2019 Kenworth T680 Transmission Fluid & Service Guide

The 2019 Kenworth T680 is a staple in the modern linehaul and regional trucking industry, heavily praised for its aerodynamic design and fuel efficiency. A critical component of that efficiency is the drivetrain. For 2019, the majority of these trucks rolled off the assembly line equipped with the Eaton Cummins Endurant 12-speed Automated Manual Transmission (AMT), though older-style Eaton Fuller manuals (like 10, 13, and 18-speeds) and Allison automatics (4000-series) remain popular options depending on the truck’s specified duty cycle.

Proper transmission maintenance is paramount to keeping your T680 on the road and avoiding catastrophic downtime. Below is a comprehensive guide to servicing the transmissions found in the 2019 Kenworth T680.

Quick answer

• Transmission make/model: Eaton Cummins Endurant 12-speed AMT (most common); Eaton Fuller Manuals (various); Allison 4000 Series (optional).
• OEM fluid spec: Eaton PS-386 Synthetic Transmission Fluid (for Eaton AMT/Manual); Allison TES 668 or TES 295 (for Allison automatics).
• Drain & fill capacity: Eaton Endurant AMT: Approx. 16 pints (8 quarts / 7.5 liters). Traditional Eaton Fuller 13/18 speeds: Approx. 22–28 pints. Allison 4000: Approx. 31–39 quarts depending on pan depth.
• Full overhaul capacity: For AMTs and manuals, overhaul capacity is nearly identical to the drain and fill capacity (no torque converter). Allison units will hold an additional 8–10 quarts over a standard drain.
• Filter OEM part number: The Endurant AMT features an air filter/regulator for the pneumatic shifts and an internal lube filter. Please verify exact part numbers with your dealer or service manual based on your specific transmission serial number.
• Recommended service interval: Up to 750,000 miles for linehaul duty (Endurant with PS-386). 500,000 miles for regional or severe duty.

Fluid type explained

For the Eaton AMTs and manual transmissions common to the 2019 T680, the gold standard is Eaton PS-386 Synthetic Transmission Fluid. This fluid specification replaced the older PS-164 rev 7 standard. PS-386 was engineered specifically to improve fuel economy, reduce operating temperatures, and provide superior gear and bearing protection. It boasts excellent shear stability and a high viscosity index, ensuring consistent performance whether you are navigating the freezing temperatures of the northern states or pulling steep grades in the desert heat.

If your T680 is equipped with an Allison transmission, you must use a fluid that meets the TES 668 (the newer standard replacing TES 295) specification. Using standard Dexron or off-brand ATFs in these heavy-duty applications can result in degraded shift quality, accelerated clutch wear, and voided warranties.

Service procedure

Servicing an Eaton Endurant AMT or an Eaton Fuller manual is a relatively straightforward drain-and-fill procedure compared to a traditional automatic with a torque converter.

1. Preparation: Drive the truck to bring the transmission fluid up to operating temperature. Warm fluid suspends contaminants and drains much faster. Ensure the truck is parked on level ground.
2. Draining: Locate the drain plug at the bottom of the transmission case. The Endurant has a single drain plug. Remove it and allow the fluid to drain completely into an appropriate catch pan.
3. Inspect the Plug: The drain plug is magnetic. A fine, dark paste of metal shavings is normal wear. Large flakes, chunks, or shards of metal are a major red flag indicating internal gear or bearing failure. Clean the plug thoroughly before reinstalling.
4. Filling: Remove the fill plug on the side of the transmission case. Pump in the approved PS-386 fluid until the fluid level is perfectly even with the bottom threads of the fill hole. Overfilling can cause aeration and blow-out through the breather; underfilling leads to catastrophic gear failure.
5. AMT Specifics: Because the Endurant is an Automated Manual, you should also check and replace the pneumatic transmission air filter if equipped. This keeps the air supply to the shift actuators clean and dry.

For full exchanges on Allison units, dropping the pan, replacing internal/external spin-on filters, and measuring fluid levels carefully via the dipstick and electronic shift pad is required.

Service interval

The beauty of modern synthetic fluids like PS-386 is their incredibly long life.

• Normal Duty (Linehaul): If you are running long distances on the highway with standard gross weights (up to 80,000 lbs), the Eaton Endurant transmission can typically go up to 750,000 miles before a fluid change is required.
• Severe Duty (Regional / Heavy Haul): If your route involves frequent stops, heavy city traffic, steep grades, or gross vehicle weights exceeding 80,000 lbs, the interval drops to 500,000 miles or less.
• Allison Automatics: For TES 668 fluids with Allison genuine filters, normal linehaul service intervals often range around 300,000 miles, but you should always rely on the Allison oil life monitor if equipped.

Always check fluid levels at every preventive maintenance (PM) interval (typically every 25,000 to 50,000 miles) and inspect for leaks around output seals and PTO covers.

Common transmission problems

The 2019 Kenworth T680 is generally highly reliable, but the Eaton Endurant and other drivetrain components are not immune to issues:

• Pneumatic / Air Supply Issues: AMTs rely heavily on the truck's air system. Moisture, oil bypass from the air compressor, or debris in the air lines can ruin the transmission’s shift actuators and valves. Regularly draining your truck's air tanks and servicing the air dryer is critical to transmission longevity.
• Software Glitches: Like any computer-controlled system, the Transmission Control Module (TCM) can occasionally experience faults, leading to rough shifting, skipping gears, or getting stuck in gear. Often, a software flash or calibration update from the dealer resolves these gremlins.
• Clutch Actuator Wear: The automated clutch actuator manages thousands of engagements over its life. As it wears, you might notice jerky starts or rough shifting. Recalibrating the clutch via diagnostic software can sometimes extend its life before replacement is needed.
• Output Shaft Seal Leaks: High mileage and driveline vibrations can eventually wear out the rear output seal, leading to fluid loss. Always inspect the yoke area during routine greasing.

Tackling heavy-duty transmission maintenance requires the right tools, large-capacity fluid pumps, and diagnostic software. When it's time for your interval service or if you are experiencing shift faults, don't leave it to chance. Find a trusted shop on Repairius.