Electrical Signals and terms

Activation voltage (also known as Pull in voltage)-The voltage required to make the solenoid armature move full stroke.  When testing solenoids,  use 8 volts, 8 volts at room temp =12 volts at hot temperature
Hertz-(or frequency) – A term used for PWM solenoids. Hz means cycles per second. A solenoid run at 64 hz will be turned on and off 64 times a second.
The duty cycle represents what % of time the solenoid is actually on during that 1/64th of a second cycle.
Peak and hold signal (modified PWM signal)– Solenoid is turned on for brief period of time, long enough to move armature.  Supply voltage is then dropped  to keep armature in the upstroked position. It takes much less energy to keep the armature upstroked, like pushing a car. Once it is rolling it takes less energy to keep it moving. This greatly reduces electrical consumption, and reduces solenoid heat generation. The hold portion of the signal will have a varying duty cycle to control solenoid output. For example, AX4S early model pwm 1.3 ohm solenoid would consume over 110 watts of power if treated like on/off valve. Try holding a 100watt light bulb in your hand!  Many early low ohm peak and hold solenoid have been converted to higher ohm replacements with slower activation signals.
Current Averaging Signals-Vary frequency and duty to keep the current supplied to the solenoid within a given range. Problems with vehicle voltage levels can cause havoc with EPC solenoids.

Peak and hold illustration

Peak and hold illustration

Peak and Hold Hydraulic response-What is happening in the VB.
The peak and hold signal is shown in the box upper left. Solenoid fired with 12 volts. Then held to 3 volts. The duration of the 3 volt hold time gradually increases with duty cycle.
Pressure Profile The pink line is the feed oil. The light blue line is the inductive trace of the solenoid itself. The yellow line is the pressure in the oil circuit controlled by the solenoid. This solenoid bleeds off pressure to the pressure control valve in the VB. Black line is the average pressure in the circuit.

How it works:
12 volts applied is indicated by the spikes. Yellow, oil pressure starts to fall cause solenoid is open Voltage drops to zero. (kink in curve shows when armature has closed the seat.) Pressure starts to rise again. As the hold portion of the signal gets longer, the armature stays open longer, letting more oil bleed out and the system pressure gets lower.

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