CONVERTER CLUTCH CONTROL
OVERVIEW
The converter clutch is an electronically controlled wet clutch which essentially bypasses the torque converter when actuated. This eliminates torque converter slippage and excess heat generation leading to fuel economy and vehicle performance benefits. The converter clutch is also used during transmission gear changes to minimize the customer perceived engine RPM change during upshifts and downshifts.
The converter clutch strategy is broken down into four basic parts:
1) Unconditional Unlock Logic - The converter clutch is unconditionally released under a number of conditions such as closed throttle, brake applied, high tip-in or tip-out rates, etc. A timer is then loaded to allow re-application of the converter clutch after the condition is no longer true. All times are independent such that the longest converter clutch release time for multiple release conditions controls re-application of the clutch.
2) Shift Unlock Logic - Shift unlock logic is used to control the converter clutch during upshifts and downshifts. For power-on upshifts, speed ratio is monitored after the gear change begins and is used to release the converter clutch. In this way, the drop in RPM caused by the ratio change is offset by an RPM rise due to the release of the converter clutch. After the converter clutch is released, speed ratio is monitored again to reapply the converter clutch. The end result is that the RPM change during the shift has been minimized and converter clutch control has been imperceptible to the driver. If speed ratio conditions are not met, a default timer controls the converter clutch. The speed ratio check, due to the accuracy required for proper control, is done during the 1 msec interrupt. Power-off upshifts and all downshifts are controlled by the default timer due to the lower required timing accuracy. Downshifts are all performed on an open converter to aid shift quality.
3) Scheduled Lock/Unlock Logic - When there are no unconditional releases in effect and the converter clutch is not being controlled during shifts, converter clutch application and releases are scheduled as a function of throttle position versus vehicle speed for each gear. An identical set of fox functions exists for altitude as well (BP_INTR is the interpolation factor). Vehicle speeds are modified by the learned N (engine speed) over V (vehicle speed) of the vehicle. Some additional features exist for scheduled converter clutch applies.
a) Speed ratio must be greater than a minimum value. This prevents application of the converter clutch while significant torque multiplications are taking place.
b) Throttle rate must be less than some maximum rate. This prevents application of the converter clutch when driver business would continually release and apply the converter clutch.
c) Intermediate altitude scheduled apply delay. This delays re-application of the converter clutch when driving in mountainous terrain to prevent business due to constantly changing throttle position associated with driving up and down hills.
d) W.O.T. Lockup Logic - WOT lockup logic is used to apply the converter clutch when at Wide Open Throttle to realize a performance and efficiency benefit. The speed ratio criteria becomes increasingly more difficult so as to prevent cycling of the clutch.
4) FMEM Lock-up Logic - If a transmission sensor critical to the Converter Clutch Control Logic fails, lock-ups will be based on information from the available sensors.
a) If the TP Sensor fails, locks are based on SPD_RATIO (speed ratio across the torque converter). If both the TP Sensor and the Vehicle Speed Sensor fail, locks are based on engine RPM.
b) If the Vehicle Speed Sensor fails, locks are based on a function of NEBART (filtered engine RPM) and TP_REL (relative TP).
