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The 1 Amp circuit on pin 29 is the high side power supply for the pressure control solenoid. It makes sure the current never goes over 1 Amp. The PC pin (33) is the ground for the pressure control solenoid - it is used to pulse width modulate the solenoid to get different pressures.
The circuit looks like this:
What the 1 Amp circuit does is make the pressure control solenoid's response to changing PWM more 'linear' because once pin 33 ground the solenoid the current rises and then stays constant, it doesn't keep rising the way it would if the were no 'high side' current limiting.
Here is an LTspice simulation of the circuit (with the 4L60e solenoid's resistance and inductance used as a load, and a typical driver for low side control):
If the 1 Amp supply was not in place, the current would continue rise, and the energy in the solenoid would accumulate increasingly quickly. That would make it increasingly harder to control the solenoid with the finite number of PWM steps we have available.
Because the load climbs more slowly in the inductor than in a resistor, the time spent limiting the current by the TIP127 is fairly short even at the maximum PWM percentage. The actual time spent limiting current at the Pressure Control solenoid's max PWM percentage of 60% is just 36.6% (the 1 Amp circuit is dropping 10V during that time).
The 1 Amp high side circuit was added because this is the way General Motors runs the pressure control solenoid (and why the pressure control solenoid has its own supply pin on the transmission's connector). Note that the effect is most notable at higher PWM percentages (low line pressures) and thus it is the smoothness of the transmission that is most affected.
However, it is possible to just run switched 12V directly to the solenoid and ignore the 1 Amp circuit (just don't connect it to anything), if you like (then it would be just like a GPIO board, of course). You would give up some degree of control on the pressure control solenoid, but this may not matter to some users in many applications.
Enjoy driving your microTCU™ controller equipped vehicle! If you have any questions or problems, you can:
