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The International Harvester TD‑25B is a heavyweight crawler dozer built during an era when brute mechanical strength defined earthmoving equipment. Machines from the early 1970s, such as the 1972 TD‑25B referenced in the discussion, remain in operation today thanks to their robust engines, simple hydraulics, and durable undercarriages. However, age and wear can expose weaknesses in the powertrain—especially in the torque converter and transmission system. One unusual issue reported by operators is rapid torque converter overheating when back‑blading, even though the machine runs cool during forward pushing or normal travel.
Understanding why this happens requires examining the TD‑25B’s drivetrain design, the behavior of its range clutches, and the hydraulic flow characteristics of its torque converter.
Development Background of the TD‑25 Series
International Harvester introduced the TD‑25 series in the 1960s to compete with Caterpillar’s D8 and D9 class machines. The TD‑25B represented the second major iteration, featuring:
Terminology notes:
The reported machine behaves normally under most conditions:
Possible Causes of Converter Overheating
Several experienced mechanics have identified likely causes for this behavior.
Reverse Clutch Pack Slippage
A slipping reverse clutch pack is one of the most probable explanations. When back‑blading, the machine is under a heavier reverse load than when simply reversing without material. If the reverse clutch pack is worn, misadjusted, or suffering from low hydraulic pressure, it may slip under load, generating excessive heat.
Indicators of clutch pack issues include:
The TD‑25B uses a filter screen in the torque converter housing. If this screen becomes clogged with debris, metal particles, or degraded oil residue, converter oil flow may be restricted. Reduced flow increases heat buildup, especially during high‑load operations.
Technicians often recommend:
Low or inconsistent hydraulic pressure feeding the transmission clutches can cause slippage. A pressure test comparing forward and reverse stall speeds can reveal whether the reverse circuit is weak.
A proper stall test includes:
Gear Selection During Back‑Blading
The TD‑25B has four reverse speeds. Back‑blading in a high gear can overload the converter, especially if the machine is dragging a long windrow of material. Operators typically use low reverse gear for controlled back‑blading.
If the operator unknowingly uses a higher gear, converter heat can spike quickly.
Historical Notes and Field Stories
Older mechanics recall that some early TD‑25B units used diesel‑filled torque converters, a design that was eventually phased out. These converters were known to behave unpredictably under light reverse loads. Although rare today, surviving machines with this configuration may exhibit unusual heating patterns.
One veteran operator remembered a TD‑25B that overheated only in reverse during light grading. Despite extensive troubleshooting, the issue was never fully resolved, suggesting that some early converter designs had inherent quirks.
Another mechanic shared memories of working on TD‑25C machines, describing the difficulty of accessing hoses and bolts around the converter housing—often requiring awkward body positions and scraped knuckles. These stories highlight the physical challenges of maintaining older dozers.
International Harvester Company Background
International Harvester was a major force in heavy equipment manufacturing throughout the mid‑20th century. Known for its agricultural machinery, trucks, and construction equipment, IH built a reputation for rugged, rebuildable machines. The TD‑25 series was one of its flagship crawler tractors, competing directly with Caterpillar’s large dozers.
Although IH eventually exited the construction equipment market, the TD‑25 legacy continued under Dresser and later Komatsu‑Dresser. Many parts remain available through aftermarket suppliers, keeping these machines operational decades after production ended.
Practical Recommendations
Torque converter overheating on a TD‑25B during back‑blading is an unusual but diagnosable issue. The most likely causes include reverse clutch pack slippage, restricted converter oil flow, or improper gear selection. With systematic testing and inspection, operators can identify the root cause and restore the machine’s performance. Despite their age, TD‑25B dozers remain powerful and capable machines when properly maintained.
Understanding why this happens requires examining the TD‑25B’s drivetrain design, the behavior of its range clutches, and the hydraulic flow characteristics of its torque converter.
Development Background of the TD‑25 Series
International Harvester introduced the TD‑25 series in the 1960s to compete with Caterpillar’s D8 and D9 class machines. The TD‑25B represented the second major iteration, featuring:
- A powerful IH DT‑817 diesel engine
- A three‑element torque converter
- Four forward and four reverse speeds
- A modular transmission and steering clutch system
- A heavy, high‑capacity cooling package
Terminology notes:
- Torque converter: A fluid coupling that multiplies torque and transfers power from the engine to the transmission.
