The transmission control module (TCM) is an electronic device which monitors various inputs in order to control various transmission functions. These functions include shift timing and quality, TCC apply and release, line pressure, and transmission diagnostics. The TCM receives input from electrical sensors, switches, solenoids and other components in order to process for use within the control program of the TCM. Based on this input information, the TCM controls certain transmission output functions and devices.

The TCM gathers the vehicle operating information from the various sensors and controls listed below. The various sensors and controls are both internal and external to the transmission. The TCM processes this information and controls the following solenoids:

Electronic control of these transmission operating characteristics provides consistent and precise shift points and shift quality based on the operating conditions of both the engine and the transmission.

1. The transmission control module (TCM) must be programmed with the proper software/calibrations. Ensure that the following conditions exist in order to prepare for TCM programming:

•	The battery is fully charged.

•	The ignition switch is in the RUN position.

•	The Techline equipment cable connection at the data link connector (DLC) is secure.

2. Program the TCM using the latest software matching the vehicle. Refer to up-to-date Techline equipment users instructions.
3. If the TCM fails to program, proceed as follows:

•	Ensure that the TCM connection is OK.

•	Inspect the Techline equipment for the latest software version.

•	Attempt to program the TCM. If the TCM still cannot be programmed properly, replace the TCM.

The DLC is a 16-pin connector that provides the technician a means of accessing serial data for aid in diagnosis. This connector allows the technician to use a scan tool in order to monitor the various system parameters and display the Diagnostic Trouble Code (DTC) information. The DLC is located within the driver's compartment, underneath driver's side fuse access panel.

In the United States, the V platform uses ISO serial data and communicates with the scan tool on DLC pin 7. Most other GM OBD II vehicles use Class 2 serial data on DLC pin 2. Both the ECM and the TCM communicate with the scan tool by using the ISO serial data circuit.

The transmission control module (TCM) controls the sport mode lamp which flashes when certain TCM diagnostics report failure. The Sport Mode lamp is located in the instrument panel cluster (IPC).

The TCM has no direct control of the engine MIL, but if an emissions related transmission fault occurs, the engine MIL must illuminate. The TCM sends MIL requests to the ECM on the controller area network (CAN) bus. When the ECM receives a MIL request from the TCM, DTC P1700 sets in the ECM and the MIL illuminates.

Notice: Do not backprobe Weatherpack connectors. The connectors are sealed for operation in an exposed environment. Backprobing may damage the seal which could eventually cause the connector to fail due to corrosion.

A wiring harness electrically connects the TCM to various sensors, solenoids, and relays within the system. Many of the connectors are environmentally-protected (Weatherpack).

For any test that requires probing the TCM harness connector or a component harness connector, use the J 35616 connector test adapter kit. Using this kit will prevent damage to the harness connector terminals.

System diagnosis will often require that electrical readings at these connectors in order to observe system performance. Anytime a connector requires separation, take care so that the pins or the terminals are not damaged.

When disconnecting the transmission external connector, additional DTC(s) will be set. Clear these DTC(s) after reconnecting the external connector. Refer to Transmission Electrical Connector in this section.

The TCM uses the following information sensors in order to gather the data for electronically controlling transmission functions:

Additional information is exchanged between the ECM and the TCM on the CAN bus.

Under normal conditions, the TCM controls the transmission operation in three modes: Economy mode, Sport mode, and Winter mode. The driver determines the transmission operating mode through the Sport mode switch and Winter mode switch.

If a major electrical system failure occurs which could affect vehicle safety or damage the transmission during normal operation, the TCM enters a default mode. In the default mode, the transmission operates in the following manner:

Important: When the system failure is not due to the TCM, and the TCM is functioning properly, the transmission will operate in 2nd gear when the selector lever is in the Manual 1st position. In this situation, the TCM operates the shift solenoids in a 2nd gear state.

With both shift solenoids off, the transmission will operate in 4th gear when the gear selector lever is in the Drive range position. However, the driver has some flexibility in the gear selection during default operation by moving the gear selector lever. The following table lists the operating characteristics in default mode.