2019 Freightliner M2 106 Transmission Fluid & Service Guide

The 2019 Freightliner M2 106 is one of the most versatile medium-duty trucks on the road today. Whether you are running a box truck, a rollback wrecker, or a utility service body, keeping your transmission properly maintained is the key to preventing costly downtime. Because the M2 106 is highly customizable, it can be spec'd with several different transmissions, including Eaton Fuller manuals and automated manuals (AMTs). However, the vast majority of these trucks are equipped with the Allison 2000 Series (specifically the 2500 RDS) or the heavier-duty Allison 3000 Series automatics.

For the purpose of this guide, we will focus on the most common setup: the Allison 2500 RDS automatic transmission. Always verify your specific transmission model via the data plate on the side of the transmission housing before beginning any service.

Quick answer: 2019 Freightliner M2 106 (Allison 2500 RDS)

• Transmission Make/Model: Allison 2500 RDS (Rugged Duty Series)
• OEM Fluid Spec: Allison TES 668 (backward compatible with TES 295)
• Drain & Fill Capacity: Approximately 14.8 quarts (shallow pan) or 15.7 quarts (deep pan)
• Full Overhaul Capacity: Approximately 20 quarts (varies based on cooler size and plumbing)
• Filter OEM Part Number: Allison 29539579 (Standard external spin-on filter for 2000/2500 series). Always verify with your dealer or service manual using your transmission serial number.
• Recommended Service Interval: 150,000 miles or 48 months (normal duty with TES 668/295); 75,000 miles or 36 months (severe duty/refuse).

Fluid type explained

When it comes to Allison transmissions in medium-duty applications, fluid specification is absolutely critical. For years, the gold standard for Allison was the TES 295 specification (like TranSynd). Recently, Allison introduced the TES 668 specification, which is the direct, fully backward-compatible replacement for TES 295.

Using a fully synthetic TES 668 fluid ensures superior oxidative stability, excellent anti-wear performance, and the longevity required for commercial applications. You may still find TES 295 fluids on the shelf, which are perfectly fine to use, but TES 668 is the modern factory standard. Avoid using standard Dexron III fluids unless it is an absolute emergency, as this will drastically reduce your service intervals (down to as little as 12,000 miles) and can compromise the durability of the clutches.

Service procedure

Maintaining the Allison 2500 RDS involves replacing the external spin-on filter and performing a fluid service. We generally recommend a traditional drain and fill over a pressurized full exchange machine.

Drain & Fill:

1. Park the truck on a level surface, set the parking brake, and allow the transmission to cool slightly (but remain warm to help suspend contaminants).
2. Clean the area around the drain plug and the spin-on filter.
3. Remove the drain plug and allow the fluid to drain completely into a large catch pan.
4. Remove the external spin-on filter. Ensure the old magnet is removed from the old filter, cleaned, and installed onto the new filter (Allison 29539579).
5. Pre-fill the new filter slightly, lube the gasket with fresh TES 668 fluid, and install it.
6. Reinstall the drain plug with a new washer and torque to spec.
7. Refill the transmission through the dipstick tube using the calculated drain and fill capacity (start with about 14 quarts for a deep pan, then check).
8. Start the engine, shift through all gears (while holding the brake), and check the fluid level hot using the dipstick or the shift pad (if equipped).

Full Exchange: While flush machines exist, Allison strongly advises against using flushing chemicals or solvents. If you need to completely exchange the fluid (e.g., switching from Dexron to TES 668), the approved method is the "double drain and fill." You drain the fluid, fill it, drive it to circulate the new fluid through the torque converter and cooler, and then perform a second drain and fill.

Service interval

Service intervals for the Allison 2500 RDS depend heavily on the type of fluid used and the duty cycle of the vehicle. When using 100% synthetic TES 668 or TES 295 fluid:

• Normal Duty (Highway / General Freight): Replace the fluid and filter every 150,000 miles, 4,000 hours, or 48 months—whichever comes first.
• Severe Duty (Refuse, Towing, Stop-and-Go City, P&D): Replace the fluid and spin-on filter every 75,000 miles, 3,000 hours, or 36 months.

Note: The initial spin-on filter replacement on a brand-new transmission is no longer required at 5,000 miles if the factory fluid is TES 295/668, but always check your specific Allison manual for any warranty requirements.

Common transmission problems

The Allison 2000/2500 series is renowned for its bulletproof reliability, but it is not entirely immune to issues in a hard-working truck like the Freightliner M2 106.

1. Shift Quality Issues / Limp Mode: Often caused by a dirty spin-on filter, degraded fluid, or failing speed sensors. If the input or output speed sensors fail, the Transmission Control Module (TCM) will command limp-in mode (usually locking the truck in 3rd gear).

2. Leaking Cooler Lines: The transmission cooler lines running from the transmission to the radiator/cooler up front are prone to chafing and corrosion on the M2 chassis. Inspect these lines at every PM service. A blown line can pump the transmission dry in minutes.

3. Shift Selector / Wiring Harness Failures: Medium-duty trucks see a lot of moisture and road debris. Corrosion at the main transmission wiring harness connector can cause communication codes (like U-codes) between the TCM and the engine ECM, resulting in a no-shift or no-start condition.

Maintaining your 2019 Freightliner M2 106 transmission isn't just about changing the fluid; it's about protecting your livelihood. Keep up with your intervals and always use the correct spec fluids.

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