- Range clutch pack: A hydraulic clutch assembly that engages a specific gear range.
- Back‑blading: Pulling the blade backward to smooth or level material.
- Stall test: A diagnostic procedure measuring engine RPM and converter load under full resistance.
The reported machine behaves normally under most conditions:
- No overheating during forward pushing
- No overheating during reverse travel
- Rapid overheating only when back‑blading
- Temperature drops quickly when the operator stops and idles the engine
Possible Causes of Converter Overheating
Several experienced mechanics have identified likely causes for this behavior.
Reverse Clutch Pack Slippage
A slipping reverse clutch pack is one of the most probable explanations. When back‑blading, the machine is under a heavier reverse load than when simply reversing without material. If the reverse clutch pack is worn, misadjusted, or suffering from low hydraulic pressure, it may slip under load, generating excessive heat.
Indicators of clutch pack issues include:
- Overheating only in reverse under load
- Normal temperatures during forward pushing
- Rapid temperature rise during back‑blading
The TD‑25B uses a filter screen in the torque converter housing. If this screen becomes clogged with debris, metal particles, or degraded oil residue, converter oil flow may be restricted. Reduced flow increases heat buildup, especially during high‑load operations.
Technicians often recommend:
- Removing floor plates
- Inspecting and cleaning the converter filter screen
- Checking for metal contamination in the oil
Low or inconsistent hydraulic pressure feeding the transmission clutches can cause slippage. A pressure test comparing forward and reverse stall speeds can reveal whether the reverse circuit is weak.
A proper stall test includes:
- Measuring stall RPM in forward
- Measuring stall RPM in reverse
- Comparing temperature rise between the two tests
Gear Selection During Back‑Blading
The TD‑25B has four reverse speeds. Back‑blading in a high gear can overload the converter, especially if the machine is dragging a long windrow of material. Operators typically use low reverse gear for controlled back‑blading.
If the operator unknowingly uses a higher gear, converter heat can spike quickly.
Historical Notes and Field Stories
Older mechanics recall that some early TD‑25B units used diesel‑filled torque converters, a design that was eventually phased out. These converters were known to behave unpredictably under light reverse loads. Although rare today, surviving machines with this configuration may exhibit unusual heating patterns.
One veteran operator remembered a TD‑25B that overheated only in reverse during light grading. Despite extensive troubleshooting, the issue was never fully resolved, suggesting that some early converter designs had inherent quirks.
Another mechanic shared memories of working on TD‑25C machines, describing the difficulty of accessing hoses and bolts around the converter housing—often requiring awkward body positions and scraped knuckles. These stories highlight the physical challenges of maintaining older dozers.
International Harvester Company Background
International Harvester was a major force in heavy equipment manufacturing throughout the mid‑20th century. Known for its agricultural machinery, trucks, and construction equipment, IH built a reputation for rugged, rebuildable machines. The TD‑25 series was one of its flagship crawler tractors, competing directly with Caterpillar’s large dozers.
Although IH eventually exited the construction equipment market, the TD‑25 legacy continued under Dresser and later Komatsu‑Dresser. Many parts remain available through aftermarket suppliers, keeping these machines operational decades after production ended.
Practical Recommendations
- Perform a stall test comparing forward and reverse performance.
- Inspect and clean the torque converter filter screen.
- Check hydraulic pressures for the reverse clutch circuit.
- Verify that the operator is using low reverse gear for back‑blading.
- Inspect clutch pack components for wear or glazing.
- Evaluate converter oil condition and replace if contaminated.
- Monitor temperature rise during different operating modes to isolate the cause.
Torque converter overheating on a TD‑25B during back‑blading is an unusual but diagnosable issue. The most likely causes include reverse clutch pack slippage, restricted converter oil flow, or improper gear selection. With systematic testing and inspection, operators can identify the root cause and restore the machine’s performance. Despite their age, TD‑25B dozers remain powerful and capable machines when properly maintained.
