[Verdoro 68]

Now that I have my a/c working again, I'm trying to adjust the idle speed for the drag the running compressor puts on the engine.

My preferred idle setting in gear with the a/c off is around 750rpm. At that base RPM, when the compressor is on the engine drops about 200 rpm and bucks and runs rough. If I bump the idle up to get it to run smoothly with the compressor running, the idle is higher than I would prefer when the compressor is off - about 950 in gear and a little over 1000rpm in park.

To get the engine to run smoothly with the compressor on and off is it just about finding an idle speed that's a sweet spot that's maybe a little higher than I'm used to?

I've got a regular old '68 Quadrajet with the stock idle stop solenoid to prevent dieseling when the car is shut off. Seems like an idle compensator that kicks in when the compressor is on would be ideal.

[78w72]

Dont most cars with a/c have an a/c idle bump solenoid? that would be the fix for your issue.

Or just split the difference in idle speeds & live with a slightly higher idle during the summer months, then drop it down for cooler months.

[Verdoro 68]

Originally Posted by 78w72

Dont most cars with a/c have an a/c idle bump solenoid? that would be the fix for your issue.

Or just split the difference in idle speeds & live with a slightly higher idle during the summer months, then drop it down for cooler months.

I think they switched to the idle compensator in the ‘70s. I was wondering if I could wire up the solenoid to the a/c compressor circuit and make it an idle compensator instead.

As of right now, I’m taking your approach of splitting the difference in the Summer and retuning for the cooler seasons. I was just wondering if there’s a more uniform solution.

[dataway]

I'm going to be in the same boat before long. I had the lurch into and out of gears with the idle high enough to handle the AC.

I too thought the idle stop solenoid on a 68 was for an auto with AC.

[vertigto]

Similar issue here, except that I have a hydroboost system that adds to the mix/drag. For now, we just bumped the idle up to @850-900.

[Verdoro 68]

Originally Posted by dataway

I'm going to be in the same boat before long. I had the lurch into and out of gears with the idle high enough to handle the AC.

I too thought the idle stop solenoid on a 68 was for an auto with AC.

Unless I’ve got mine set up wrong, the solenoid activates with the ignition and it’s where you set the idle from. The carb is set lower than than the idle with the solenoid. It’s not about idle compensation for a/c, it’s about anti dieseling when you shut the engine off.

I’m sure someone will correct me if I’m wrong, but this aligns to the emissions decal instructions.

[PunchT37]

Allstate Carburetor might have a kick up sol. I bought one for a Holley.

[nUcLeArEnVoY]

Get an AC Idle solenoid. My '79's solenoid from the factory was for the sole purpose of AC load compensation.

Just bear in mind, most, if not all, of those solenoids DO NOT have the strength to actually move the throttle lever to the desired idle automatically . They need some driver input - give it some gas so that the plunger fully extends, and then when you let it back down, it will hold it at the desired setting until the AC is turned off. Another compromise with these solenoids is that they don't de-energize until the AC is actually switched off, meaning if you have a newer system that cycles, the idle will be higher when the clutch cycles off (but AC is still on) since there will be no load on the engine but the solenoid will still be energized and deployed. This essentially puts you in the same situation you're describing about having too high an idle, only difference is that once you actually turn the AC off, everything goes back to normal. Maybe you can wire it into the clutch wire so that it retracts and deploys in sync with the AC cycling on and off, but then you'd have to give gas every time the AC cycles back on to fully deploy the plunger. Kind of annoying little buggers, but it is what it is.

My compressor in my '79 is a remanufactured and I think it's either already bad or the system the needs a recharge, because it does blow cold(ish) air but it bogs down the motor a lot even with my solenoid adjusted, and honestly I've never let it run long enough to see if it eventually cycles off after exchanging all the heat in my cabin, because it raises my engine temp quick and I get paranoid, even though I know that's normal lol.

[OG68]

The Idle Stop solenoid came with every engine in 1968, OHC and V8s. (Not sure about the RAII)
Its sole purpose was to prevent engine run on after the ignition was turned off and is wired to energize in the ignition switch 'RUN' position.

I would say that most original QJets and engines have been modified to where the solenoid is no longer needed for its original purpose. Unless of course it's for show cars or accurate restorations.
With that being said, I don't know why the solenoid couldn't be used as an AC idle booster. Simply rewire to the compressor clutch circuit.
But as mentioned previously, if the clutch circuit has been modified from stock to cycle on and off, the solenoid may not have the power to push the linkage back during idle conditions.

[Goatracer1]

Use a solenoid. Towards the end of the 1970's they came on almost everything. I even found one that fit my Tripower center carb.

