Miscellaneous Controls: Transmission

Powertrain Control Module (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. 1-2 & 2-3 Shift Solenoid Valves
   
   The shift solenoid valves are two identical, electronic exhaust valves that control upshifts and downshifts in all forward gear ranges. These valves are normally open. These shift solenoid valves work together in a combination of ON and OFF sequences to control the positions of the 1-2, 2-3, and 3-4 shift valve trains. The Powertrain Control Module (PCM) monitors numerous inputs to determine the appropriate solenoid state combination and transmission gear for the vehicle operating conditions. See Fig 1 .

   The solenoid states are normally ON (1-2) and OFF (2-3) in the Park, Reverse, and Neutral gears. However, these may change based on the vehicle speed and the throttle position. The PCM energizes the shift solenoid valves by providing a ground to the solenoid's electrical circuit. This sends current through the coil winding of the solenoid, thereby creating a magnetic field. The magnetic field repels the plunger inside the solenoid. This seats the solenoid metering ball against the fluid inlet port. This action prevents the exhaust of fluid through the solenoid. It then provides an increase in fluid pressure at the end of the shift valves. This fluid pressure initiates an upshift by moving the shift valves. See appropriate OIL CIRCUIT DIAGRAMS in HYDRA-MATIC 4T40-E/4T45-E article in TRANSMISSIONS for a complete description of the hydraulic control of the shift valves for each gear range.

   The resistance on the shift solenoid valves should measure between 19-24 ohms when measured at 68°F (20°C) and between 24-31 ohms when measured at 190°F (88°C). The shift solenoid valves should energize when the voltage is greater than 7.5 volts. The shift solenoid valves should de-energize when the voltage is less than one volt.

   SHIFT SOLENOID OPERATION
   
   

   Gear	Solenoid 1-2	Solenoid 2-3
   Park, Reverse & Neutral	ON	OFF
   First	ON	OFF
   Second	OFF	OFF
   Third	OFF	ON
   Fourth	ON	ON

2. A/T Fluid Pressure Sensor Manual Valve Position Switch Assembly
   
   The automatic Transmission Fluid Pressure (TFP) manual valve position switch assembly is attached to the control valve body. This assembly contains six fluid pressure switches and the automatic transmission fluid temperature sensor. Five of the fluid pressure switches are normally-open. These are used to indicate the position of the manual valve. The PCM uses this information to control line pressure, TCC apply and release and shift solenoid operation. See Fig 2 .

   The release pressure switch is used as a diagnostic tool to confirm that the TCC is actually OFF when it has been commanded OFF by the PCM. This switch is normally-closed. Each fluid pressure switch produces either an open or a ground to the PCM depending on the presence of fluid pressure at the switches. The sequence of open and closed switches produces a combination of voltage readings. The PCM monitors these readings.

   The PCM measures TFP manual valve position switch signal voltage from each pin to ground and compares the voltage to a TFP manual valve position switch combination table stored in the PCM memory. If the PCM does not recognize the switch sequence, a Diagnostic Trouble Code (DTC) will be set as a result. A DTC may also set if the TFP manual valve position switch sequence indicates a gear range selection that conflicts with other sensory inputs to the PCM.

3. Manual Shaft Detent with Internal Mode Switch Lever Assembly (Impala & Monte Carlo)
   
   The lever assembly-manual shaft detent with Internal Mode Switch (IMS) is a sliding contact electrical switch assembly that corresponds to the PRNDL position selected. See Fig 3 . Each of the PRNDL positions has a unique ground pattern on four wires from the PCM.

   The assembly consists of two major components:

   • The internal mode switch that consists of the housing, which makes up the stationary contacts and the insulator, which makes up the moving contacts and is secured on the detent lever.
   • The lever assembly which consists of the manual shaft and the detent lever.

   The range detection is accomplished by securing the moving contacts to the detent lever. When the driver selects a PRNDL position, the detent lever inside the transmission rotates. This rotates the insulator which in turn grounds the four wires in a unique pattern for each gear selection, corresponding to the PRNDL position selected. The IMS is electrically connected by five wires (four of which the PCM supplies voltage to and one wire that is a common ground) to the transmission pass through connector.

4. A/T/ Fluid Pressure Manual Valve Position Switch (Impala & Monte Carlo)
   
   The automatic Transmission Fluid Pressure (TFP) switch is attached to the valve body and consists of one fluid pressure switch that monitors TCC release pressure. This switch is used as a diagnostic tool to confirm that the TCC is actually OFF when it has been commanded OFF by the PCM.

   The TCC release switch is a normally-closed pressure switch. A normally-closed switch allows current to flow from the positive contact through the switch to ground when no fluid is present. Fluid pressure moves the diaphragm to disconnect the positive and ground contacts, opening the switch and stopping current flow. This change in switch state electronically signals the PCM that the TCC is released.

5. Automatic Transmission Fluid Temperature Sensor (Impala & Monte Carlo)
   
   The automatic transmission Fluid Temperature (TFT) sensor is a negative temperature coefficient thermistor (temperature sensitive resistor) that provides information to the PCM regarding the transmission fluid temperature. The TFT sensor clips on to the valve body and is replaced as a separate component and not as a part of the AT wiring harness assembly. The TFT sensor monitors non-pressurized fluid in the sump to determine the operating temperature of the transmission fluid.

   The internal electrical resistance of the sensor varies in relation to the operating temperature of the transmission fluid. The PCM sends a 5 volt reference signal to the TFT sensor and the PCM measures the voltage drop in the electrical circuit. A lower fluid temperature creates a higher resistance in the TFT sensor, which produces a higher voltage signal.

