A/C Idle solenoid?

[ChevyGuyNC]

72 Cheyenne Super, 350/350.

Apparently a prior owner did something creative with the wire for this. It runs from the switch to the fuse panel, into one of the unfused always on blades. So, essentially, it's always engaged, keeping the idle higher even if the AC isn't on. I'm guessing that wire should go to a different source, one that is tied to the A/C somehow.

I've searched and seen different wiring diagrams; maybe I'm just not seeing where it should go.

Thoughts?

Have to replace the AC Diverter box and heater core... oh joy. But at least from what I've seen, the AC heater core is easier to do, and can be done from inside.

[Davidf]

The solenoid should be always active with key in on/run position. The solenoid is not intended for the a/c. It is an anti dieseling solenoid. Curb idle should be set using the solenoid plunger, not the carb idle screw.

I BELIEVE that curb idle should be set to 550 rpms with a/c on and trans in gear. As a result, with a/c off the higher idle when in park can cause dieseling. The solenoid thus fully closes off air supply so dieseling cannot occur.

So, I think people think the solenoid is an a/c idle compensator. Well, it sort of is since it allows a higher base idle setting to compensate for a/c and then prevents dieseling due to the higher base idle setting.

[Rich72C10]

I BELIEVE that curb idle should be set to 550 rpms with a/c on and trans in gear. As a result, with a/c off the higher idle when in park can cause dieseling. The solenoid thus fully closes off air supply so dieseling cannot occur.

Interesting, I didn't think of adjusting it this way. Though when the A/C off or not cycling at idle it seems you'd have a higher idle than you would want.

I am getting my new NOS solenoid to see if I can get it to work for the a/c... easy enough to test with jumpers before dorking with the OEM wiring.

Though I've read the solenoid isn't strong enough to push the idle up on its own. But testing will be fun regardless

[ChevyGuyNC]

Originally Posted by Rich72C10

Interesting, I didn't think of adjusting it this way. Though when the A/C off or not cycling at idle it seems you'd have a higher idle than you would want.

I am getting my new NOS solenoid to see if I can get it to work for the a/c... easy enough to test with jumpers before dorking with the OEM wiring.

Though I've read the solenoid isn't strong enough to push the idle up on its own. But testing will be fun regardless

My truck is running so good now with the idle set by the screw, mixture set by vacuum and dwell just right, I am not sure I want to mess with it any more. I'm taking it down to Daytona for the Turkey Run this weekend.

Ordered the heater core, A/C selector box, and a new glove box liner since mine is all messed up from the prior owner. First order with LMC...

[Davidf]

Originally Posted by Rich72C10

Interesting, I didn't think of adjusting it this way. Though when the A/C off or not cycling at idle it seems you'd have a higher idle than you would want.

I am getting my new NOS solenoid to see if I can get it to work for the a/c... easy enough to test with jumpers before dorking with the OEM wiring.

Though I've read the solenoid isn't strong enough to push the idle up on its own. But testing will be fun regardless

If the solenoid is used as designed, it does not have to be strong enough to overcome the throttle return spring. It just has to be able to hold against it. when you start a cold engine, the choke holds the throttle open, so the solenoid is initially pushing against nothing. It is then engaged and will hold the idle after warm up. When the engine is warm/hot, the proper starting procedure is to apply part throttle before cranking. In doing so, you allow the solenoid to extend to its proper position. If you are used to starting a warm engine with no throttle applied, then the solenoid will not extend and you will not be able to start the engine. Alternatively, you can turn the ignition on, slightly depress and return throttle and then start the engine with no throttle applied.

If you don't want the high idle with a/c off, set idle the way you like it via the solenoid. When the compressor is engaged, idle speed will drop regardless. Or, you can wire the solenoid through a relay to engage with the a/c compressor. But, the idle will not increase unless you tap the throttle or the compressor engaged while the throttle was already depressed. Usually not a problem if using the stock compressor which does not cycle. But, if you were at idle (say at traffic light) and switched on the a/c then the idle will drop until you blip the throttle or move off. All this assuming the solenoid will not overcome the return spring.

Those are my thoughts anyway.

[Rich72C10]

Hi Davidf - yes I do understand all of that 100% (I do not disagree with any of your points, at all) but it is nice to see it written out they way that I been thinking how this solenoid works from the factory and how I *might* want to try to make it operate. I have been mulling this over for quite a few months and reading up on this solenoid. A lot of people are used to how this would have worked with newer cars/A/C systems and not one where it was designed for anti-dieseling (me included at the beginning).

My stock R12 system as been replaced with R134a with a smaller and cycling compressor, for the record.

[ChevyGuyNC]

I may try this after the Turkey Run this weekend. Don't want to mess with it as it's running so good.

Saw on a Corvette forum, that you should adjust the solenoid to the idle you want (600RPM for my truck per the label) and then use the screw on the carb to set the idle BELOW that. Then when you disconnect the wire, idle should drop to 4-500 RPM.

My A/C has also been converted to 134, but using original compressor.

[Rich72C10]

Had a little drama with the eBay solenoid. While it was clean as a whistle and the solenoid worked perfectly, the plunger was loose as a goose. Meaning it was finger loose and I mean loose to just touching it. I tried loctite to hold it in place and let it sit overnight but when I return from a test drive it backed in so much, I could hardly get into my garage because of no idle.

I figured I'd not hear anything positive from the seller and bought another one (the sale clearly said no returns/refunds on electrical parts). Then to my surprise the original seller reached out, understood my problem, was sorry, and sent me a replacement! He reported in his message to me that he double checked all the ones he had and sent me the best of the lot and more or less guaranteed it will stay put after adjustments. That's a fine eBay seller!

I have my 2nd one installed - working great and requires a wrench to adjust it. Now I have the replacement coming in the mail, so I'll have a spare!

Currently in OEM/Factory mode as anti-dieseling.