Commonly Used With Eaton AutoShift, Eaton UltraShift, UltraShift PLUS, Fuller Advantage automated transmissions.

These transmissions were installed in many Class 6–8 trucks from manufacturers such as: Peterbilt, Kenworth, Freightliner, International, Volvo Trucks, Mack Trucks, Western Star

1. Controls gear shifting

The TCM decides:

• When to upshift
• When to downshift
• Which gear is best for speed and load

It constantly adjusts based on:

• Vehicle speed
• Engine RPM
• Throttle input
• Driving conditions

2. Improves driving smoothness

The TCM manages shift timing and hydraulic pressure so shifts feel:

• Smooth
• Quick
• Comfortable

It helps prevent:

• Harsh shifts
• Jerking
• Gear hunting

3. Increases fuel economy

By selecting the most efficient gear, the TCM helps:

• Lower engine RPM at cruising speeds
• Reduce fuel consumption
• Improve overall efficiency

4. Protects the transmission

The TCM can detect problems and reduce damage by:

• Entering “limp mode”
• Limiting certain gears
• Adjusting pressure to prevent slipping

This can prevent expensive transmission failure.

5. Adapts to driving style

Many modern TCMs “learn” how you drive:

• Aggressive driving → later shifts, higher RPM
• Gentle driving → earlier shifts for economy

Some vehicles also include:

• Sport mode
• Tow/haul mode
• Manual shift mode

6. Coordinates with the engine computer

The TCM communicates with the engine control system to:

• Reduce engine torque during shifts
• Improve acceleration
• Reduce drivetrain stress

This coordination is important in modern vehicles.

1. Hard or jerky shifting

The transmission may:

• Slam into gear
• Shift very aggressively
• Feel rough between gears

2. Delayed shifting

You press the gas, but the transmission:

• Hesitates before changing gears
• Takes too long to engage Drive or Reverse

3. Transmission stuck in one gear (“limp mode”)

Many vehicles enter a protection mode where:

• The car stays in 2nd or 3rd gear
• Speed and acceleration are limited

This helps prevent transmission damage.

4. Gear slipping

The vehicle may:

• Randomly change gears
• Rev high without accelerating properly
• Feel like it “falls out” of gear

5. Check Engine Light or transmission warning light

A bad TCM often triggers codes such as:

• P0700
• P0600 series
• Shift solenoid codes

A scan tool is usually needed to read them.

6. No shifting at all

The transmission may:

• Refuse to upshift or downshift
• Stay neutral-like
• Fail to engage gears properly

7. Poor fuel economy

Improper shifting can cause:

• Higher RPMs
• Inefficient gear selection
• Increased fuel consumption

8. Erratic or unpredictable shifting

The car may:

• Shift randomly
• Shift at unusual RPMs
• Change behavior from one drive to another

9. Trouble going into Drive or Reverse

You may notice:

• Long delays
• Hard engagement “clunks”
• No response initially