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Repurposing idle-stop solenoid to be air conditioning idle compensating solenoid

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Repurposing idle-stop solenoid to be air conditioning idle compensating solenoid Empty Repurposing idle-stop solenoid to be air conditioning idle compensating solenoid

Post by zucchi Tue 15 Aug 2023 - 13:02

Couldn't find a post that addressed this topic so…

Background — skip to next paragraph to get to the meat:
Back in the mid-1980s, I built my first engine using a pair of tricked-out original double-hump fuelie heads. Because those heads didn't have the bolt holes for the AC compressor, I tossed it. Besides, I was living in Monterey CA at the time, it never got blazing hot there. While I was at it, I installed a tricked-out 1968 Q-jet which doesn't have a provision to mount the idle-stop solenoid so it had to go. Flash forward to the late 1990s… With a new engine that has a pair of phase 2 bowtie heads, and the bolt holes for a compressor, I reinstalled it and had a shop get the AC working. Flash forward to 2010… I had an AC shop upgrade my VIR (valves in receiver which has check balls, springs and valves that regulated the flow of liquified refrigerant) AC system to a cycling clutch orifice tube (CCOT with no moving parts) system like what GM introduced in 1977. I noticed that when the AC compressor would cycle, the engine would surge or drag just a tiny bit while driving but it didn't bother me. Flash forward to early 2020… car needs engine rebuilt and, while I was at it, I got a later model Q-jet tricked-out. Flash forward to last month… I got fed-up with having to choose between low idle that's borderline stall whenever I turn on the AC, or adjusting the idle accordingly, then have it idle too high when the AC is off. As picky as car buyers are, I can't imagine that they would have tolerated that back then so I looked into this issue. I learned that, sure enough, they didn't. In essence, the car companies designed the system so that the compressor was engaged all the time. Then came the 1970s and all the fuel conservation & environmental awareness, etc.

In 1977 when GM introduced the CCOT AC system, they repurposed the idle-stop solenoid to be an AC idle compensation solenoid. The shop that upgraded my VIR to CCOT didn't install an idle compensating solenoid, then again, the Q-jet I had back then couldn't accommodate one. I show how I "completed" the upgrade in this video:



As you'll see near the end of the video, the solenoid keeps the throttle sufficiently open when the engine slows to idle but it's not strong enough to push the throttle open from curb idle. This doesn't bother me in the least. However, there are a couple of things one could try to perhaps make that work. One would be to use a lighter throttle return spring. The other would be to feed more current/higher voltage to the solenoid; I used the superheat cutout switch circuit which is only 10 volts. Of course, combining both may achieve the desired results.
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Repurposing idle-stop solenoid to be air conditioning idle compensating solenoid Empty Re: Repurposing idle-stop solenoid to be air conditioning idle compensating solenoid

Post by 73ss Tue 15 Aug 2023 - 17:57

I did basically the same mod years ago on my 73. I have a later model q-jet with the later model idle solenoid. I spliced a lead into the AC clutch harness to kick the idle up when the AC is on. Mine doesn't have enough umph to push the throttle open either. Mine has the added spring tension from the TV cable for the OD trans. I lightly blip the throttle when I turn the AC on and all is good. I'm using the original idle stop solenoid wire for the electric choke now.
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