Idle Solenoid - Chevelle Tech
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post #1 of 16 (permalink) Old Jun 2nd, 19, 8:43 PM Thread Starter
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Idle Solenoid

I need some help understanding the idle solenoid on my 72 chevelle.

I thought that this solenoid would bump the RPM up a few hundred when I turned on the the AC to keep the car from stalling due to the extra load. However I used a test light last week and found out that the solenoid is engaged any time the key is in the run position.

My car Idles smoothly at 750 rpm, but If I turn the AC on the extra load will cause the car to stall. I bump my RPMs up to around 850-900 and the car runs fine with the AC on, but the it does not want to shut off at that RPM. My next thought is to set the RPM at 750 without the solenoid and have the solenoid bump the Idle up to around 900, but even still the car does not want to shut off.

Can someone explain what I am doing wrong and if I am even thinking about this in the right way.

Thanks in advance.

1972 Chevelle SS 454 "W" code
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post #2 of 16 (permalink) Old Jun 2nd, 19, 9:04 PM
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Re: Idle Solenoid

It is actually designed to help prevent "dieseling" or after run with the early emissions controlled engines. It allows the throttle blades to completely shut when you turn the ignition off.

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post #3 of 16 (permalink) Old Jun 2nd, 19, 9:12 PM
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Re: Idle Solenoid

Billk is correct, it is probably an anti dieseling solenoid. However you could wire it to the AC compressor to get power whenever the AC clutch is engaged. Some I have seen needed a blip of the throttle to activate, must not be strong enough to push up the throttle? But can hold it once engaged. My 85 GMC 1/2 ton works that way.
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post #4 of 16 (permalink) Old Jun 2nd, 19, 9:16 PM
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Re: Idle Solenoid

Also to address the run on or dieseling, on any of mine that are prone to that I simply shut it off in gear at idle and then push up to park when its stopped, or manual let the clutch out in a higher gear to stop the motor.
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post #5 of 16 (permalink) Old Jun 2nd, 19, 9:19 PM Thread Starter
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Re: Idle Solenoid

I understand that it is supposed to allow the blades to fully close. As it stands right now I need my rpms to be at least 850 to keep the engine from stalling when my AC is on.

I guess my question is what RPM should I be setting the carb for and what should the solenoid be adding? The way I have it now is that that carb alone idles at 750 and when the solenoid is engaged it pushed it to 850, but the car is still dieseling. Should I lower the RPM on the carb only?

The solenoid is not strong enough to move the throttle itself so it does need a blip of the throttle to engage.

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post #6 of 16 (permalink) Old Jun 2nd, 19, 9:37 PM
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Re: Idle Solenoid

Jon, do you have vacuum advance connected to full vacuum? If so try it on ported. Check timing as well.

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post #7 of 16 (permalink) Old Jun 2nd, 19, 10:29 PM
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Re: Idle Solenoid

is your carb original to your car? 2 or 4 barrel?
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post #8 of 16 (permalink) Old Jun 2nd, 19, 11:05 PM
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Re: Idle Solenoid

I just added one of these to my '70 Elky today, using a bracket that I fabbed up. You will be trial and error to get it set where you want it, but it works very well,
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post #9 of 16 (permalink) Old Jun 3rd, 19, 8:11 AM Thread Starter
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Re: Idle Solenoid

This motor is the numbers matching motor to the car. The carb is the original Q-jet and the vacuum advance is hooked up as it would have come from the factory.

1972 Chevelle SS 454 "W" code

Last edited by LevonH; Jun 3rd, 19 at 10:47 AM.
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post #10 of 16 (permalink) Old Jun 3rd, 19, 8:16 AM
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Re: Idle Solenoid

When I used the solenoid I set my idle off it whenever the car was running and I backed my idle set screw out all the way so the throttle blades shut fully.
Now Maybe not all the way out as you can shut the blades so far that they can stick a bit in the closed position.
So i screwed in my idle set screw just enough to keep that from happening.
I also made a bracket for mine.
It was adjustable and I could set idle speed to whatever.

Now I see you run a 5 speed stick shift Legend trans so killing in gear won't be a good idea like you can do with an automatic.

Hard to believe your car wants to die from the compressor being turned on.

Stuff happens though.
I had one car with a huge cam that anything below 1200 rpm it would die.

I do not think you are having an issue like that.
I almost feel your initial timing could be too low.


If it is less than 18 initial you may benefit from getting it there and backing out the idle set screw as it should idle higher with 18 initial vs say 10 initial timing.
You probably know that.

