IAC valve question

[Yves]

I'm using the Jeep Valve with the alu block to mount it from DIYAUTOTUNE.

I have never been able to make it work (wouldn't move or irratically). Since I have changed to the new MS2 firmware I found I'm able to switch the individual coils. When doing so, I noticed that each circuit displays close to 12V, but one is negative and the other is positive. When switching them via the I/O test mode the polarity changes, in the sense that it goes from +12V to -12V. Is this the normal thing ? I would have assumed that it would go to 0...

[dontz125]

The stepper pins are push-pull; when IAC1 is ON, IAC1A is at +12v, while IAC1B is at GND. When IAC1 is OFF, 1A is at GND while 1B is at +12v.

Make sure the wires are connected correctly. Measure the resistance across your pins; matched pins will give some resistance, while unmatched pins will show an open circuit. Try swapping the 1A and 1B wires, then the 2A and 2B wires. If this has no effect, try adjusting the first three "Stepper Idle" settings in TS.

[billr]

That is normal. Voltage polarity on each coil (A or B) is reversed in a specific sequence to effect the motor "stepping". Is your IAC motor 4-wire or 6-wire?

Edit: I may be confusing things with my last answer, the coils may be called "1" and "2" in MS, not "A" and "B".

[Yves]

I already checked the resistance across the coils and they are connected the right way. Should the polarity on the coils be the same (in that case I could switch the wires respectively for IAC1 and IAC 2?

[billr]

If you switch polarity (swap the wires) on one coil, the motor rotation is reversed. Swapping wires on both coils probably has no effect, I'm not sure. Swapping the both wires from one coil to the other probably also reverses the motor, again I'm not sure.

[Yves]

Ok, but I'm still wondering if it is supposed to show the + 12V on one coil and -12V on the other and when switching them, they change polarity.
So I need to know if both coils should have either + or - 12V...

d***, why is this so hard. I really need this IAC as idle speed is too low on a cold motor and it would greatly improve things.

[billr]

Both coils should have 12V across their terminals (not referenced to ground), and it will change polarity as the motor steps; there really is no + or - that can be defined for those coils as that polarity is changing all the time on both coils as the motor steps. At any particular time that the motor is stopped, with current set for "always on" or "holding", the coil voltages may be "reversed' from another time the motor is stopped. There are four possible combinations for those coil polarities.

[Yves]

I tried switching wires around and it seemed to have had the effect that I can actually move the valve now.

It has some effect on idle rpm (like a 100 rpm or so, but not enough to keep the idle around the 900-1000 range that I need when cold. I think the block for the idle valve that I got from DIYautotune and the associated 1/2 hoses (3/8 inners) are too small to do anything. Not sure how to solve that one though.

[ChevytoyR1]

The way you solve a small air leak iac is by setting the idle rpm at normal running temperature without any air leak from the iac and letting idle off of just the throttle body by adjusting the throttle blade. That way when the engine is cold it only uses minimal iac leak to make it idle..

[Yves]

I already adjusted the idle rpm hot. The drop from hot to cold is apparently too big for the IAC to handle.

[BigLou240sx]

If you remove the valve from the 3/8" I.D. line feeding your manifold, does your RPM increase to a suitable amount for cold? If so, then your issue lies in the setup of the IAC valve or tuning it

Seems like a line that big would affect a fairly large change in RPM if it was free to feed as much as it could (no valve in the way)

[Yves]

I think a year ago when I fiddled with that valve before I tried that and the rpm was not much more than what I'm seeing when the valve is pulled totally open.

Imo it has something to do with the ITB's I'm using. The iac is connected to a common vacuum chamber underneath the runners. Each runner has a 5mm hole in them below the throttle plate that connects them to the runner. The plenum is pretty large. All in all the plenum is around 100 cm³ I think.

[dontz125]

100cc doesn't sound like much volume for something hooked up to the various intakes of a V8! What size engine were those ITBs originally from?

[Yves]

Specifically for a SBC V8. It's a crossram manifold based on an original GM ITB manifold used in the 60' for the grand sports. Since it's a crossram, the space formed between the lower lip of the runners at the head interface and the triangle formed by the runners forms the vacuum chamber.