Transmission Control Systems: Description and Operation

The Powertrain Control Module (PCM) and its input/output network control the following transmission operations:
^ shift scheduling
^ line pressure (shift feel)
^ torque converter clutch

The transmission control is separate from the engine control strategy in the PCM, although some of the input signals are shared.

The PCM receives some input signals from engine-related sensors, including:
^ Mass Air Flow Sensor (MAF sensor).
^ Engine Coolant Temperature Sensor (ECT sensor).
^ Barometric Pressure Sensor (BARO sensor).

These signals provide information to the powertrain control module about the load and climate under which the engine is operating. Some other inputs are based on driver inputs, such as accelerator pedal position which is related to the powertrain control module by the Throttle Position Sensor (TP sensor). Still other inputs are provided by:
^ transmission components.
^ Output Shaft Speed (OSS) sensor.
^ Digital Transmission Range (TR) sensor (controlled by driver placement of the shift lever).
^ Transmission Fluid Temperature Sensor (TFT sensor).

Using all of these input signals, the powertrain control module can determine when the time and conditions are right for a shift or torque converter clutch application. The PCM can also determine the line pressure needed to optimize shift feel. To accomplish these functions, the PCM uses hydraulic solenoids to control transmission operation:
^ two on/off shift solenoids
^ one pulse-width modulated Torque Converter Clutch Solenoid(TCC solenoid) for Torque Converter Clutch (TCC) control or "controlled slip" of the torque converter clutch
^ an Electronic Pressure Control (EPC) solenoid for line pressure control

The powertrain control module receives and sends electrical signals that are used to make the transmission more responsive to driver and vehicle needs.

The driver demands are transmitted to the powertrain control module through four components:
^ Transmission range selector lever-The driver's demand for a particular gear range is translated into an electrical signal for the powertrain control module by the digital Transmission Range (TR) sensor.
^ Accelerator pedal-The driver's demand for torque and acceleration are sent mechanically to the throttle body on the engine. A throttle position sensor then translates this mechanical motion into an electrical signal and sends it to the powertrain control module.
^ Brake pedal-A Brake On/Off (BOO) switch tells the powertrain control module when the brake is applied, indicating the driver demand to disengage the torque converter.
^ Transmission Control Switch-A momentary contact switch, located on the side of the transmission range selector that allows the driver to manually select three or four speed operation.

POWERTRAIN CONTROL MODULE (PCM)
The operation of the transmission is controlled by the powertrain control module. Many input sensors provide information to the powertrain control module. The powertrain control module then controls actuators which determine transmission operation.
DTCs: P0605, P1605, P0603

ENGINE COOLANT TEMPERATURE (ECT) SENSOR
The engine coolant temperature sensor detects temperature of engine coolant and supplies information to the powertrain control module. The engine coolant temperature sensor is installed into the heater outlet fitting or cooling passage on the engine. For engine control applications, the ECT signal is used to modify ignition control, EGR flow and air-to-fuel ratio as a function of engine coolant temperature. On electronic instrument clusters, the ECT output is used to control the coolant temperature indicator. The ECT sensor is used to control torque converter clutch operation.
Symptoms: Torque converter clutch will always be off, resulting in reduced fuel economy.
DTCs: PO117, PO118, P1116, P1117

INTAKE AIR TEMPERATURE (IAT) SENSOR:
The intake air temperature sensor provides the Sequential Fuel Injection (SFI) system with mixture (fuel and air) temperature information. The intake air temperature sensor is used both as a density corrector for air flow calculation and to proportion cold enrichment fuel flow. The intake air temperature sensor is installed in the air cleaner outlet tube and provides the fuel system with mixture temperature information. The intake air temperature sensor is used in determining EPC pressure.
Symptoms: Incorrect EPC pressure, either high or low, will result in either harsh or soft shifts.
DTCs: PO112, PO113, PO114

