7.4 Startup / Idle 7 ADDITIONAL ITEMS: BEYOND BASIC FUEL AND IGNITION CONTROL
– Stepper valve: This uses a rotating motor to move a pintle. The motor takes a discrete step every time
one of the wires is pulsed.
• Algorithm
– Open-loop (warmup): The MS3Pro commands how far to open the valve as a function of coolant tem-
perature. Note that on/off valves are always open loop, so this setting is grayed out when an on/off valve
is selected.
– Closed-loop: The MS3Pro adjusts the idle valve opening to match an RPM target.
The different valve types use different settings, described in detail below.
7.4.4.1 On/Off Valve
• Fast Idle Temperature - The MS3Pro will energize the on/off valve until the engine reaches this coolant
temperature.
• Hysteresis - The amount of temperature drop below the fast idle temperature needed for the on/off valve to
turn back on. This prevents the valve from toggling on and off rapidly when the engine is right at the fast idle
temperature.
7.4.4.2 Stepper valve settings
• Time Step Size - This is the number of milliseconds the firmware will wait between each step if the idle valve
moves more than one step. If a valve does not move reliably, this setting should be increased. Setting this
valve too high will make the valve move slowly.
• Initial Time Step Size - This is the number of milliseconds for the first step. It is often set longer than the
other steps to help overcome friction.
• Minimum # of steps to move - This is the minimum number of steps the controlling piece of code has to
command before the code that moves the valve will actually try to move the valve. We’ve found some stepper
valves, particularly if they have significant wear, may stick if commanded to move fewer than 5 to 8 steps.
• Homing steps - This is the number of steps the valve will move (in the homing direction) on boot so that the
rest of the code can rely on the start position being accurate. After homing, the IAC valve will then move to
the number specified in the Cranking Steps.
• Homing direction - You can set the valve to go through its homing routine by either opening or closing.
• Wide Open Steps - If the homing direction is set to open, this number defines the amount of steps from the
closed position to wide open.
• Crank-to-Run Taper Time - This is the amount of time the IAC valve uses to transition after starting from its
crank position to its running position.
• Power between steps
– Moving only: Power is turned off in between steps. Do not use if the valve freewheels when unplugged.
– Hold current: Applies partial current in between steps.
– Always on: Leaves the valve at full power between steps. This produces the most heat but will prevent a
4 wire stepper from moving when not commanded. Use this if the valve spins with almost no resistance
when unplugged.
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 182
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