Transmission Fluid Pressure Manual Valve Position Switch
The Transmission Fluid Pressure (TFP) manual valve position switch is attached to the valve body, and consists of six fluid pressure switches. Three of the fluid pressure switches D4, Low, and Reverse) are normally open, while the other three (D3, D2 and TCC Release) are normally closed. These six switches indicate the position of the manual valve. (Scheme 287) The PCM uses this information to control line pressure, TCC apply and release, and shift solenoid operation.
The release pressure switch is a normally closed pressure switch. This switch is used as a diagnostic tool to confirm that the TCC is actually off when the PCM issues the command to turn the switch off.
Each fluid pressure switch produces either an open or a ground to the PCM depending upon the presence of fluid pressure at the switches. The sequence of open and closed switches produces a combination of voltage readings that are monitored by the PCM. The PCM measures the TFP manual valve position switch signal voltage from each pin to ground and compares this voltage to a combination table which is stored in PCM memory. If the PCM does not recognize the switch sequence, then a diagnostic code will be set as a result. A diagnostic code may also be set if the TFP manual valve position switch sequence indicates a gear range selection that conflicts with other sensory inputs to the PCM. (Scheme 288)
Resistance should be measured with the engine running. When the transaxle in-line 20-way connector is disconnected from the vehicle harness and the engine is running, then multiple diagnostic codes will be set. Clear these codes when you have finished with this procedure.
Scheme 287
Scheme 288
Transmission Fluid Temperature Sensor
The 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 transaxle internal 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 transaxle shift schedules and the TCC apply in an attempt to reduce the fluid temperature by reducing the transaxle heat generation. During hot mode the PCM applies the TCC at all times in 3rd and 4th 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).
Note. For transmission fluid temperature sensor replacement, see TRANSAXLE INTERNAL WIRING HARNESS .