Theory Of Operation
The purpose of the diagnostic is to detect a functional failure of the pressure control solenoid output stages in the Transmission Control Module (TCM). The diagnostic monitors for an incorrect current feed to the pressure control solenoids which control the friction clutches. The monitoring is performed using two independent methods of comparing the commanded current to the measured current:
Current Summation Check: The sum current (total current of Pressure Control Solenoids B, C, D, E) is measured using a shunt resistor. The measured value is compared to the summed commanded currents of Pressure Control Solenoids B, C, D, E (Sum Current Calculation).
PWM Feedback Signal Check: The Pulse Width Modulation (PWM) feedback signal generated by the current regulator in the output stage Application Specific Integrated Circuit (ASIC) identifies a deviating output current for each current regulator. To perform a functional check of the output stage current, the TCM calculates the actual current based upon the PWM feedback signal, the output stage power supply voltage and the resistance of the corresponding pressure control solenoid. This calculated current is compared to the commanded current from the current regulator.
The electrical resistance of any of the solenoids can cause the commanded current summation to deviate from the calculated current summation.
