses a carburetor, clean the idle jets and carburetor passages to remove any buildup.
  
• Check the idle control solenoid and air control valve for proper function, and replace them if necessary.
  
• Inspect the carburetor for any signs of wear or damage and rebuild it if needed.
  

1. Throttle Not Returning to Idle
   

In some cases, the throttle lever on the John Deere 120 may fail to return to the idle position, which results in the engine running at a high RPM even when the throttle lever is in the idle position.
Symptoms:

• The throttle lever does not move freely to the idle position.
  
• The engine continues running at a high speed, even when the throttle is in the idle position.
  
• The machine may become difficult to control due to the high RPM.
  

Possible Causes:

• The throttle return spring may be broken or worn out.
  
• A binding in the throttle linkage or cable.
  
• Dirt or debris in the throttle assembly.
  

Solutions:

• Inspect the throttle return spring for damage or wear and replace it if needed.
  
• Check the throttle linkage and cable for any binding or friction points. Lubricate or replace the parts as necessary.
  
• Clean the throttle assembly and ensure that no dirt or debris is affecting the movement of the throttle lever.
  

Preventative Maintenance for Throttle System Issues
To prevent throttle-related problems on the John Deere 120, regular maintenance is crucial. Below are some tips for keeping the throttle system in good working order:

1. Regularly clean the throttle linkage: Ensure that the throttle linkage and cable are free from dirt and debris. Clean and lubricate these components to keep them functioning smoothly.
   
2. Replace the air filter: A clogged air filter can cause a variety of engine performance issues, including erratic throttle behavior. Replace the air filter regularly.
   
3. Check fuel and air delivery systems: Periodically inspect the fuel system, including the fuel filter, injectors, and fuel lines, to ensure proper fuel flow. Also, inspect the intake system for any air leaks or blockages.
   
4. Test the throttle position sensor: Ensure that the throttle position sensor is working correctly. If the sensor is malfunctioning, it can cause incorrect engine speeds.
   
5. Inspect throttle return spring: Regularly check the throttle return spring for any signs of wear or damage and replace it when needed.
   

Conclusion
Throttle problems on the John Deere 120 can significantly affect the machine’s performance and productivity. By understanding the common throttle-related issues and knowing how to troubleshoot them, operators can keep their machines running efficiently and minimize downtime. Regular maintenance, including cleaning the throttle components, checking fuel and air systems, and inspecting critical parts like the throttle position sensor, can prevent many common issues from arising. Keeping the throttle system in optimal condition will ensure the John Deere 120 continues to perform well on the job site.