What is TCM and how does it work?

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TCM stands for Transmission Control Module. The transmission control module does exactly what it defines.

A transmission control module controls the automatic transmission system and gear shifting by collecting data from different sensors of the vehicle.

Unlike manual transmission systems, automatic transmission systems are more capable of processing data and information through the transmission control module.

Transmission Control Module

What is TCM?

A Transmission Control Module(TCM) is an electromechanical device that controls the transmission and gear shifting of an automatic car.

The whole process is done by collecting and processing data from various sensors found in the mechanical system of an automotive vehicle.

A transmission control unit can also be known as a Transmission Controller, a Transmission Control Unit (TCU), or a Powertrain Control Module.

What are the functions of the TCM in automatic cars?

1. Controlling the transmission

The primary function of a Transmission Control Module is to control the transmission or gear shifting of an automatic car. That means a TCM unit helps to control both upshifting and downshifting.

2. Problem diagnosis

At the same time, the electrical component of a TCM unit can process data collected from the sensors of the vehicle and send signals to the dashboard if anything is misaligned.

The dashboard usually shows us the problems through TCM codes that I will be discussing in the latter part of this article.

3. TCM also communicates with other important vehicle units

Smooth gear shifting and controlling the automatic transmission system of a vehicle are not the only things that a Transmission Control Module does.

A TCM unit also communicates with the engine control unit and all the other important sensors that are directly connected to the Transmission Controller Module device.

As a result, problem finding and diagnosis become much easier. 

How does the Transmission Control Module (TCM) work?

A series of sensors are attached directly to the TCM mainboard. The function of the TCM is to collect information and data from those sensors and store them in the onboard memory of the unit.

Once specific data is collected from specific sensors, the TCM unit processes those data and sends signals accordingly.

This we call the output signal.

The output signal afterward prompts the vehicle to act according to the signals sent by the Transmission Control Module.

That is how a modern automatic car works in a more controlled fashion.

What are the units that send input to the TCM?

The three broad units that send inputs to the TCM are-

  • Engine control unit
  • Transmission sensors
  • All the rest electronic controllers

Where does the TCM send output signals?

The transmission control module after processing the data collected from the input units sends the output signal to-

  • The shift solenoids
  • The pressure control solenoids
  • Torque converter lockup solenoids
  • And other important electronic controllers

What can the Transmission Control Module control?

The complex electromechanical processing capability of the TCM can control a variety of parameters.

Here are some of the many-

  • Transmission
  • Gear shifting
  • vehicle and wheel speed
  • The throttle  position
  • Transmission fluid temperature
  • Switches
  • Shift lock
  • Pressure control

What can happen if the transmission control module fails?

A faulty transmission control module is not that common but not avoidable all the time.

This is the exact unit that controls the overall automatic transmission system and determines the next move.

Any faults to the unit mean you will face problems while shifting gears from one point to the other.

Here are some of the common problems you may face if the TCM fails-

  • Downshifting and upshifting will not be regular. You may notice certain changes while downshifting and upshifting the gears.
  • An automatic gear shift to a certain parameter may happen again and again. In most cases, the gear tends to move to the neutral mode again and again.
  • While shifting from one gear to another, you may notice a delay.
  • Speed acceleration will not be normal
  • Fuel efficiency may deteriorate
  • Transmission fuel temperature may rise

Common codes for diagnosing automatic transmission systems and what the codes mean

Some common codes can be found through auto-transmission system diagnostic tools. For example-

  • PO218-Transmission over temperature
  • PO613-Faulty TCM processor     
  • PO614-ECM/TCM misaligned     
  • PO700-Faulty control system
  • PO706-Faulty transmission range sensor circuit
  • PO715- Faulty input turbine speed sensor circuit

Common problems and problem diagnoses of Automatic gear transmission system

  • Hard shifting into gear

Probable reasons


Clutch is not releasing

Free the clutch

A faulty gear shift linkage


Bent shift fork

Fix the bent or replace

Faulty synchronizing unit

Fix or repair the unit

  • Transmission jumps out of gear

Probable reasons


A faulty gear shift  linkage


Lose gear above the primary shaft

Replace the shaft or gear

Broken gear teeth

Replacing the gear

Excessive  end-play between the gears

Replacing the loose parts

Low tension on the shift liver spring

Replacing the shift liver spring with a new one

Damaged bearing


  • Gear clashes when shifting in 2nd gear

Probable reasons


Faulty gear shaft linkage


Lose gear or drum above the primary shaft

Repairing or fixing

Excessive primary shaft end-play

Need to repair the damaged parts

Lose tension in the shift liver or lockout spring

Replacing the spring with a new one

Broken gear teeth

Replacing the gear

  • Transmission  slippage when shifting in high gear

Probable reasons


A faulty gear shift linkage


Misaligned engine transmission

Alignment is necessary

Higher primary shaft end-play

Repair the loose parts

Broken gear teeth

Gear replacement

Broken or damaged synchronizing unit


  • Transmission noisy in neutral

Probable reasons


Misaligned engine and transmission connection

Ensuring the correct alignment

Broken or heated  bearings

Repairing or greasing the bearings

Damaged gears


Bent or broken countershaft


Excessive  counter shaft end-play

Replacement of the broken or damaged parts

How much does it cost to repair the Transmission Control Module(TCM)?

Repairing the TCM usually costs over $300 in most situations. Anyway, replacing the transmission control module unit with a new one can cost over $600.

In most cases, the TCM can be found near Engine Control Module.