Symtomatics

How is the IAC valve controlled on 1.8 NA?
It seem to me, that this is a simple valve, which opens, if current is supplied?
Which frequency is used to control the valve? Is the frequency changed if the engine gets warm? How much pulsewidth is applied?
Is the pulsewidth changed to hold the idle-RPM or the frequency?

Thanks

P.S.: I am not using Megasquirt, but another ECU. But you seem to be able to answer this question.

Reverant

iTrader: (10)

Correct me if I am wrong but the NA cars don't have an IAC valve but a cold enrichment valve; ie it allows more air to enter the cylinders at idle (to raise the idle revs) while the coolant temperature is still low. On a NA, the idle is statically set by the idle screw on the AFM.

olderguy

iTrader: (19)

The NA's have an air valve that allows more airflow based on coolant temperature being low and an ISC valve that reacts to engine loading(like A/C on/off) The idle is set with a screw on the throttle body.

bripab007

And I believe only the '90-'93 cars have the air valve that olderguy described. It's on the drivers' side of the top of the intake manifold.

olderguy

iTrader: (19)

Yes, it is probably only on the 1.6

Symtomatics

Below the trottle body, there is the cold start valve, which opens in dependence of the coolent temperature.
The other 2-wire valve is controlled by the ECU. If A/C is turned on, this valve has open to stabilize the idle rpm. How is this one controlled?
The 90-93 NA seem to have the same valve as the Mazda BG-Series, but the 1.8 NA valves are different.

Aussie Driver

I'm controlling the ISC on the throttle body of my NB 1.8 engine with the PWM idle control mods that are listed in the MegaManual. I'm also running keithmac's idle code (which also uses spark to control the idle) and the idle on my car is rock solid.

matt_fulghum

hmmm, I hooked my IAC valve up today and it just made horrifyingly loud noises. I'm running the MS-II, and I've done the mods necessary to drive the PWM idle valve, but I dunno, it just sounds like it's going crazy. I don't remember it sounding so nutty when I was on the stock ECU?

In the shop manual I have, it shows that normally, the valve is controlled by varying the duty cycle between 40% (closed) and 100% (fully open), but it doesn't say what the frequency is supposed to be... does anyone know? I'd like to get my IAC valve working.

Aussie Driver

I have mine running at 33 ish.

matt_fulghum

yeah, but I'm running MS-II, where you set the frequency directly. I've got mine at 160 Hz, and it's not humming or buzzing or anything, but changing the duty cycle does nothing... it just runs really fast. Unplugging the IAC brings engine speeds back to normal, if I plug it back in it goes really fast again, unplug, back to normal. *sigh*

By the way, I found in my shop manual for my MX-3 (same idle valve) a graph of duty cycle -> opening correlations. Dunno if it's useful to anyone.
http://img255.imageshack.us/img255/1...levalvedz4.jpg

matt_fulghum

anyone have any ideas?

Aussie Driver

Sorry Matt

Almost all of us here are running MSnSE and not MSII. It looks like the settings are totally different and we know ours but not yours.

Symtomatics

@Aussie Driver:
So the frequency is fixed to 33 Hz, but the pulsewidth is changed. What are the minimum and maximum pulsewidths (in %), so that the valve does not make a loud noise?

The FM-Manual says the pulsewidth at idle should be around 35%.

By the way, can the Pressure Valve (for fuel pressure, BP engine only) be used to control the boost?

MiataMath

Not very usefull for you.. but I let the IAC controlled by the stock ECU.

Worked like a charm.

akaryrye

iTrader: (5)

I have a 90' and have disassembled the intake manifold out of sheer curiosity. There seems to be two componants which control idle function:

The first and most simple is the one on the drivers side of the manifold. It has no wires attatched to it, so there is no ecu control over it. It simply has coolant in and out and has a valve that is open more when the coolant is cooler, and when it is warmer closes (either partially or completely, idk).

The second is on the bottom of the throttle body and also has coolant lines in and out, however this one has a wiring harness, meaning this is the one connected to the ecu if anything is. The wiring harness is attached to a black box that has a shaft coming out of it. on that shaft there is a valve that swings back and fowarth, allowing more or less air through.

Hope this is sort of helpful - ryrye