Once the vehicle is moving, the primary regulator valve needs to reduce line pressure in the interests of smooth gear shifting and economy.
This is obtained by directing modulator valve pressure to the end of the valve opposite to the regulating spring.
The modulator valve assembly is in two sections and consists of a modulator valve and a modulator plug and spring.
The modulator valve is operated by throttle pressure but is also subjected to governor pressure, so that its output is sensitive both to accelerator position and to vehicle speed.
Throttle pressure is directed to the modulator valve and separates the valve and plug by moving the valve to the right and forcing the plug to the left against the compression of the spring.
This action re-regulates the throttle pressure to modulator pressure, which is then directed to the end of the primary regulator valve opposite to the spring.
Therefore if the accelerator is fully depressed for heavy acceleration, high modulator pressure will force the regulator valve upwards to oppose throttle pressure and spring pressure,
This prevents development of excessive line pressure under stall conditions.
However, as the vehicles' road speed rises, governor pressure is developed and this is directed onto the large land area at the end of the modulator valve.
A road speed will be reached when governor pressure will be high enough to overcome throttle pressure and force the modulator valve to the left which in turn forces the modulator plug fully to the left.
In this position, the modulator plug can no longer reduce the incoming throttle pressure.
This means that the undiminished throttle pressure will now exist in the modulator circuit and will be applied to the lower end of the primary regulating valve.
The pressure moves the regulator valve upwards to suitably reduce line pressure for a smooth upshift, about to take place.
This reduction in line pressure is referred to as the "cut-back" point.
At the same time as governor pressure moves the modulator valve to the left, it also allows throttle pressure to pass between the modulator valve lands and into a circuit leading back to the smallest reaction area at the end of the throttle valve.
This pressure, acting through the orifice in the circuit, helps to move the throttle valve to the left against its regulating spring.
This reduces the initial T.V. pressure established in the throttle pressure circuit, and in turn, further reduces line pressure.
