1989 240sx: Need help understanding AAC and FICD functions

[BlackHat]

I have a 1989 240sx w/ KA24E SOHC

I'm having trouble understanding the different controls for idle control. I'm adapting a MSEFI to my 240sx and before I start re-running wires I wanted to be clear on the functions of each of these units.

I ran a search and looked through the FSM for information on the FICD (Fast Idle Control Device), Idle-up solenoid, Air Regulator and AAC (Auxiliary Air Control). I also saw mention of a Air-Cut Valve, but from what I read it doesn't need terminal input from the ECU (please correct me if I'm wrong on that).

So far as I can tell, the Idle-up solenoid supplies air through the throttle body to the FICD and AAC. I understand that the Air Regulator allows for more air during a cold start up to allow for fast idling.

I get confused on why there's a AAC and FICD... I thought they did the same thing?

The air cut valve appears to just be a safety device in case of AAC failure, I don't fully understand how it prevents problems (probably because I'm confused on how/why the AAC works), but I do understand that it determines airflow by coolant temp.

Thanks for any help you give!

[vancouverbc]

this is the iaa on my dohc/ka24de. both the aac and ficd solenoids are controlled by the cars computer. the fsm really doesnt explain stuff like this fully. The FICD is used to increase idle when air conditioning is on . I'm not sure what other times the ficd comes on. The AAC is the main valve and depends on many variables to determine what it does.

[vancouverbc]

I'm interested in the air-cut valve. I think i concluded that it was part of the AAC valve. The AAC valve is a plastic piston that moves by the aac solenoid, the spring pressure, and maybe differences in pressure . for example a deep vacuum in the intake manifold, likely would result in the AAC opening on its own from pressure difference. just guessing.