#4831, "Finally got MSII to control stock IAC stepper" Mar-25-10 07:53 PM by Corbin
The MegaSquirt guys finally came out with code that is bug free for controlling a stepper type IAC (a few months ago). That is version 2.1.0 (20090627). I've spent a couple months getting it to work. They "fixed" a few other things that forced me to change several settings unrelated to idle.
I can drop the clutch while rolling up to a light and the engine will settle to a steady idle RPM. I can turn on the A/C and defogger without stalling or bad oscillation. This is a feat with an aluminum flywheel. It does not get hung at high RPM or lose steps after boosting. It is nearly like stock.
Step one is to wire up the stepper motor. Here is the wiring scheme:
MS JS0 = IAC yellow/blue stripe MS JS1 = IAC gray MS JS2 = IAC gray/blue stripe MS JS3 = IAC orange
If you mix this up, the IAC will work in reverse or not move at all. I did solder jumpers across the 1R0 resistors on the underside of the MSII daughterboard. I think that was the only hardware mod inside the case.
Here are my settings:
If your stock ECU is still in place, then it is probably supplying the ground for all of your sensors (under the fuse box, on the body). If the ground for your MS is in a different place, you can get a lot of noise in your sensors when fans or other devices turn on. The latest code is more sensitive and this can cause havoc with your idle (TPS especially). I have my MS grounded to the firewall, where the battery grounds. I chose to run a 4 gage welding wire from the MS/battery ground to the stock ECU ground. That quieted the noise down to a reasonable level. You should play with your VE bins in your fuel map to create a nice flat area right around your normal idle and in slightly higher manifold pressures to cover A/C loading. That will make sure your A/F is stable at idle, loaded and unloaded. It should be slightly rich. I aim for 13:1 A/F, but you could probably go a hair leaner. You will probably want to reduce your ignition advance at idle down to 2 to 6 degrees, mimicing the stock ECU. This creates an artificially weak idle that requires a slightly more open IAC valve. That gives you room to close the valve more and never worry about hitting the valve seat (hitting the valve seat can cause you to lose steps and confuse the MS). I also increased the ignition advance at all RPM below my desired idle RPM. This is kind of an anti-stall feature and makes a nice valley for the idle RPM to settle into.
