2020 Freightliner M2 106: Transmission Fluid & Service Guide

Quick answer

Your 2020 Freightliner M2 106 is most commonly equipped with an Allison automatic transmission, typically from the 2000 or 3000 series. While Eaton Fuller manual transmissions are also an option, this guide focuses on the prevalent Allison automatics. Always confirm your specific transmission model before purchasing parts or fluid.

• Transmission Make/Model: Allison 2000 Series (e.g., 2500 RDS) or Allison 3000 Series (e.g., 3000 RDS). The model is identified on a tag on the side of the transmission case.
• OEM Fluid Spec: Allison TES 668 Approved Full-Synthetic ATF. This is the latest specification and is fully backward-compatible with fluids meeting the older TES 295 and TES 389 specifications.
• Drain & Fill Capacity: Varies by pan depth. For an Allison 2000 Series, approximately 15-17 quarts. For a 3000 Series, approximately 19-21 quarts.
• Full Overhaul Capacity: Varies significantly based on cooler size and configuration. For a 2000 Series, approximately 25-29 quarts. For a 3000 series, can range from 35-49 quarts.
• Filter OEM Part Number: Allison offers filter kits that include all necessary seals and filters. It is critical to verify the exact part number with your dealer using your truck's VIN, as it can vary based on shallow vs. deep pan and other factors. Do not use generic aftermarket filters.
• Recommended Service Interval: With TES 668 fluid, intervals are extended. Normal duty: 150,000 miles (fluid), 75,000 miles (filters). Severe duty: 75,000 miles (fluid), 50,000 miles (filters).

Fluid Type Explained

The lifeblood of your Allison transmission is its fluid. For the 2020 model year, the required fluid specification is Allison TES 668. This is a high-performance, full-synthetic automatic transmission fluid engineered for the demanding conditions of commercial use.

Using a TES 668 fluid is not just a recommendation; it's a requirement for maintaining transmission performance and longevity. These fluids provide superior thermal and oxidative stability, which means they resist breaking down under high heat—the number one enemy of any automatic transmission. They also maintain consistent viscosity across a wide temperature range, ensuring smooth shifts whether you're starting on a cold morning or pulling a heavy load up a steep grade in the summer heat.

It is critical to use only fluids that are officially on the Allison TES 668 approved list. Using non-approved fluids, such as generic multi-vehicle ATFs or older spec fluids like DEXRON III, can lead to premature clutch wear, overheating, and eventual transmission failure. While TES 668 supersedes the older TES 295 spec, if you are topping off a transmission already filled with TES 295, it is safe to mix the two.

Service Procedure

When it comes to servicing your transmission, you have two primary options: a standard drain and fill or a complete fluid exchange.

• Drain and Fill: This is the more traditional method. A technician removes the drain plug, allowing about two-thirds of the old fluid to drain from the pan. The pan is often removed to replace the internal sump filter(s). The system is then refilled with fresh fluid. While this is better than no service, it leaves a significant amount of old, degraded fluid in the torque converter and transmission cooler lines, which immediately contaminates the new fluid.

• Full Fluid Exchange: This is the preferred method for getting the most value and protection from expensive synthetic fluid. A specialized machine connects to the transmission cooler lines. It pumps out virtually all the old fluid while simultaneously refilling the system with new TES 668 fluid. This ensures a near-100% replacement of old fluid. During this service, the external spin-on filter (if equipped) and internal sump filter(s) should also be replaced.

Regardless of the method, replacing the filters is non-negotiable. Allison transmissions use a high-quality filtration system to trap contaminants, and these filters must be changed to ensure proper fluid flow and pressure.

Service Interval

Allison provides different service intervals based on the truck's duty cycle. Using the correct TES 668 fluid allows for significantly extended intervals compared to older fluids.

• Normal Duty: This generally applies to vehicles that operate on-highway with consistent speeds and lighter loads. For these applications, the recommended fluid change interval is 150,000 miles, with the filters being changed at 75,000 miles.

• Severe Duty / Towing: This category includes most vocational applications. If your M2 106 is a dump truck, tow truck, delivery vehicle in heavy city traffic, or frequently uses a Power Take-Off (PTO), you fall under severe duty. This type of operation generates significantly more heat. For severe duty, the fluid change interval is shortened to 75,000 miles, and the filters should be changed every 50,000 miles.

Common Transmission Problems

The Allison 2000 and 3000 series are legendary for their durability, but they are not immune to problems, especially as mileage increases.

• Solenoid Failure: The transmission uses electro-hydraulic solenoids to control shifts. Over time, these can fail, leading to harsh shifting, being stuck in a gear, or a "Check Trans" light on the dash. Solenoid issues often require removing the valve body for replacement.
• Sensor Malfunctions: The Input Speed Sensor, Output Speed Sensor, or internal mode switch (IMS) can fail. This can cause erratic or incorrect shifts, as the Transmission Control Module (TCM) is receiving bad data. These are typically less intensive repairs than internal component failure.
• Wiring and Connector Issues: Given the commercial nature of the M2 106, its wiring harnesses are exposed to the elements, vibration, and potential damage. A corroded connector or chafed wire can cause a host of intermittent electronic issues that can be difficult to diagnose.
• Overheating: While robust, these transmissions can be damaged by excessive heat. This is usually a result of operating with old, broken-down fluid, a clogged transmission cooler, or working the truck far beyond its rated capacity. Chronic overheating is the fastest way to an expensive rebuild.

Proper and timely service is the best defense against these common problems. Using the right fluid and filters on the correct schedule will ensure your Freightliner's transmission has a long and reliable service life.

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