Electronic Transmission

PCM controls transmission and other vehicle functions. PCM monitors a number of engine/vehicle functions and uses data to control shift solenoid valves and TCC solenoid. PCM also regulates TCC engagement, upshift pattern, downshift pattern and line pressure (shift quality).

• 1-2 Shift Solenoid Valve
  
  The 1-2 shift solenoid valve is a normally open exhaust valve that is attached to the valve body. The PCM controls the solenoid by grounding the solenoid through an internal quad driver. The 1-2 shift solenoid valve is ON in 1st and 4th gear. When commanded ON, the 1-2 shift solenoid valve redirects fluid to act on the 1-2 shift valve. There are 2 PCM related DTCs associated with the 1-2 shift solenoid valve: P0751 and P0753. The PCM monitors the 1-2 shift solenoid circuit for an open or short to ground condition. If PCM detects an open or short to ground condition, then DTC P0753 sets. If PCM detects an incorrect gear ratio, then DTC P0751 sets. When DTC P0753 or P0751 sets, the PCM commands maximum line pressure, freezes shift adapts from being updated, and inhibits 3-2 downshifts. To diagnose DTC(s), see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.
• 2-3 Shift Solenoid Valve
  
  The 2-3 shift solenoid valve is a normally open exhaust valve that is attached to the valve body. The PCM controls the solenoid by grounding the solenoid through an internal quad driver. The 2-3 shift solenoid valve is ON in 3rd and 4th gear. When commanded ON, the 2-3 shift solenoid valve redirects fluid to act on the 2-3 shift valve. There are 2 PCM related DTCs associated with the 2-3 shift solenoid : P0756 and P0758. The PCM monitors the 2-3 shift solenoid circuit for an open or short to ground condition. If PCM detects an open or short to ground condition, then DTC P0758 sets. If PCM detects an incorrect gear ratio, then DTC P0756 sets. When DTC P0758 or P0756 sets, the PCM commands maximum line pressure, freezes shift adapts from being updated, and inhibits 3-2 downshifts. To diagnose DTC(s), see SELF-DIAGNOSTICS - 8.1L AVALANCHE, SIERRA, SILVERADO, SUBURBAN & YUKON XL article.
• Transmission Pressure Control Solenoid
  
  The transmission pressure control solenoid is an electronic pressure regulator that controls pressure based on the current flow through its coil winding. The magnetic field produced by the coil moves the solenoid's internal valve which varies pressure to the pressure regulator valve. PCM controls the pressure control solenoid by commanding current between 0.1-1.1 amps. This changes the duty cycle of the solenoid, which can range between 5-95 percent (typically less than 60 percent). High amperage (1.1 amps) corresponds to minimum line pressure, and low amperage (0.1 amp) corresponds to maximum line pressure (if the solenoid loses power, the transmission defaults to maximum line pressure). PCM commands the line pressure values, using inputs such as engine speed and throttle position sensor voltage. The pressure control solenoid takes the place of the throttle valve or the vacuum modulator.
• Torque Converter Clutch Solenoid Valve
  
  The Torque Converter Clutch (TCC) solenoid valve is a normally-open exhaust valve that is used to control torque converter clutch apply and release. When grounded (energized) by the PCM, the TCC solenoid valve stops converter signal oil from exhausting. This causes converter signal oil pressure to increase and move the TCC solenoid valve into the apply position.
• TCC Pulse-Width Modulation Solenoid Valve
  
  The Torque Converter Clutch Pulse Width Modulation (TCC PWM) solenoid valve controls the fluid acting on the converter clutch valve. The converter clutch valve controls the TCC apply and release. This solenoid is attached to the control valve body assembly within the transmission. The TCC PWM solenoid valve provides a smooth engagement of the torque converter clutch by operating during a duty cycle percent of ON time.
• Transmission Fluid Pressure Manual Valve Position Switch
  
  The Transmission Fluid Pressure (TFP) manual valve position switch consists of 5 pressure switches (2 normally closed and 3 normally open) on the control valve body that sense whether fluid pressure is present in 5 different valve body passages. The combination of switches that are open and closed is used by the PCM in order to determine the actual manual valve position. The TFP manual valve position switch, however, cannot distinguish between PARK and NEUTRAL because the monitored valve body pressures are identical in both cases. The switches are wired to provide 3 signal lines that are monitored by the PCM. These signals are used to help control line pressure, torque converter clutch apply and shift solenoid valve operation. Voltage at each of the signal lines is either zero or 12 volts. In order to monitor the TFP manual valve position switch operation, the PCM compares the actual voltage combination of the switches to a TFP combination table stored in its memory. The Transmission Fluid Temperature (TFT) sensor is part of the TFP manual valve position switch assembly.
• Transmission Fluid Temperature Sensor
  
  The automatic Transmission Fluid Temperature (TFT) sensor is part of the automatic Transmission Fluid Pressure (TFP) manual valve position switch. The TFT sensor is a resistor or thermistor, which changes value based on temperature. The sensor has a negative-temperature coefficient. This means that as the temperature increases, resistance decreases and as temperature decreases, resistance increases. PCM supplies a 5-volt reference signal to the TFT sensor and measures voltage drop in the circuit. When transmission fluid is cold, sensor resistance is high and PCM detects high signal voltage. As fluid temperature warms to a normal operating temperature, resistance becomes less and signal voltage decreases. PCM uses the TFT sensor information to control shift quality and TCC application.
• Transmission Range Switch
  
  The Transmission Range (TR) switch is part of the Park/Neutral Position (PNP) and backup light switch assembly, which is externally mounted on the transmission manual shaft. The TR switch contains four internal switches that indicate the transmission gear range selector lever position. PCM supplies ignition voltage to each switch circuit. As the gear range selector lever is moved, the state of each switch may change, causing the circuit to open or close. An open circuit or switch indicates a high voltage signal. A closed circuit or switch indicates a low voltage signal. PCM detects the selected gear range by deciphering the combination of the voltage signals. PCM compares the actual voltage combination of the switch signals to a TR switch combination chart stored in memory.