A/C "Idle Up" With IAC?

69 1/2 Six Pack Bee · Mon Aug 21, 2006 10:31 am

Is it possible to use the GM IAC already utilized for the usual fast idle duties to also be operable with the idle up duties when operating the A/C as is used in the GM systems?
Is this already in the code or is this only do-able with one of the other configurable outputs with another externally mounted solenoid?

Thanx,
Bud

Post by **grippo** » Mon Aug 21, 2006 12:46 pm

This is not currently in the code. The problem is you need an input to determine when the AC is on and there isn't any. You could use a spare ADC, tied into the AC clutch (but not directly); then it would only take a line or two of code to increase the stepper motor position when ADC > 2.5 V and decrease it when < 2.5V. Without changing any hardware you could also look at tps near the idle stop and rpm < thresh, then increase the stepper motor position. But this is not as good as the hardware solution.

Bernard Fife · Post by **Bernard Fife** » Mon Aug 21, 2006 1:14 pm

Bud,

Another way to do this is to use a relay triggered by the A/C clutch to operate an on/off FIdle valve of the sort discussed in the manual. Then you could adjust the resultant rpm with an appropriate sized restriction in the vacuum line to that valve.

That's what I would do - it would have no connection to MegaSquirt at all.

Lance.

krisr · Post by **krisr** » Mon Aug 21, 2006 2:49 pm

grippo wrote:This is not currently in the code. The problem is you need an input to determine when the AC is on and there isn't any. You could use a spare ADC, tied into the AC clutch (but not directly); then it would only take a line or two of code to increase the stepper motor position when ADC > 2.5 V and decrease it when < 2.5V. Without changing any hardware you could also look at tps near the idle stop and rpm < thresh, then increase the stepper motor position. But this is not as good as the hardware solution.

In theory wouldn't that be how you'd have closed loop idle control except 2.5V would be replaced with a variable RPM entry configured in MT?

Post by **grippo** » Tue Aug 22, 2006 6:45 am

Yes it would, but its not as simple as it looks to get smooth, reliable and safe performance. The AC case is a bit simpler.

wes kiser · Post by **wes kiser** » Tue Aug 22, 2006 8:11 am

I handle ac idle speed with increased timing slightly below my desired idle speed. It also works to smooth out idle in general. It works great on my car (2.3 liter 4 banger, will do even better on a v8).

I start by setting the timing lower than "ideal" around idle. I make sure this value doesn't change for a few cells for stability. In my case, 20 is probabably max vaccum, so I have it set to 15. Then you make a map point about 100 rpm lower than your desired idle speed, and put the "ideal" timing value there. When load increases on the engine, it will transition into that timing value (I use 20), and accelerate up very close to the desired idle speed. If you get it right it will do it with very slight oscilation. My "desired" speed is 900 rpm. Warm with no AC, I idle at 920, if I turn the AC on it drops to 850. For me this is plenty acceptable and as good as I could ever get it with the factory computer.