   The PCM uses this input to help determine the proper line pressure, shift schedules and TCC apply. When the transmission fluid temperature reaches 266°F (130°C), the PCM enters Hot Mode. Above this temperature the PCM modifies the transmission shift schedules and the TCC apply in an attempt to reduce the fluid temperature by reducing the transmission heat generation. During Hot Mode the PCM applies the TCC at all times in third and fourth gears. Also, the PCM performs the 2-3 and the 3-4 shifts earlier to help reduce fluid heat generation. The PCM stays in Hot Mode until the temperature drops below 248°F (120°C).

6. Transmission Fluid Temperature Sensor (Alero & Grand Am)
   
   The automatic Transmission Fluid Temperature (TFT) sensor is a negative temperature coefficient thermistor that provides information to the Powertrain Control Module (PCM) regarding transmission fluid temperature. The TFT sensor is integrated in the automatic Transmission Fluid Pressure (TFP) manual valve position switch assembly which is bolted to the control valve body. The sensor monitors main line pressure from the inside of the control valve body to determine the operating temperature of the transmission fluid. The sensor uses an "O" ring seal to maintain fluid pressure in the control valve body. See Fig 4 .

   The internal electrical resistance of the sensor varies in relation to the operating temperature of the transmission fluid. The PCM sends a 5-volt reference signal to the sensor. This measures the voltage drop in the electrical circuit. A lower fluid temperature creates a higher resistance in the TFT sensor, thereby measuring a higher voltage signal. When the transmission fluid is cold, the sensor resistance is high and the PCM senses a high signal voltage. As the transmission fluid warms, the sensor resistance lowers and the PCM senses lower voltage. The PCM uses the TFT reading to control the Torque Converter Clutch (TCC), line pressure adjustments, and temperature compensated shifts. The TFT range is -40 to 305°F (-40 to 151°C).

   The PCM measures this voltage as another input to help control line pressure, shift schedules and TCC apply. When the TFT reaches 284°F (140°C), the PCM enters hot mode. Above this temperature the PCM modifies the transmission shift schedules and TCC apply in an attempt to reduce fluid temperature by reducing the amount of heat generated by the transmission. During hot mode the PCM applies TCC at all times in Third and Fourth gears. Also, the PCM performs the 2-3 and the 3-4 shifts earlier to help reduce the generation of fluid heat.

7. Torque Converter Clutch Pulse Width Modulated Solenoid Valve
   
   The Torque Converter Clutch (TCC) solenoid valve is a normally closed, Pulse Width Modulated (PWM) solenoid used to control the apply and release of the converter clutch. The Powertrain Control Module (PCM) operates the solenoid with a negative duty cycle at a fixed frequency of 42 Hz to control the rate of TCC apply/release. The solenoid's ability to ramp the TCC apply and release pressures results in smoother TCC operation. See Fig 5 .

   When vehicle operating conditions are appropriate to apply the TCC, the PCM immediately increases the duty cycle to approximately 42 percent. The PCM then ramps the duty cycle up to approximately 90 percent to achieve full TCC apply pressure. The rate at which the PCM increases the duty cycle controls the TCC apply. Similarly, the PCM also ramps down the TCC solenoid duty cycle to control the TCC release.

   Some operating conditions prevent or enable the TCC apply under various conditions. Also if the PCM receives a high voltage signal from the brake switch, indicating that the brake pedal is depressed, the PCM immediately releases the TCC. Note that the duty cycles in the graph are for example only. Actual duty cycles vary depending on the vehicle application and the vehicle operating conditions. The TCC solenoid valve resistance should measure between 10-11 ohms when measured at 68°F (20°C). The resistance would measure approximately 16 ohms at 300°F (150°C).

8. Pressure Control Solenoid Valve
   
   The Pressure Control (PC) solenoid valve is a precision electronic pressure regulator that controls the transmission line pressure. See Fig 6 . This control is based on the flow of current through the coil windings of the valve. As the flow of current is increased, the magnetic field which is produced by the coil moves the solenoid's plunger further away from the exhaust port. Opening the exhaust port decreases the output fluid pressure regulated by the PC solenoid valve, which ultimately decreases line pressure. The Powertrain Control Module (PCM) controls the PC solenoid valve based on various inputs, including throttle position, fluid temperature, MAP sensor, and gear state.

   The PCM controls the PC solenoid valve on a positive duty cycle at a fixed frequency of 614 Hz. Duty cycle is defined as the percent of time current is flowing through the solenoid coil during each cycle. A higher duty cycle provides a greater current flow through the solenoid. The high, positive, side of the PC solenoid valve electrical circuit at the PCM controls the PC solenoid valve operation. The PCM provides a ground path for the circuit, monitors average current and continuously varies the PC solenoid valve duty cycle to maintain the correct average current flowing through the PC solenoid valve. The resistance on the PC solenoid valve should measure between 3-5 ohms at 68°F (20°C). See PC SOLENOID VALVE DUTY CYCLE/AVERAGE CURRENT SPECIFICATION  table.

   PC SOLENOID VALVE DUTY CYCLE/AVERAGE CURRENT SPECIFICATION
   
   

   Duty Cycle	Current - Amps	Line Pressure
   +5 Percent	0.02	Maximum
   +40 Percent	1.1	Maximum

9. Up-Shift Indicator (M/T)
   
   The Instrument Panel Cluster (IPC) illuminates the upshift indicator when the PCM determines that the vehicle should be shifted to the next higher gear or the IPC receives a class 2 message from the PCM requesting illumination. The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.

Fig 1: Identifying Shift Solenoid Valves 1-2 & 2-3

Courtesy of GENERAL MOTORS CORP.