TRANSMISSION CONTROL SWITCH (TCS) AND TRANSMISSION CONTROL INDICATOR LAMP (TCIL):
The transmission control switch is a momentary contact switch. When the switch is pressed, a signal is sent to the powertrain control module to allow automatic shifts from first through fourth gears or first through third gears only. The powertrain control module energizes the Transmission Control Indicator Lamp (TCIL) when the switch is off. The TCIL indicates overdrive cancel mode activated (lamp ON) and EPC circuit shorted (lamp flashing) or monitored sensor failure.
Sensor: Transmission Control Switch (TCS)
Symptoms: No overdrive cancel when switch is cycled.
DTC: P1780 tested during Key On Engine Off (KOEO) Self-Test.
Actuator: Transmission Control Indicator Lamp (TCIL)
Symptoms:
^ FAILED ON-Overdrive cancel mode always indicated, no flashing for EPC circuit shorted or monitored sensor failure.
^ FAILED OFF-Overdrive cancel mode never indicated, no flashing for EPC circuit shorted or monitored sensor failure.
DTC: N/A

TRANSMISSION FLUID TEMPERATURE (TFT) SENSOR:
The Transmission Fluid Temperature (TFT) sensor is located on the lead frame assembly near the shift solenoids on the main control valve body. It is a temperature sensitive device called a thermistor. It sends a voltage signal to the Powertrain Control Module (PCM). The voltage signal varies with transmission fluid temperature. The PCM uses this signal to determine whether a cold start shift schedule is necessary. The shift schedule is compensated when the transmission fluid temperature is cold. The PCM also inhibits Torque Converter Clutch (TCC) operation at low transmission fluid temperatures and corrects electronic pressure control.
Symptom: Torque Converter Clutch (TCC) engagement and stabilized shift schedules happen too soon after a cold start.
DTCs: P0712 P0713,P1711,P1783,P1710

DIGITAL TRANSMISSION RANGE (DTR) SENSOR:
The digital Transmission Range digital (TR) sensor is located on the outside of the transmission at the manual lever. The digital TR sensor also opens and closes a set of four switches that are monitored by the powertrain control module to determine the position of the manual lever (P, R, N, D, 2, 1).
Symptoms: Engagement concerns, wrong gear, no shifts, increase in EPC pressure.
DTCs: P0705, P0708, P1705, P1704

BRAKE PEDAL POSITION (BPP) SWITCH:
The Brake On/Off (BPP) switch tells the powertrain control module when the brakes are applied. The switch closes when brakes are applied and opens when they are released. The BPP switch will also disengage torque converter clutch when the brake is applied.
Symptoms:
^ Failed ON-torque converter clutch will not engage at less than one-third throttle.
^ Failed OFF (or not connected)-torque converter clutch will not disengage when brake is applied.
DTCs: P1703

ELECTRONIC IGNITION (EI) SYSTEM:
The electronic ignition system consists of the powertrain control module, a crankshaft position sensor and two 4-tower ignition coils. The crankshaft position sensor sends a crankshaft position signal to the powertrain control module. The powertrain control module then sends the appropriate ignition signal to the ignition coils. The powertrain control module also uses this signal in the transaxle strategy as well as Wide Open Throttle (WOT) shift control, torque converter clutch control and electronic pressure control.
Symptoms: Harsh engagement and shifts, late WOT shift and no torque converter clutch engagement.
DTCs: P0300, P0308, P0340, P1351 through P1364, P0320

AIR CONDITIONING CLUTCH (A/C CLUTCH) 2884: OEM FACTORY INSTALLED
An electromagnetic clutch is energized when the A/C cycling switch closes. The A/C cycling switch is located on the suction accumulator/drier. The closing of the switch contacts completes the circuit to the A/C clutch and draws it into engagement with the compressor driveshaft. The powertrain control module uses this signal to adjust EPC pressure when the air conditioning clutch is engaged to compensate for additional load on the engine.
Symptoms:
^ Failed ON-EPC pressure slightly low with A/C OFF.
^ Failed OFF-EPC pressure slightly high with A/C ON.
DTC: P1460

