As long as everything was functioning properly before the gear swap, your TCC will still lockup. You just need to make sure to update your tire size/rear axle ratio in the VID of the PCM so that lockup will occur at the correct time.
The PCM/EEC compares the turbine speed to the input shaft speed and they need to be within a certain percentage of each other as calculated by the TCC strategy dependant upon the following: As long as you are under a certain throttle angle (this changes as the strategy changes), over a certain speed, in at least 3rd gear, and under a certain load, the TCC will be commanded to engage.
I am not positive, but from my tuning experience, I have seen ECT and trans temp data also influence the TCC lockup strategy, so I suspect that they are also a factor used to control the TCC.
When the TCC is commanded to engage, the ECM completes a ground circuit to energize the TCC solenoid. That moves a check ball in the fluid line between the front pump and the stator. This allows the TCC to apply when hydraulic pressure is sufficient.
The older converters didn't use PWM (Pulse Width Modulation), and they also suffered from turbine wash really bad because of the turbine design and front cover design, and increased rotating mass of the turbine, stator, sprag, and clutch assembly.
Older technology converters were also using clutches that were not designed to work well with a technology called "Pulse Width Modulation". This is a technology which does not allow the converter clutch to lock up 100%, but instead just 99.5% locked. Newer converters have special clutches made from a hybrid carbon-fiber that is designed to work well with PWM OBD-II (some older Ford OBD-I) systems on cars. The PWM system was engineered to assist in smoothing out power transfer from the motor to the trans by allowing that 0.5% percent slippage in the lock-up clutch. It also makes the lockup less noticeable.
So basically with these revised torque converters for the PWM systems, they are better suited for higher stalls on the street due to their design which allows for a more efficient transfer of low RPM power.
