Troubleshooting the S-720 3-Speed Powershift in the IH 520B Loader

[MikePhua]

The IH 520B Loader, also known as the International Harvester 520B, is a classic piece of heavy equipment designed for a variety of material handling and excavation tasks. Known for its rugged durability, the 520B has been a reliable workhorse in many industries, including construction, mining, and agriculture. One of the standout features of the 520B is its S-720 3-speed powershift transmission, which provides excellent control and power transfer for efficient operation. However, like all mechanical systems, the powershift transmission can develop problems over time.
In this article, we will dive into the common issues faced by the S-720 3-speed powershift in the IH 520B loader, how to diagnose these problems, and what solutions can help keep your machine running smoothly.
Understanding the S-720 3-Speed Powershift Transmission
Before diving into potential issues, it’s important to understand how the S-720 3-speed powershift transmission works. A powershift transmission is designed to allow the operator to change gears without the need to manually disengage the clutch, making it a more efficient and smoother way of shifting compared to manual transmissions.
The S-720 system is a 3-speed transmission, meaning it allows three forward speeds and a reverse gear. Powershift transmissions are often used in heavy equipment like loaders and backhoes because they provide smoother shifts, better torque management, and allow the machine to maintain high power levels even under load.
While the S-720 transmission is generally robust, problems can arise as the machine ages or as maintenance is neglected. Below are some of the most common issues and their potential causes.
Common Problems with the S-720 3-Speed Powershift
1. Slipping Gears or Difficulty Shifting
One of the most common problems with the S-720 powershift transmission in the IH 520B is slipping gears. When a transmission slips, the vehicle loses the ability to maintain a consistent speed, making operation erratic and inefficient.

• Symptoms: Loss of power while driving, noticeable engine revving without the corresponding increase in speed, or the inability to maintain gear.
  
• Possible Causes:
  • Low or contaminated transmission fluid: If the fluid levels are too low or the fluid is dirty, it can prevent the clutch packs from fully engaging, causing slipping.
    
  • Worn clutch packs: Over time, the clutch packs within the transmission can wear out, leading to inadequate power transfer and gear slippage.
    
  • Faulty solenoids or shift valves: These components control the engagement of gears and, if malfunctioning, can cause improper shifting or gear slippage.
    
• Solution: Begin by checking the transmission fluid for both level and condition. If the fluid is low or contaminated, replace it with the recommended type and quantity of fluid. If the issue persists, the clutch packs may need to be inspected and replaced. For more complicated issues involving solenoids or shift valves, consult a professional mechanic.
  

2. Transmission Overheating
Transmission overheating is another issue that can affect the S-720 3-speed powershift. Overheating can cause a variety of issues, including slippage, erratic shifting, and even complete transmission failure if not addressed.

• Symptoms: Transmission fluid temperature gauge reading higher than normal, difficulty shifting, and the machine losing power.
  
• Possible Causes:
  • Insufficient cooling: The transmission may not be properly cooling due to a blocked cooler, damaged cooling lines, or a malfunctioning transmission cooler.
    
  • Dirty fluid: Contaminated transmission fluid can cause the transmission to overheat by increasing friction and reducing the fluid’s cooling effectiveness.
    
• Solution: Inspect the cooling system to ensure the transmission cooler is not clogged and that all lines are intact. Check the fluid condition and replace it if necessary. Also, ensure the machine is not overworking and that the oil temperature is within the recommended range.
  

3. Poor Acceleration or Hesitation
Another issue that can arise with the S-720 transmission is poor acceleration or hesitation when shifting between gears.

• Symptoms: The machine hesitates when moving from a stop, or the acceleration feels sluggish, especially when switching between gears.
  
• Possible Causes:
  • Low fluid pressure: The transmission relies on fluid pressure to engage the gears. If there is a drop in fluid pressure, the gears may not engage correctly.
    
  • **Faulty shift valve: A malfunctioning shift valve can cause hesitation in gear changes, leading to poor acceleration.
    
• Solution: Check the fluid pressure and inspect the shift valve. If the fluid pressure is low, check for leaks in the transmission or pump, and replace any faulty components. If the shift valve is malfunctioning, it will likely need to be replaced.
  

4. Grinding or Unusual Noises
Unusual noises, such as grinding, rattling, or clunking sounds, are often an indication that there is a mechanical issue within the transmission.

• Symptoms: You may hear grinding noises when shifting, or the machine may emit a loud clunk when changing gears.
  
• Possible Causes:
  • Worn gears or bearings: Over time, the internal components of the transmission, including the gears or bearings, can wear out, causing friction and grinding noises.
    
  • Low fluid levels: If the fluid is too low, it can cause the internal components to wear prematurely and create loud grinding sounds.
    
• Solution: Inspect the transmission for any damaged gears or bearings. If you hear grinding sounds, it's crucial to address the issue quickly to avoid further damage to the transmission. Check the fluid levels and ensure the transmission is properly lubricated.
  

Maintenance Tips for the S-720 3-Speed Powershift
Preventative maintenance is essential to keep the S-720 3-speed powershift in good working order. Here are a few tips to extend the life of your transmission:

1. Regularly check fluid levels: Ensure that the transmission fluid is at the correct level and is free from contaminants. Change the fluid according to the manufacturer’s recommended intervals to prevent clogging and overheating.
   
2. Inspect the cooling system: Make sure that the transmission cooler is working effectively to prevent overheating. Clean or replace any clogged filters and inspect the cooler lines for leaks.
   
3. Check for leaks: Inspect the transmission and cooling lines for leaks, as fluid loss can result in performance issues. Replace any damaged seals or hoses.
   
4. Avoid overloading: Operating the IH 520B loader beyond its rated capacity can put unnecessary stress on the transmission, leading to premature wear.
   
5. Use the correct transmission fluid: Always use the fluid recommended by the manufacturer to ensure the transmission operates efficiently.
   

Conclusion
The S-720 3-speed powershift transmission in the IH 520B loader is an integral part of the machine’s powertrain, providing smooth gear shifts and efficient power delivery. While it’s a reliable system, it can experience issues like slipping gears, overheating, or poor acceleration over time. Regular maintenance, including fluid checks, cooling system inspections, and proper fluid replacement, is essential to keep the system running smoothly.
By staying on top of these common issues and following the provided solutions, you can ensure that your IH 520B continues to perform at its best, reducing downtime and maintenance costs.