1986 CAT 518 Skidder Transmission Troubleshooting

[MikePhua]

Introduction
The 1986 Caterpillar 518 Skidder, equipped with a 3304 engine and a three-speed powershift transmission, is a robust machine designed for demanding forestry tasks. However, like any heavy equipment, it is susceptible to transmission issues that can impede its performance. Understanding common problems and their solutions is essential for maintaining the skidder's operational efficiency.
Common Transmission Issues

1. Loss of Power or Gear Engagement
   

Operators have reported instances where the skidder loses power or fails to engage gears, particularly the first gear, after operating for a period. This issue often arises when the machine is warm, and the problem may be linked to hydraulic fluid contamination or low hydraulic pressure. For example, a user noted that after operating the skidder for half an hour, the first gear stopped driving, and the transmission exhibited no forward or reverse motion. Upon inspection, the transmission oil appeared milky, indicating possible water contamination, which can cause clutch slippage and overheating. In such cases, draining and replacing the contaminated oil and filter is recommended. Additionally, inspecting clutch packs No. 2 and No. 5 for wear or damage is crucial. Allowing the transmission to dry and refilling it with manufacturer-specified fluid can help restore proper function.

1. Hydraulic Pressure Loss
   

Another prevalent issue involves a complete loss of hydraulic pressure, leading to the failure of the transmission, PTO, and winch. This problem may manifest suddenly, often during a coast down a hill. For instance, a user reported that during a downhill coast, the transmission stopped working, and there was no pressure from the torque converter inlet, outlet, or lubrication pressure taps. Despite testing pressures at various points and finding them normal, the absence of pressure at critical points suggests a malfunction in the valve spools. In such scenarios, it's advisable to inspect the valve bodies for any issues. Removing the valve bodies may be possible without disassembling the entire engine and transmission, but this depends on the specific configuration of the skidder.

1. Torque Converter Problems
   

Torque converter issues can also contribute to transmission failures. A common problem is the gear on the output shaft of the converter sliding out of place after the retaining bolt breaks. This gear drives the transmission on the bottom side and the winch shaft on the top side. Such bolt breakage usually occurs from the shock when the operator continuously throws the machine in the opposite direction without allowing it to come to a complete stop. To address this, it's recommended to check the charge pump pressure at the filter housing and regulator to ensure the pump is moving oil. If the charge pump is functioning correctly, but issues persist, inspecting the torque converter for gear misalignment or damage is necessary.
Maintenance Tips

• Regular Fluid Checks: Regularly monitor hydraulic fluid levels and quality. Replace fluids as per the manufacturer's recommendations to prevent contamination and ensure optimal performance.
  
• Clutch Pack Inspection: Periodically inspect clutch packs for wear or damage. Clutch packs No. 2 and No. 5 are particularly crucial for gear engagement.
  
• Torque Converter Maintenance: Ensure that the torque converter's retaining bolts are intact and that the gear is properly aligned. Address any issues promptly to prevent further damage.
  
• Temperature Monitoring: Install and monitor the transmission temperature gauge to detect overheating early. Overheating can lead to fluid degradation and component wear.
  

Conclusion
The 1986 CAT 518 Skidder is a durable and reliable machine when properly maintained. Understanding common transmission issues and their solutions can help operators keep the skidder in optimal working condition. Regular maintenance, timely repairs, and adherence to manufacturer guidelines are key to prolonging the life of the skidder and ensuring its efficient operation in the field.