Idle control experts ....check in
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Racer
Joined: Sep 2002
Posts: 388
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From: somewherein Ar
Idle control experts ....check in
My son and I removed the throttle body to clean it on my son's 92 Z28 (TPI). When we put it all back together it was getting dark and we somehow managed to reconnect the electrical connector to the IAC 180 deg opposite than it should have been 
In other words...upside down!! Upon starting the car, this screw-up caused the idle to surge slowly up about 500 rpm and then back down, then up, then down etc... Once we figured out what was wrong, re-connected it properly and the idle was back to normal. This got me wondering, which is usually dangerous 
. My question is this. How does the ECM control the "counts" on the IAC motor? What does the ECM use as input to calculate how many counts to either extend or retract the pintle to control idle?
In other words...upside down!! Upon starting the car, this screw-up caused the idle to surge slowly up about 500 rpm and then back down, then up, then down etc... Once we figured out what was wrong, re-connected it properly and the idle was back to normal. This got me wondering, which is usually dangerous . My question is this. How does the ECM control the "counts" on the IAC motor? What does the ECM use as input to calculate how many counts to either extend or retract the pintle to control idle?
Burning Brakes
Joined: Nov 2004
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The idle speed (in RPMs) is programmed into the computer, in your case via the PROM/chip. During warmup, or open loop, let's say stock is 1,000 RPM and warmed up, or closed loop, let's say it is 700 RPM. The computer quickly reads RPMs as an ignition signal and moves the IAC pintle in or out until the RPMs match what is in the program. During warmup the counts will be a little higher (bypassing more air for a faster idle) than during normal operation. When the Throttle Position Sensor senses a change in throttle angle it tells the computer to ignore the IAC and to 'feed' the engine based on throttle movement and the effect it has on oxygen sensor feedback (a whole other set of 'orders' programmed into the computer). The IAC pintle has a limited range of movement and a maximum count number it must stay below. You can reprogram the computer to change the idle speed, but it must be obtainable within those parameters, which are better understood by the programmer or chip burner. The reason your car was still able to idle is because the throttle blades pass a small amount of air when closed and there is a small orifice in the center of the throttle body to bypass just enough air, on a stock engine, in the event that the IAC malfunctions.
Drifting
Joined: Mar 2005
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From: GA
To build on the above.. The IAC is a stepper motor with a total of 255 steps or positions. The ECM reads the current position of the motor and commands X number of steps to achieve the programmed idle.
And there's that number 255 again...what was GM's affinity with 255?
And there's that number 255 again...what was GM's affinity with 255?
Drifting
Joined: Mar 2005
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From: GA
Originally Posted by Red90Coupe
Thanks for the input, guys.
Does the ECM use any sensor info from the ECT (engine coolant) to step the IAC, or is it just the RPM from the ign module?
Does the ECM use any sensor info from the ECT (engine coolant) to step the IAC, or is it just the RPM from the ign module?
