Idle speed, IAC and TPS ?

How does computer know that engine is idling, and that IAC should be used to adjust idle speed (for example to cut off some air) ?

Is TPS used in this process ?

I think I have some strange problems, because somebody (including me) messed out with throttle stop ( ) to 'adjust' idle speed. I guess that engine control module is now confused about what is idle and what is not.

Is there some procedure to readjust this non-adjustable thing, and to reestablish at least near the factory idle speed setup ?

Thanks!

The ECM keeps tabs on engine RPM via the CPS (crankshaft position sensor). Internally, the ECM's program decides what the desired idle RPM should be--based on engine temp and other operating concerns. When the TPS indicates the throttle is closed, the ECM compares Desired RPM to actual RPM and commands the IAC to either add or subtract air, in order to control idle speed. Generally, the idle speed adjustment your talking about is to establish "minimum air". This setting is important since the ECM assumes minimum air is correct, and acts accordingly with respect to setting idle speed via the AIC. I'm not sure what the procedure for setting minimum air is for your engine, but in general, insure the IAC is fully extended its piston, so that its not allowing air to bypass the throttle plate. Then unplug the AIC; this will keep the ECM from varying the idle while you adjust "minimum air". Next, using a calibrated tach, adjust the "throttle stop" for "minimum air". To be correct you need to know what the engine mfg specifies for your engine; not knowing, I'd try 450 RPM. Note: "minimum Air" adjustment affects the TPS voltage setting. You may need to re-adjust the TPS to get the correct balance between the two. For a Renix, 4.0L engine, this setting is .8 volts.

This is exactly the info I needed!

Thanks!

Btw, it is 4.0 Renix engine, with AW4, so I guess 0.8V at closed throttle TPS is for Manual transmission, correct ?

You're welcome. Yes-.8v. Not positive, but I'd think the dual TPS used on the autos would still use .8v for the ECM input--the other half inputs to the TCU. I don't know how that is adjusted.

Note the second TPS signal on the Renix autos runs backwards from the engine signal (5->0 volts versus 0->5 volts). When I swapped the throttle arm on my throttle body, I had to adjust the screw about 1/4 turn to get rid of the high idle.