Okay, a little progress.
First order of business was to figure out how the big solenoid on the side of the Ford V6 throttle body operates. It's called the idle air control (IAC) motor by Ford and it works in closed-loop mode with the Ford computer. The connector has four wires - red, black, yellow and purple. NB these may not be the same on what you get from the junkyard - Ford identifies these wires by the color going to the car (into the plug), not into the motor. Check the following manual page I got from a post on the 4-eye Ford forum (not a great transfer, but readable):
There are 4 leads from the car to the connector - ISC+ (white or white with light-blue trace), ISC- (yellow with black or red trace), ITS (light green with white trace) and SIG RTN (black with white trace). The various plug styles are also shown. My ISC motor has the early style pluggage. On the ISC motor (not to the car), the color code is ISC+ red, ISC- black, ITS and SIG RTN yellow and purple.
The idle tracking switch (ITS) tells the computer that the throttle arm is against the idle stop, and that it should calculate whether to adjust the idle speed. The ISC motor also acts as a dashpot, extending fully at cruise and pulling back slowly when the throttle is closed. The ITS uses the ITS and SIG RTN wires.
The ISC motor has four states - running out, running in, fully extended or fully retracted. Fully extended or retracted it draws no current. So to move it in and out fully, you only need to switch the polarity between ISC+ and ISC-. When running, the motor draws about 250 mA with no load, and takes maybe 1-2 s to go from stop to stop.
Using this with the MegaSquirt (MS), it's too complicated to try and use the intended closed-loop mode. So I'll use the MS fast idle (FIdle) on and off mode. There is a FIdle pin on the DB37 connector, pin 30, that goes low (ground) when the coolant temperature is low enough to trigger fast idle, and then floats (does not conduct) above that temperature.
I'm abandoning the ITS functionality, so I removed the ITS and SIG RTN pins from the original socket, leaving only 2 wires coming out ot the connector (IAC+ and IAC-). This is easy to do on the Ford plugs - just pull out the red plastic block in the plug that's held with a couple of springy plastic detents and carefully pry the connector shell away from the pin to be removed.
The fast idle will work by extending the pintle completely at fast idle, and withdrawing it completely at curb idle. There is another idle stop screw on the throttle body that can be used for curb idle. Note that the pintle moves about 1/4" stop to stop, which is way more than you need for fast idle. So the pintle, when fully withdrawn, will be too far from the throttle arm to affect the curb idle.
Note that the FIdle on a FI system only affects the amount of air available for running when cold. You still have to make a fuel map that accounts for the change in idle during warm-up.
Next big task - design and build my relay board. Here's the schematic:
[ 2/16/15 - Diagram modified slightly so that the fast idle connection only powers one relay coil. The relay coils have a fairly low resistance (75 ohms) and pin 30 is only supposed to sink a maximum of about 500 mA of current. Running two relay coils from that does not leave a comfortable safety margin, so I changed the wiring slightly so that the coil of RY4 is grounded by the contacts of RY3, along with the IAC Motor Negative. The 2A fuse should be ok, since I've measured that the IAC motor uses about 250 mA running, and that plus the relay coils 2 * 13.4 V / 75 ohms = 357 mA is less than an amp. That's more than 2x margin and should be fine. ]
I'm using two of the standard automotive SPDT relays to change polarity on the IAC motor. I looked for a suitable DPDT relay, but did not find one. A single device would be better, but this should be fine - nothing bad will happen if one of the relays hangs up.
IAC+ to positive and IAC- to negative fully extends the pintle. So the default state (pin 30 floating) is retracted.
Next up - building the relay board. Should be an afternoon's work; if I get all my parts by Saturday there will be an update soon.