IAC

[dn]

Dan wrote:

>      On the subject of the GM IAC(idle air controller) for the TPI engines. 
>      Does anyone know of a replacement, working of sensing the temperature 
>      or similar, ie. some sort of bi-metal spring system.  The problem is 

The IAC is a stepper motor controlled by the ECU.  The ECU stores a reference
position when the engine is shut off, and closes the valve so it knows where
it is on start up (open loop, no position feedback from the IAC).

The IAC is controlled by engine RPM, it adjusts idle speed according to engine
load.  Think of it as a small throttle plate bypassing the main throttle body.
Only way I can think of to make it work without ECU control is to design a 
small processor board to emulate the ECU signals.  This would require an input
from the ignition system (to determine RPM) and stepper motor driver software
with a driver chip or discrete transistor output.  Simply write the code to 
adjust the IAC in or out (in real time) until the idle speed is what you want,
after that anything that affects idle (temperature, A/C, alternator load, auto
transmission in gear, etc.) would be compensated for by the processor.  
(this would be a good application for a BASIC stamp).
 
I don't know for sure if you could just bypass it, I think it is necessary
to have it hooked up in order for the engine to get enough air at idle.  Can't
remember if there is a separate idle speed screw on the throttle body.  You 
could maybe adjust the thing by removing it and rotating the pintle until
you get the idle speed you want, and just leave it there, but you won't have
any warmup fast idle or other compensation.  You might also be able to hack 
off the motor end of the thing, so you can rotate the shaft and have a manual 
adjustment |-(  

regards,
dn


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 Darrell A. Norquay              Internet: dn at dlogtech.cuc.ab.ca     
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