After going down several paths (Bosch 3 wire PWM IACV, decode PWM signal with an Arduino and then run a stepper from the Arduino, etc.) to look at how to do idle air control (IAC) for my Honda Pacific Coast PC800 fuel injection project, I’m coming back around to the idea of modifying my MicroSquirt V2 to include a stepper motor control. Stock the MicroSquirt ECU doesn’t come with stepper control because there wasn’t enough space on the printed circuit board for the half dozen components needed and there wasn’t enough room in the ECU wiring harness.
I’ve talked previously with Matt at DIY Auto Tune about adding stepper control to a MicroSquirt. It is possible but it’s also hard because of one specific solder connection that needs to be made to the MC9S12C32 IC that is the brains of the unit. The DIYPNP Upgrade Kit for stepper motor control from DIY Auto Tune has the parts necessary. There is a schematic showing how to assemble the parts on a DIYPNP ECU board. From the schematic of the MicroSquirt V2 board, two of the three necessary pins are available from the IC. The third pin, IOC6/PT6, is soldered to the board but doesn’t have a trace coming away from it that I could tap into somewhere else.
Based upon the Motorola datasheet for MC9S12C32, it looks like the pin pitch on the 48 pin IC package that appears to be on the MicroSquirt is only 0.5mm. That’s TINY. I will have a very hard time finding a fine enough solder tip and solder to be able to work with a very thin wire and solder it to that pin. The pin on the 48 pin IC package is #9 I believe.
Luckily I have a couple motorcycle-specific IAC valves that are stepper motor-driven in my vast inventory of spare parts. One is off of a CBR600RR throttle body and I believe can be modified to work with my specific setup.
Another problem will be implementing vacuum bungs below the butterfly valves in the throttle bodies. I’m already using the two vacuum ports for my MAP sensor. If I were to double-duty those vacuum ports to also serve the IACV, I believe that I would probably get erroneous MAP readings. Based upon the pressure difference here in metro area Denver with sea level and with the top of mountain passes in Colorado, I need rock-solid IACV performance to make the bike rideable across that wide range of altitudes and atmospheric pressures. There is very little room to install another set of vacuum bungs. I am going to have to find some good stainless steel bungs that are bent at appropriate angles that I can press fit into place or perhaps JB Weld into place. It will be tricky. But then again… what part of this project hasn’t been tricky?
I’m going ahead and ordering the kit to have stepper motor control. I also have to order a new MAP sensor for the broken pressure sensor that I am using for barometric correction.