The PCM controls the TCC solenoid in the transmission, which controls pressure flow to the converter clutch. The PCM program has a number of scenarios where the TC clutch is partially or fully unlocked, which use a variety of the sensors. TP, MAF, RPM, VSS, even the crank sensor contribute in some way to what state the TCC is in.
- The TC is always unlocked completely in 1st and 2nd gear. In 1st, it is hydraulically impossible for it to be locked, even if the TCC solenoid was commanded full on. For 2/3/4 the TCC is responsible for the locked/open state. In the factory program, the TC does not lock in 2nd gear.
- If the transmission fluid is very cold (typically under 55), the PCM will not lock the converter. This is to generate heat to allow the drivetrain to more quickly warm up. After the transmission fluid reaches 95F the converter and shift schedule return to normal.
- The converter will lock after the 2-3 shift completes. The PCM slowly ramps up pressure to the TCC to smooth the application and avoid shocking the driveline.
- The PCM is monitoring the converter in/out speed ratio. A ratio of 1.00 means the speed of the input shaft of the transmission is the same speed as the engine. A ratio of .5 means the engine is spinning twice as fast as the transmission input shaft because of the converter being unlocked.
When applying the TCC to lock the converter, the ratio has to be at least .8 otherwise the PCM will leave leave the converter unlocked until the ratio gets closer to 1.00 again. This is for better performance because of the torque multiplication through the unlocked converter at low speed. An example would be if you had the throttle open to, say, 30-40% to maintain moderate acceleration from a stop while going uphill. Under these conditions it would be detrimental to performance for the PCM to try and lock the converter when shifting into 3rd or 4th gear as the engine RPM would drop to a speed too low to produce enough torque to keep the vehicle accelerating as driver input demands (lugging).
- The PCM considers how quickly the driver changes throttle position. If a lot of throttle is applied quickly (high throttle rate), the converter will stay unlocked for a moment to keep from shocking the driveline.
- The PCM has a relationship chart of vehicle speed and throttle position that determines whether it will try to lock or keep the converter unlocked. At low throttle positions, the converter generally is locked. At medium throttles the TC may stay unlocked until a higher vehicle speed is reached (see my notes above about lugging).
- When the transmission shifts between gears, the TC is at least partially unlocked. About 3 seconds after the shift completes it reapplies.
- When you let off the throttle and re-apply it, the TC is unlocked briefly (tip-in). This is to reduce driveline shock and improve ride quality.
- When you are decelerating from higher speeds for a sufficient length of time (coasting) the PCM will re-lock the converter and shut fuel to the injectors off completely, allowing the momentum of the car to keep the engine from stalling. This is known as CFSO, coasting fuel shutoff.
- When you depress the brake pedal the PCM unlocks the converter (and if it was in CFSO before turns fuel back on).
- There are other parameters in TCC operation that relate to the amount of calculated torque the engine is producing. The converter will unlock, for example, if the PCM thinks the engine is making over 280 ft-lbs of torque as a precaution to protect the driveline.
- The PCM will command the TCC to slip slightly as the A/C clutch cycles. As with many other behaviors, this is to reduce NVH on the driver.