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Duty Solenoid A - controls line pressure. Normally closed (i.e. when disconnected from power). TCU keeps it fully open from
0 to 2000 RPM. At 2001 RPM it applies 50% duty cycle that quickly goes to 0% as throttle position and engine RPM are increased.
This keeps line pressure at 60/80 (forward/reverse) PSI at idle
and increases it to 140+ PSI at 5000rpm. Lower pressure at idle slows down shifts to make them smoothers and improves fuel economy (less energy wasted pumping ATF). Higher pressure accelerates shifts and prevents slipping of clutches when engine develops high power/high torque.
Possible failures -
a) coil short circuit or disconnect (detected by TCU - you get flashing green light when starting engine)
b) mechanical wear of the moving parts that keeps solenoid always open or open much wider than it should be. This is not detected by TCU, the only reliable diag is attaching oil pressure gauge to the test port in transmission.
Worn solenoid A unfortunately this leads to lower line pressure, excessive clutch slippage and dramatically shortens transmission life span (what do you expect if clutches slip at full load - the maximum possible wear).
On my car I saw high/forward or lockup clutches (no way of knowing for sure) slip at >50% throttle if I increased throttle gently to keep the car in lockup. The rpm would jump from 2500 to 2700-3000 but jump back immediately as I released throttle a bit. This was not an unlock of lockup clutch, because TCU does not lock it back immediately - it waits 5 seconds or so.
Duty solenoids are more likely to fail this way than shift solenoids, because they work much more (cycle 60 times per sec compared to one cycle per gear shift).
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