MASS AIR FLOW (MAF) SENSOR:
The mass air flow sensor directly measures the mass of air flowing into the engine. The sensor output is a DC (analog) signal ranging from 0.5 volt to 5 volts used by the powertrain control module to calculate fuel injector pulse width. For transmission strategies this mass air flow sensor is used for EPC pressure control, shift and torque converter clutch scheduling. The powertrain control module uses this signal for EPC pressure control, shift and torque converter clutch scheduling.
Symptoms: High/low EPC pressure, incorrect shift schedule, incorrect converter engagement scheduling and symptoms similar to a Throttle Position (TP) sensor malfunction.
DTCs: P0102, PO103, P1100, P1101

THROTTLE POSITION (TP) SENSOR:
The throttle position sensor is a potentiometer mounted on the throttle body. The TP sensor detects the position of the throttle plate and sends this information to the powertrain control module as a varying voltage signal. The powertrain control module uses this signal to control shift scheduling, EPC pressure control and torque converter clutch control.
Symptoms: Harsh engagements, firm shift feel, abnormal shift schedule, torque converter clutch does not engage, torque converter clutch cycling.
DTCs: P0122, P0123, P1120, P1124

OUTPUT SHAFT SPEED SENSOR (OSS):
The Output Shaft Speed (OSS) sensor is a magnetic pickup, located at the output shaft ring gear that sends a signal to the powertrain control module to indicate transmission output shaft speed. The powertrain control module uses this signal for torque converter clutch control, shift scheduling, used in determining EPC pressure.
Symptoms: No converter engagements, harsh shifts, abnormal shift schedules.
DTC: P0720, P0721

VEHICLE SPEED SENSOR (VSS):
The vehicle speed sensor is a magnetic pickup that sends a signal to the powertrain control module. This VSS signal tells the PCM vehicle speed. The PCM uses this signal to modify upshift schedules.
Symptoms: Shift engagement/disengagement (hunting) on grades.
DTC: P0500, P0503, P1500, P1501, P0501

SHIFT SOLENOID ASSEMBLY, SHIFT CONTROL SOLENOIDS 1 AND 2 (7G484):
Two ON/OFF shift solenoids are used for electronic shift scheduling. One unit containing the two shift solenoids is located in the main control valve body. The shift solenoids are two-way, normally closed style. Shift solenoids (SS-1 and SS-2) provide gear selection of first through fourth gears by controlling the pressure to the three shift valves.
SS-1 Symptoms: Improper gear selection depending on failure mode and manual lever position.
^ Failed ON-first and fourth gear only.
^ Failed OFF-second and third gear only.
DTCs: P0750, P0751, P0781, P0782, P0783, P1751, P1714
SS-2 Symptoms: Improper gear selection depending on failure mode and manual lever position.
^ Failed ON-third and fourth gear only.
^ Failed OFF-first and second gear only.
DTCs: P0755, P0756, P0782, P0783, P1756, P1715
SS-1 and SS-2 Symptoms:
^ Both failed ON-fourth gear only.
^ Both failed OFF-second gear only.
DTCs: P0750, P0751, P0755, P0756

TORQUE CONVERTER CLUTCH (TCC) SOLENOID:
The torque converter clutch solenoid is used to control the apply and release of the torque converter clutch.
Symptoms:
^ Failed ON-engine stalls in second gear (D, 2 range) at low idle speeds with the brake applied.
^ Failed OFF-torque converter clutch never engages.
DTCs: P0741, P0743, P1744, P1741, P1742, P1743, P1767, P1740

ELECTRONIC PRESSURE CONTROL (EPC) SOLENOID:

Reference: Solenoid Operation Chart #701:

The Electronic Pressure Control (EPC) solenoid regulates transmission EPC pressure. EPC valve pressure is used to control line pressure.
Symptoms:
^ Failed ON-Minimum EPC pressure (minimum capacity), limit engine torque (alternate firing).
^ Failed OFF-Maximum EPC pressure, harsh engagements, harsh shifts.
DTCs: P1746, P1747