General Motors' Transmissions

General Motors' Transmissions

General Motors is using several different transmission models in their current line of automobiles. Although these transmissions are significantly different in design and operation, most of the electronically controlled units are controlled in the same way. A look at some of the more commonly used transmissions should provide an accurate description of all General Motors' electronic-controlled transmission systems.

4L80-E Controls The 4L80-E was introduced in 1991 and is based on the same design as a THM 400 with a THM 200-R4 overdrive assembly and with an electronic control system. All automatic upshifts and downshifts are electronically controlled by the powertrain control module (PCM) for gasoline engines or the transmission control module (TCM) for diesel engines.

Park, neutral, and drive ranges are hydraulically controlled according to the position of the manual shift valve, as are selector-operated, forced downshifts. The PCM/TCM also has a self-diagnostic mode that is capable of storing transmission fault codes. The PCM is programmed to adjust its operating parameters in response to changes within the system, such as component wear. As component wear and shift overlap times increase, the PCM adjusts line pressure by controlling the VFM to maintain proper shift timing calibrations.

A variable force motor (VFM) controlled by the PCM is used to change line pressure in response to engine speed and vehicle load. By responding to current operating conditions, the VFM is able to match shift timing and feel with the current needs of the vehicle. The lockup converter is hydraulically applied and electrically controlled through a pulse width modulated (PWM) solenoid and the PCM/TCM.

Operation Once the PCM has processed the input signals, it controls transmission operation through two on/off solenoids, a pulse width modulated solenoid, and a variable force motor located in the valve body. The two shift solenoids are attached to the valve body and are normally open. There are four possible on/off combinations of the solenoids, which determine fluid pressure flow to the shift valves in the valve body.

The shift solenoids receive voltage through the ignition switch and are grounded through the PCM. When a solenoid is energized by the PCM, a checl ball held in place by the solenoid plunger blocks the fluid pressure feed. This closes the exhaust passage and causes signal fluid pressure to increase. When the solenoid is de-energized, fluid pressure moves the check ball and plunger off the check ball seat. This allows fluid to flow past the check ball and exhaust through the solenoid, decreasing signal pressure.

The PWM is a normally closed valve installed in the valve body. It controls the position of the TCC apply valve. When the solenoid is off, TCC signal fluid exhausts and the converter clutch remains released. Once the solenoid is energized, the plunger moves the metering ball to allow TCC signal fluid to pass to the TCC regulator valve. The PCM cycles the PWM solenoid on and off 32 times per second but varies the length of time it is energized in each cycle, or 1/32 second. The VFM is an electro-hydraulic actuator made of a variable force solenoid and a regulating valve. The VFM is installed in the valve body and controls main line pressure by moving a pressure regulator valve against spring pressure. The VFM operates by a constantly changing current from the PCM. The PCM turns the solenoid on and off 292.5 times per second but varies the solenoid's duty cycle. When the duty cycle is zero, line pressure is at maximum. Under normal operating conditions, the PCM pulses the VFM about every 10 seconds to either 100 percent duty cycle or 0 percent duty cycle. This prevents the VFM valve from sticking in any given position due to contamination. The torque signal compensator valve spring prevents major fluctuations in line pressure by absorbing the pulsations caused by the temporary full duty of the solenoid.

4T60-E Controls The 4T60-E transaxle is a four-speed automatic transmission with electronically controlled shifting and converter lockup. the torque converter clutch is controlled by two electronic solenoids, one for apply and release and the other to control the feel of the apply and release of the TCC. Some 4T60-E transaxles use a PWM solenoid to control converter lockup. The operation of this transaxle is very similar to the 4L80-E transmission. The transaxle relies on the energizing and de-energizing of two solenoids to cause a change of gears.

The primary inputs for transmission operation are from the throttle position sensor, vehicle speed sensor, engine coolant temperature sensor, manifold air pressure sensor, and from the cruise control, brake, low gear pressure, and fourth gear pressure switches. With this information, the PCM is able to calculate the optimum time for gear changes.

On some models, the PCM is also connected to one or more other computers, which operate the climate control system, antilock brake system, driver information center, and supplemental restraint system. Through multiplexing, these computers are able to share information and use common input sensors.

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