11 hours ago
A persistent issue with the International 6+ transmission failing to hold 5th gear under load is most often caused by worn synchronizer teeth, degraded slider engagement, or weakened detent springs. This condition typically worsens during uphill pulls and high torque demand, while remaining stable during downhill coasting.
Transmission Background and Model History
The International 6+ transmission, commonly found in 1990s International 4700 trucks equipped with the T444E diesel engine, is a 6-speed manual gearbox with an additional low gear for heavy hauling. Produced during Navistar’s peak years in vocational truck manufacturing, this transmission was designed for durability and simplicity. However, like many medium-duty gearboxes, it relies on mechanical synchronizers and spring-loaded detents to maintain gear engagement.
International Harvester, later rebranded as Navistar International, was a major player in North American truck production. The 4700 series was widely used in delivery, utility, and municipal fleets, with tens of thousands sold between 1990 and 2001. The T444E engine, a derivative of the Ford Power Stroke, paired well with the 6+ transmission for moderate-duty applications.
Terminology Note
Operators report that 5th gear engages briefly under load but pops out within seconds. Re-engagement requires shifting fully into neutral before retrying. The gear holds during downhill coasting, suggesting that torque direction affects retention. Grinding occurs if the shift is attempted without full neutral reset.
This behavior points to a loss of mechanical retention rather than hydraulic or electronic fault. The gear teeth may still be intact, but the back-cut on the slider or the detent mechanism is no longer sufficient to hold the gear under torque.
Root Causes and Mechanical Analysis
Repair Recommendations
The International 6+ transmission’s failure to hold 5th gear is a classic symptom of mechanical wear in the synchronizer and slider assembly. While the truck may still operate in other gears, ignoring the issue risks further damage and unsafe operation. A targeted rebuild with attention to detents, forks, and gear teeth can restore full functionality and extend the life of this workhorse drivetrain.
Transmission Background and Model History
The International 6+ transmission, commonly found in 1990s International 4700 trucks equipped with the T444E diesel engine, is a 6-speed manual gearbox with an additional low gear for heavy hauling. Produced during Navistar’s peak years in vocational truck manufacturing, this transmission was designed for durability and simplicity. However, like many medium-duty gearboxes, it relies on mechanical synchronizers and spring-loaded detents to maintain gear engagement.
International Harvester, later rebranded as Navistar International, was a major player in North American truck production. The 4700 series was widely used in delivery, utility, and municipal fleets, with tens of thousands sold between 1990 and 2001. The T444E engine, a derivative of the Ford Power Stroke, paired well with the 6+ transmission for moderate-duty applications.
Terminology Note
- Synchronizer (Synchro): A friction-based mechanism that matches gear speeds before engagement.
- Slider: A splined collar that moves to engage the gear teeth.
- Back-Cut: A tapered tooth profile that helps lock the slider in place under load.
- Detent Spring: A spring-loaded ball or plunger that holds the shift rail in gear position.
- Grinding: Audible gear clash caused by mismatched speeds or incomplete engagement.
Operators report that 5th gear engages briefly under load but pops out within seconds. Re-engagement requires shifting fully into neutral before retrying. The gear holds during downhill coasting, suggesting that torque direction affects retention. Grinding occurs if the shift is attempted without full neutral reset.
This behavior points to a loss of mechanical retention rather than hydraulic or electronic fault. The gear teeth may still be intact, but the back-cut on the slider or the detent mechanism is no longer sufficient to hold the gear under torque.
Root Causes and Mechanical Analysis
- Worn back-cut teeth on the slider allow the gear to slip under load. This is common in high-mileage transmissions or those used for frequent towing.
- Degraded synchronizer rings fail to match gear speeds, causing grinding and incomplete engagement.
- Weak or broken detent springs allow the shift rail to drift under vibration or torque reversal.
- Bent shift forks or worn bushings can prevent full gear engagement, especially in 5th and reverse.
Repair Recommendations
- Transmission removal and teardown is required to inspect the 5th gear assembly.
- Replace the slider and synchro ring as a matched set. Use OEM or high-quality aftermarket parts.
- Inspect the shift rail and detent springs for wear or breakage.
- Check the shift fork alignment and replace bushings if excessive play is found.
- Flush and refill transmission fluid with manufacturer-recommended gear oil after rebuild.
- Avoid resting your hand on the shifter during operation—this can wear detents prematurely.
- Shift fully into neutral between gears to allow synchros to reset.
- Replace transmission fluid every 30,000 miles or annually.
- Monitor gear engagement feel—early signs of wear include vague shift resistance or audible click loss.
The International 6+ transmission’s failure to hold 5th gear is a classic symptom of mechanical wear in the synchronizer and slider assembly. While the truck may still operate in other gears, ignoring the issue risks further damage and unsafe operation. A targeted rebuild with attention to detents, forks, and gear teeth can restore full functionality and extend the life of this workhorse drivetrain.
