The way this type of valve works is that the center pin is connected to +12V.  When you ground one side pin the valve opens, when you ground the other side the valve closes.  By pulsing the ground connections you can vary the valve's position.

Megasquirt only has a single PWM output for controlling an idle valve.  One common way to run a three wire valve with Megasquirt is to connect one side of the valve to ground via a resistor.  This ground circuit will provide a constant opposing force in the opposite direction as the side controlled by Megasquirt.  This is similar to installing a spring to pull the valve one way and using the PWM output to pull against the spring.  Here is a diagram taken from the Megasquirt site.

This resistor circuit is what I am currently using.  It works well but there is a little bit of variability in the absolute valve position for a given PWM.  A more sophisticated way to control the three wire valve is with a circuit that will toggle the two ground connections on opposite directions based on the input from one PWM signal.  Glenn's Garage offers such a driver circuit.  I plan on upgrading to this circuit in the near future.

Balancing The Idle Air Circuit

After driving with the above setup for about a year, I decided to try to fix some of the tuning issues I was still having with my mixture when the idle valve was open for fast idle.  The problem I was experiencing was that the fast idle mixture was always going too rich or too lean.  I could not get a stable fast idle mixture across the fast idle RPM range and the coolant temperature range.  I could play with the warm up and VE table values and get a narrow range of RPMs and temperatures to work but not the entire operating range.

One of these problems was a miss-match in the air flow between the inlet to the idle air valve which is about 16mm in diameter and the individual hoses supplying the ITBs with air which are about 6mm in diameter.  What was happening was when the idle air valve was opened beyond a minimal amount, the inlet to the valve could flow much more are than could be supplied to the throttles via the 6mm hoses.  The MAP sensor is connected between the 6mm hoses and the idle air valve so the vacuum sensed by the MAP sensor was much lower then the true vacuum within the manifold.  For example, the MAP sensor might indicate a value of 90% atmospheric in the vacuum manifold, but the true MAP inside the manifold might have been 60% or 70%.  The idle valve was basically creating a vacuum leak within the vacuum manifold causing a false MAP reading when the idle valve was opened too far.  This imbalance would generate a MAP reading that was too high and therefore deliver too much fuel causing an over-rich condition when the idle valve was open too far.

The solution to this problem was pretty straight forward.  I added a flow restrictor at the idle valve inlet to limit the air flow into the idle valve so that no more air volume can be delivered than can be consumed by the engine via the 6mm hoses to the ITBs.  I used a rubber stopper that I drilled out to match the inside diameter of the individual ports on the vacuum manifold.  The rubber stopper was then cut to length and inserted into the inlet of the idle air valve hose at the intake plenum.

Rubber stopper I picked up from a hardware store to used as a flow restrictor.	Rubber stopper was cut to length and inserted into the inlet of the idle valve.	The inside diameter of a port going to the ITBs was used to set the correct dimension for the flow restrictor.
Hole drilled into rubber stopper.	Flow restrictor installed into idle air valve. Stopper was cut to length and hole diameter double checked.

This simple addition to the idle air circuit immediately improved the tuning of fast idle but was not the complete solution.  The other piece of the problem involved the issue that my blended tuning tables did not work correctly when the MAP was raised by the idle valve while the throttle position was still fully closed.  The solution to this tuning problem is described in the Tuning For Idle Air section of my ITB Tuning page.