[Skip Fix]

I would think the "idle stop solenoid" could do the same thing as the A/C solenoid, depending how those were wired. The AC solenoid drops idle back down also so it won't diesel. Holley even makes a bracket(that actually takes the same one as on my 78 from the factory). i think the demons are drilled for the same bracket. Comes on with the AC as factory wired or you could even wire it to a toggle switch.

[lust4speed]

I'm using a stock idle stop solenoid on my added Vintage Air setup. With the AC off the idle is at 850 (fair sized cam) and about 975 RPM with the AC running. I could have matched the off idle speed but the AC system likes a few more RPM. Probably keeps the alternator and the engine cooling system a little happier also.

I have my solenoid in the compressor circuit and it only energizes when the compressor is running. Most of the time I'm coasting to a stop and the compressor is running and the idle is at the 975 mark. If the compressor shuts off the solenoid drops out and idle returns to the 850. Now if the compressor kicks on and I'm sitting at a light then it will pull down the engine RPM because as said above the solenoid isn't strong enough to push through, but it's become second nature to just quickly tap the gas pedal and the higher RPM returns. Before adding AC I never had a problem with run-on and didn't use a solenoid. I see they are available at Summit although they are a little pricy. Luckily I had a few on the shelf from my old pick-a-part scrounging days.

[dataway]

Surprised none of the aftermarket vintage AC companies offer a solenoid suitable for this purpose.

[grivera]

Agreed - I have Vintage Air in both the 69 Bird and 64 Lemans. Luckily with EFI it’s a non-issue in my bird as the ECU maintains the idle RPM but not the case in the Lemans, which has dual AFB carbs. Would like to add a solenoid to one of the carbs.

[Trevor78]

Same thing, the adjustment changed and when the unit was powered. It either gained access to power and came on when AC was turned on, bumper up idle speed, or it was at full length when running and backed off when engine shut off to prevent dieseling or run-on. There's possibly even cars that used both, pushing on different areas of throttle linkage, but I don't know of them.

[bhill86]

For the record the late 70s AC solenoids were wired after the low pressure switch so if the compressor cycles off the solenoid loses power.

[Verdoro 68]

Originally Posted by lust4speed

I have my solenoid in the compressor circuit and it only energizes when the compressor is running.

How do you have this wired? Is it as simple as running a wire from the compressor to the solenoid?

I don't have a run on problem unless I raise the idle to compensate for a/c so repurposing the solenoid for a/c would be more useful to me. I may experiment and give this a shot.

[dataway]

So .... if the solenoid on 68s was solely for the purpose of preventing dieseling ... what did 68s with AC do to deal with this problem?

https://www.jegs.com/i/Edelbrock/350/8059/10002/-1

And I see a ton of reasonably priced solenoids at Jegs, Summit etc. Sold as "universal" carb solenoids .... build a bracket, wire to the compressor clutch?

I bet someone could draw up a print for a bracket to fit a common universal solenoid and have it laser cut at one of the many online services, couple bends later you have a bracket. My experience is they'd be cheap as dirt ordered a couple dozen at a time.

[PunchT37]

I had mine wired to the clutch. If the clutch disengaged at a light or something, the idle was very low and rough [246/253 cam] when the clutch came back on as these solenoids need a "tip in" input from the driver to pop out to their extended position. I got tired of that and wired it to the compressor switch side of the circuit. It idles high when the clutch kicks off, but it`s better than the clutch side. The solenoid will stay out during clutch cycling.



This is really not an issue with a POA valve as the compressor runs all the time. So, one could wire it to the clutch side.

[Verdoro 68]

Originally Posted by dataway

So .... if the solenoid on 68s was solely for the purpose of preventing dieseling ... what did 68s with AC do to deal with this problem?

This is what I'm wondering. The emissions label states idle is 650 for automatics and 850 for manuals with the air conditioning off. Maybe with a smaller 067/068 cam and a generally more tame setup it wasn't an issue?

I also just answered my question above about wiring the solenoid. I ran a jumper wire from the compressor to the solenoid and sure enough the solenoid kicks on when the compressor clutch is activated. The only drawback is I have a stray connector from the factory harness for the original purpose of the solenoid, but this may be the cleanest solution.

Looking around at aftermarket solenoids, these look like they *might* fit a factory solenoid bracket which is available.

https://www.amazon.com/Standard-Moto...dp/B000C83KUM/

https://www.amazon.com/ACDelco-214-2...dp/B0010HG1E6/

Bracket:
https://www.ebay.com/itm/195783308299