[Trevor]

The long story and correct story, is that the resistor does not, and can not, separate two signals. It does not, and can not, prevent one from shorting out the other. It is a fact that only one signal is involved and therefore the impossible is not required.

Pulse width control of solenoid “A” involves the application of voltage from two circuits, but these act in combination and together form a single means of control.

The TCU combines each and every available input required and involved in adjusting line pressure via solenoid ”A”. A single signal is then computed and delivered to the solenoid via two circuits. As is logical and practical, there is no separation of the signal, as has been wrongly claimed.

SVX Transmission Line Pressure Control.

Line pressure is initially controlled via pulse width modulated, (PWM) electrical, normally closed, solenoid valve “A”. When open, this valve bleeds off pressure, rather than interrupts pressure, as a means of control. It provides precise control, but has only limited capacity. Therefore amplification is necessary, in order to achieve final control of the overall operative line pressure.

The adjusted pressure from solenoid “A” is applied as pilot control pressure, to a fully hydraulic pressure modifier valve. The pressure modifier valve, in turn controls the main pressure regulator valve. The result is a system of amplification, in two stages.

Solenoid valve “A” is controlled by means of a PWM signal, delivered by the transmission control unit, (TCU) via a direct circuit. This electrical signal comprises a series pulses, delivered at a fixed frequency of nominally 50 cycles per second. The length of the pulses, rather their frequency, controls fluid output from the valve.

The dropping resistor circuit.

It will be immediately apparent that the sudden on off pulse width modulated duty, to which normally closed solenoid valve “A” is subject, tends to cause what could be called a hammering of the valve seat, even though this is largely reduced/damped by the flow of the controlled fluid.

The dropping resistor introduces a second series of current pulses, applied in parallel with the control signal. These pulses are applied across the off cycles, so as to check the travel of the armature as it moves, thus reducing both shock and noise. These secondary parallel signals mean that in effect, during the closing/closed period, the voltage does not fall completely to zero.

This second series of pulses must be at a lesser level than the control signal, hence the dropping resistor. A resistor with a high current rating is required, which can not be mounted within the TCU enclosure.

Full voltage from the direct circuit operates the solenoid and quickly opens the valve. The low voltage dropping resistor circuit, holds in the solenoid and thus controls the point at which the valve is allowed to close. Therefore controlling the length of the low voltage pulse, sets the overall pulse length.

The full voltage direct circuit signal, comprises a very short fixed length pulse. This is immediately followed by an independent low voltage pulse, from the resistor circuit. The sum of the two provides the total pulse length delivered during each cycle.

It will be appreciated that increasing the resistance in the circuit, or opening the circuit by omitting the dropping resistor, will upset the normal pulse length, thus increasing the line pressure and making shifts more abrupt. Secondly, as an undesirable issue, shock loads applied to solenoid valve “A” are increased.