Attempting to Build FIdle (MS) to 3-Wire Idle Air Motor controller converter

[DrJeff]

As part of my EFI project I plan to continue using the stock Idle Air Motor (IAC). The stock IAC uses a 3-wire input signal to determine the degree to which the Idle Air Valve is open (one 12v wire, and two wires that are selectively grounded). The MegaSquirt controller uses the FIdle signal, which is a more modern PWM (Pulse Width Modulation) approach to set the degree to which the IAC is open.

I found a circuit online (here (http://www.glensgarage.com/3wire/)) that claims to do the job of converting from a PWM signal to a 3-wire Bosch format and thus I set about building it. I'm not sure if the website is active anymore, but the owner did post the details of the circuit. I found several other references to much more simple approaches, but the discussions suggested that they could cause problems with the IAC eventually.

I understand electronics in a binary sense... i.e. switches, relays, etc., but when it comes to circuits I basically just copy what I see. Let's hope that method is successful in this case.

220882208922090220912209222093

Despite my best efforts to layout the circuit as cleanly as I could, I still seemed to end up with wires running back and forth. It doesn't have the heat sink fitted here. I got the thing built and next up is bench testing it.

[Bitsyncmaster]

Looks like that circuit should work. If working properly you would not need heat sinks. If the drivers get hot you may need some extra clamping diodes. The three wire motors use PWM but the other driver just switches opposite of the first driver. One thing to note is you want the PWM frequency to be 100 Hz. or less. Lower frequencies makes the motors less prone to sticky (dirty) valves.

[Elvis]

I'd stay wit the original ECU - just deactivate the original circuit and feed the power stage with your new signal,
keep the original wiring, mounting position ... well if it makes sense with the rest of your mods. You decide.


Like Dave said - stay with ~100Hz.

I don't know what kind of signals the megastuff delivers, but it's also possible to feed the circuit with an analog signal.
Especially if the new megathing delivers higher frequencies, they can easily be filtered and transformed into to an analog signal.

I'm not a fan of just copying a circuit somebody posted somewhere. Very often adaptions are needed, even if
those are only little details - they can make a big difference.

PM me if you need details of the original circuit.