PCM Control

The Powertrain Control Module (PCM) monitors and controls the speed control, charging, A/C, and automatic transmission. The Engine Control Module .(ECM) monitors and controls the Fuel Engine System.The Transmission Control Module (TCM) in your car oversee the operation of the transmissionElectronic transmissions are getting smarter all the time. Most of the current generation units have fully adaptive control systems that "learn" the best shift points based on real-time sensor inputs and feedback. The transmission computer adapts the shift strategy to compensate for changes in engine performance and wear in the transmission friction elements in the clutches. By making subtle changes to the shift points and engagement, the computer tries to maintain consistent shift quality.

The TCM monitors what's happening inside the transmission with various speed and gear range sensors that tell it if the gears are shifting correctly and at what speeds. But to pick the shift points, it needs certain inputs from the ECM and PCM. The TCM needs to know how fast the vehicle is traveling. This information is provided by the vehicle speed sensor. It also needs to know engine rpm and load.

On some applications, the rpm signal is hard-wired directly to the transmission computer as well as the PCM. There's a dedicated circuit between the crankshaft position sensor and TCM to supply the rpm signal. On other applications, the rpm signal goes only to the PCM and the PCM forwards it to the TCM via a data bus circuit.

Depending on what type of fuel injection system is used (speed density or airflow), engine load inputs may come from the throttle position sensor (TPS), manifold absolute pressure sensor (MAP) and/or a vane airflow sensor (VAF) or mass airflow sensor (MAF). As with the rpm signal, the information may be shared directly with the TCM or it may go through the PCM and forwarded to the TCM over the data bus.


  • Car may not shift smoothly. Engine rpm's flare up when the engine is started
  • Engine may shift at the wrong rpm. The transmission on the car occasionally shifts rather harshly
  • Engine may even go into a "limp-in" mode and remain frozen in second or third gear.
  • Powertrain related sound(s) which may be described as front axle whine or as 2,200 to 2,400 engine rpm moan


  • Engine shut down before turning off ignition switch. Engine would not start again.
  • Failure to start or vehicle is stalling.
  • Engine stalling/poor performance, engine temp rising when car was not moving, and transmission shuddering.
  • No start at times; Intermittent stalling while driving during warm-up or when hot, usually restarts; Power "hitch" at cruise or under load
  • Erratic shifting and power loss due to the lock-up torque converter engaging in third gear
  • Car will run when stationery but after one or two miles the engine cuts out.
  • The vehicle engine is operating at higher than normal engine temperatures
  • The driver may experience a drone like noise while the vehicle is idling at a stop. The drone like noise


  • The ECM also performs the diagnostic function of the fuel injection system. When the ECM senses an operational problem, it will illuminate the "Check Engine" lamp to identify the problem area
  • The customer may experience an incident of engine misfire during certain vehicle operating conditions
  • While operating in speed control, a loss of vehicle speed may be experienced prior to transmission down shift

Three essential components in many vehicles are the Engine Control Module (ECM), Transmission Control Module (TCM), and the Powertrain Control Module (PCM). The Fuel Engine System is monitored and controlled by the ECM, while the transmission is governed by the TCM. The PCM is responsible for monitoring and controlling the automatic transmission, speed control, and air conditioning.

With each new model year of vehicles, electronic transmissions seem to become more "intelligent". The majority of today's vehicles have control systems that are adaptive; that is, they can identify ideal shift points according to the real-time data and feedback received from sensor inputs.

The TCM is able to accommodate any changes detected in the performance of the engine, as well as any wear in the clutches' transmission friction elements. The TCM attempts to have consistently high shift quality through slight changes to the shift points and engagement.

The computer tracks everything within the transmission with the help of gear range and speed sensors, which inform the TCM as to how fast the gears are shifting and if they are doing so properly. However, in order to choose the best shift points, the TCM has to communicate with the PCM and ECM. The computer must know the vehicle's speed, which is transmitted by the vehicle speed sensor. The TCM also requires data regarding engine load and RPMs. In some situations, both the PCM and the transmission computer are hard wired directly to the RPM signal, via a dedicated circuit between the TCM and the crankshaft position sensor.Otherwise, the data is forwarded to the TCM from the PCM using a data bus circuit.

With respect to engine load inputs, this information can come from the mass airflow sensor (MAF), vane airflow sensor (VAF), throttle position sensor (TPS), and/or a manifold absolute pressure sensor (MAP), depending on whether the vehicle has an airflow or speed density fuel injection system. Similar to the RPM signal, the data may be transmitted via data bus from the PCM to the TCM, or it may go directly to the TCM.

Symptoms of TCM Failure

  • Sounds from the powertrain: 2200-2400 RPM engine "moan" or front axle whine
  • Engine may freeze in 2nd or 3rd gear
  • Harsh or rough shifting, or shifting at the wrong RPM
  • RPMs spike when the engine starts

Symptoms of PCM Failure

  • Vehicle fails to start or stalls easily
  • Engine shuts off even though ignition is still in "on" position and fails to restart
  • Engine temperature rises even when vehicle is not in motion or is consistently higher than usual
  • Shuddering transmission
  • Engine dies after only a few miles of driving
  • Buzzing noise while vehicle idles at a stop
  • Engine will not start at all
  • Frequent stalling while warming up or hot
  • Inconsistent shifting and/or loss of power (caused by the lock-up torque converter going into 3rd gear)
  • Power feels like it stutters when the vehicle is under a heavy load Symptoms of ECM Failure
  • "Check Engine" light is on, as the ECM detects an operational issue
  • Engine seems to "misfire" under certain conditions
  • While using cruise control, transmission downshift may be preceded by a decrease in speed