Now one more thing you can try is to screw out the idle mix screws just a little bit.
This will many times help with the dying issue when the AC or a load is put on it.

Yea I know it will richen the mixture and may not be what you like too see on a wide band meter but it is worse to have a vehicle die and restart or diesel than it is to have a tiny bit richer idle.

You will or may also readjust the idle set screw after messing with the mixture screws.

I like 850 or higher idle on my V8 cars.
Flat tappet cams I like a bit higher than that.
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post #11 of 16 (permalink) Old Jun 3rd, 19, 9:23 AM Thread Starter
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Re: Idle Solenoid

Quote:
Originally Posted by jeff swisher View Post
When I used the solenoid I set my idle off it whenever the car was running and I backed my idle set screw out all the way so the throttle blades shut fully.
Now Maybe not all the way out as you can shut the blades so far that they can stick a bit in the closed position.
So i screwed in my idle set screw just enough to keep that from happening.
I also made a bracket for mine.
It was adjustable and I could set idle speed to whatever.

Now I see you run a 5 speed stick shift Legend trans so killing in gear won't be a good idea like you can do with an automatic.

Hard to believe your car wants to die from the compressor being turned on.

Stuff happens though.
I had one car with a huge cam that anything below 1200 rpm it would die.

I do not think you are having an issue like that.
I almost feel your initial timing could be too low.


If it is less than 18 initial you may benefit from getting it there and backing out the idle set screw as it should idle higher with 18 initial vs say 10 initial timing.
You probably know that.

Now one more thing you can try is to screw out the idle mix screws just a little bit.
This will many times help with the dying issue when the AC or a load is put on it.

Yea I know it will richen the mixture and may not be what you like too see on a wide band meter but it is worse to have a vehicle die and restart or diesel than it is to have a tiny bit richer idle.

You will or may also readjust the idle set screw after messing with the mixture screws.

I like 850 or higher idle on my V8 cars.
Flat tappet cams I like a bit higher than that.
Thank you. This is exactly what I was looking for. I just got back from a 1000 mile road trip so I doubt my wife is going to appreciate me messing with the car tonight, but I am sure I can find some time later this week to test a few things and see what works.

1972 Chevelle SS 454 "W" code
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post #12 of 16 (permalink) Old Jun 3rd, 19, 9:29 AM
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Re: Idle Solenoid

the factory anti deseiling specs for a 72 qjet call for 600in drive with the sol energized and 450 de energized using the base idle screw. of course this is with the factory camshaft. you could bump it up a bit for the ac but it shouldnt kill it much more than 50 rpm
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post #13 of 16 (permalink) Old Jun 7th, 19, 9:15 AM
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Re: Idle Solenoid

Quote:
Originally Posted by JChilders View Post
I need some help understanding the idle solenoid on my 72 chevelle.

I thought that this solenoid would bump the RPM up a few hundred when I turned on the the AC to keep the car from stalling due to the extra load. However I used a test light last week and found out that the solenoid is engaged any time the key is in the run position.

My car Idles smoothly at 750 rpm, but If I turn the AC on the extra load will cause the car to stall. I bump my RPMs up to around 850-900 and the car runs fine with the AC on, but the it does not want to shut off at that RPM. My next thought is to set the RPM at 750 without the solenoid and have the solenoid bump the Idle up to around 900, but even still the car does not want to shut off.

Can someone explain what I am doing wrong and if I am even thinking about this in the right way.

Thanks in advance.
More advance at idle with the vacuum advance can will make the motor stronger, make more hp, at idle, much harder for the air cond. compressor to bring the idle way down. It will also allow you to close the butterflies more, both primaries and secondaries, which stops dieseling at shutoff. so, both problems can be solved with 10 more degrees at idle from your vacuum can.

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post #14 of 16 (permalink) Old Jun 7th, 19, 11:44 AM
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Re: Idle Solenoid

I would disconnect the solenoid and set your curb idle speed to approx. 600-650 in drive when hot.
then shut the engine off to see if your run on issue is still occurring. If ok, reconnect the solenoid, turn on the ac, and adjust the solenoid hex head screw to maintain a satisfactory ac rpm level.
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post #15 of 16 (permalink) Old Jun 10th, 19, 1:23 PM Thread Starter
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Re: Idle Solenoid

Quote:
Originally Posted by LevonH View Post
Jon, do you have vacuum advance connected to full vacuum? If so try it on ported. Check timing as well.
You were right with the timing. It has slipped a couple of degrees. I changed it back and it runs and shuts off fine now.

Thanks for the help everyone!
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1972 Chevelle SS 454 "W" code
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