[Beav]

I'll answer this without looking at a schematic, so it might be wrong. If necessary I'll look it up. Most automotive electronic systems have power applied with the ignition and circuits are controlled by switching the ground on and off. Adjusting the resistance only changes the 'strength' of the voltage at the solenoid, it doesn't change the time on/off. So, adjusting the resistance will vary the line pressure constantly, as a result of the solenoid exerting moreor less force per cycle, unless you twiddle the knob between each shift.