Transmission Control Systems: Testing and Inspection
Diagnosis of Automatic Transmission1. General Description
ECM and TCM exchange information during the engine operating time between power on and power off. The TCM checks the wiring of all input (e.g. rpm-sensors) and output (e.g. solenoids) components continuously. The TCM computes the different rpm- information from engine speed sensor, transmission output sensor and rpm-sensor for the large tooth-wheel. The transmission input RPM is taken from the engine RPM signal calculated by ECM and the output RPM from the vehicle speed sensor. The TCM calculates the actual transmission slip in each gear and compares this information with stored values (map of admissible slip) for actual input rpm. If the actual transmission slip is higher, then the TCM stores an error code. During computation of this information the TCM checks the correct gear (success of commanded gear). In the case the torque converter clutch is closed, then the slip has to be lower or equal than the commanded slip as seen when the torque converter clutch is open.
During shifting, the two planetary-gear trains react according to hydraulic pressure / flow commanded by TCM via solenoids. The rpm-sensor located at the large tooth-wheel detects movement of it during shifting and submits rpm information to TCM. Therefore the TCM can decide whether the commanded switching will be carried out or not (Plausibility check). The shift solenoids EV1...EV3 and EV5 are simply on/off valves for wet discclutches within the Automatic Transmission and the EM4 and EM6 valves are pressure regulation valves.
2. Diagnosis of Torque Converter Clutch (TCC)
During closing of TCC the actual pressure has to be lower than a threshold pressure. The system pressure is a special calibration of the current supplied to the modulation valve and the system pressure within the automatic transmission (no pressure sensor needed). In cases of not achieving the desired slip with the predetermined system pressure, then the TCC is opened and closed again, with an applicable delay time. After several attempts without success in achieving predetermined values, a fault is stored and the diagnosis for the TCC is locked for this driving cycle.
The diagnosis of the TCC is disabled during coasting, for a short time during shifting and at predetermined transmission oil temperatures.
3. Diagnosis of Gear Control
The Gear-Control is based on slip detection during open TCC. The TCM watches the actual slip values and compares this information with stored values. In cases of slips greater than threshold values a fault is detected and a DTC is stored.