4.7 IAC valves 4 WIRING
4.7 IAC valves
IAC stands for Idle Air Control. These valves work like a small throttle under ECU control to allow the MS3Pro to
adjust idle speed. Sometimes you will see these called an ISC valve, for Idle Speed Control, or BAC, for Bypass
Air Control. Idle air control valves come in several varieties. The MS3Pro supports most designs out there, with a
few exceptions.
4.7.1 Stepper IAC valves
Stepping motors come in two varieties, permanent magnet and variable reluctance (there are also hybrid motors,
which are indistinguishable from permanent magnet motors from your MS3Pro controller’s point of view). Lacking
a label on the motor, you can generally tell the two apart by feel when no power is applied. Permanent magnet
motors tend to "cog" as you twist the rotor with your fingers, while variable reluctance motors almost spin freely
(although they may cog slightly because of residual magnetization in the rotor). You can also distinguish between
the two varieties with an ohmmeter. Variable reluctance motors usually have three (sometimes four) windings, with
a common return, while permanent magnet motors usually have two independent windings, with or without center
taps. Center-tapped windings are used in unipolar permanent magnet motors.
There are a variety of stepper motor types in use in OEM IAC systems. Most domestics use bipolar stepper
motors, while some Toyota and Mitsubishi engines use unipolar stepper motors. Either one can run off the MS3Pro.
For both permanent magnet and variable reluctance stepping motors, if just one winding of the motor is ener-
gized, the rotor (under no load) will snap to a fixed angle and then hold that angle until the torque exceeds the
holding torque of the motor, at which point, the rotor will turn, trying to hold at each successive equilibrium point.
Unipolar stepping motors (5 or 6 wire) are usually wired as shown in the schematic below, with a center tap on
each of two windings. In use, the center taps of the windings are typically wired to the positive supply, and the two
ends of each winding are alternately grounded to reverse the direction of the field provided by that winding.
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 46
To Next Page
Loading...