Idle increase with A/C?

M

mike ballard

New member
May 23, 2018
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On the 1975 455, was there a mechanism to bump up the idle speed when the A/C was turned on? My carb doesn't seem to have anything like that and
turning on the A/C will often kill the engine.
--
Mike & Apryl Ballard
Fair Play, Missouri
1975 Glenbrook
 
What's your idle speed?
Maybe it's too low.
--
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
 
I agree that if it kills the engine when the A/C comes on, the idle may be too low, or otherwise misadjusted. However, I also agree that the GMC
SHOULD have some sort of "idle up" system. My early '75 455 doesn't have one either, and idling in traffic or waiting in a parking lot on one of our
brutal Colorado summer days, the A/C (2 evaporator system) loses much of its cooling power. I'm thinking about switching the vacuum advance to full
manifold vacuum when the A/C compressor kicks in, if I can find an appropriate solenoid vacuum switching valve. A throttle kicker is another
possibility. Maybe both! (I hate sweating in the cab!)
Good luck, and stay cool.
Rick Staples

--
Rick Staples, '75 Eleganza, Johnstown, CO

"Advice is a dangerous gift, even from the Wise to the Wise, and all paths may run ill." -Tolkien
 
I think the Cadillacs of the early "70s were the pioneers of using the idle up solenoid on the throttle. Might require that particular carb base
though.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.
 
The original carb on my early 1974 GMC did not have an idle up solenoid.

I made the following mod about 10 years ago. My memory sucks so the specifics may be a bit off but the mod still works fine to this day.

So what I did was use (I think) an egr solenoid from a GM 4.3 and blocked off the vent port. This left me with a 2 port air valve, flow when power is
applied, blocked when power is off. Wired the coil to be energized when the ac compressor is on. Vacuum lines, one from manifold to supply vacuum and
the other teed into the line going to the vacuum advance. On the ported vacuum port in the carb base, I filled it with jb weld and drilled it to
something small, like maybe a number 58 or so drill bit.

What this does is at idle with no ac the timing is not vacuum advanced. RPM's are 580 to 600 in drive. Off idle and the port is exposed to vacuum and
the vacuum can operates like it always did and the timing vacuum advances. At idle if the ac is turned on the electric valve opens and applies full
vacuum to the vacuum can and the timing vacuum advances. Idle RPM's go to 620 to 640 in drive. The slight vacuum leak from the ported vacuum port
leans the mixture about .1 (point one) on the fuel ratio gauge. Off idle the ported vacuum port gets vacuum and, in addition to the vacuum coming thru
the electric valve, the vacuum can works like you would expect from a full-time vacuum setup (which it is). If the ac cycles on and off at idle I can
barely notice the difference in engine speed, listening very carefully I can hear the change in the exhaust note.

I'm using a Crane adjustable vacuum can so everything is tuneable. I couldn't tell you how much advance or at what vacuum it starts at, I didn't write
that down. All I know is that it still works fine after all these years and wonder why GM didn't do something like this originally (money?).

JWID

--
Bob Heller
1974 X-Canyonlands 26ft
Original 455 exc for timing chain,
Rockwell intake, valve covers. 141k miles.
Winter Springs FL
 
Another solution might Be to convert from R134 to Duracool refrigerant. System pressure is so much lower that when my AC comes on at idle, there is
little noticeable change in load engine RPM. Just what I have noticed.
--
Larry
78 Royale w/500 Caddy
Menomonie, WI.
 
The 403 has a vacuum canister that bolts to the intake manifold and the
nylon plunger tip aligns with the throttle. It is part of the emission
package on the California equipped coaches. It works to increase the idle
rpm under spark retard. I bet that it could be used on the 455 with a bit
of tweaking to increase idle when the A/C is on.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> I think the Cadillacs of the early "70s were the pioneers of using the
> idle up solenoid on the throttle. Might require that particular carb base
> though.
> --
> Terry Kelpien
>
> ASE Master Technician
>
> 73 Glacier 260
>
> Smithfield, Va.
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
During the warmer season, one should set the idel speed with the a/c
compressor running and in drive and on a slightly higher rpm.
Being a larger displacement engine, we find it does the job.
​Also you should NEVER remove the Thermostatic Vaccume Switch as if you get
in a traffic and engine with A/C​ on a warm day the engine will over heat
even with a HD fan clutch. TheTVS will pull full vaccume on the distributor
and increase the rpm to 1,500, which is enough to allow tthe fan to to cool
the engine.

> The original carb on my early 1974 GMC did not have an idle up solenoid.
>
> I made the following mod about 10 years ago. My memory sucks so the
> specifics may be a bit off but the mod still works fine to this day.
>
> So what I did was use (I think) an egr solenoid from a GM 4.3 and blocked
> off the vent port. This left me with a 2 port air valve, flow when power is
> applied, blocked when power is off. Wired the coil to be energized when
> the ac compressor is on. Vacuum lines, one from manifold to supply vacuum
> and
> the other teed into the line going to the vacuum advance. On the ported
> vacuum port in the carb base, I filled it with jb weld and drilled it to
> something small, like maybe a number 58 or so drill bit.
>
> What this does is at idle with no ac the timing is not vacuum advanced.
> RPM's are 580 to 600 in drive. Off idle and the port is exposed to vacuum
> and
> the vacuum can operates like it always did and the timing vacuum advances.
> At idle if the ac is turned on the electric valve opens and applies full
> vacuum to the vacuum can and the timing vacuum advances. Idle RPM's go to
> 620 to 640 in drive. The slight vacuum leak from the ported vacuum port
> leans the mixture about .1 (point one) on the fuel ratio gauge. Off idle
> the ported vacuum port gets vacuum and, in addition to the vacuum coming
> thru
> the electric valve, the vacuum can works like you would expect from a
> full-time vacuum setup (which it is). If the ac cycles on and off at idle I
> can
> barely notice the difference in engine speed, listening very carefully I
> can hear the change in the exhaust note.
>
> I'm using a Crane adjustable vacuum can so everything is tuneable. I
> couldn't tell you how much advance or at what vacuum it starts at, I didn't
> write
> that down. All I know is that it still works fine after all these years
> and wonder why GM didn't do something like this originally (money?).
>
> JWID
>
> --
> Bob Heller
> 1974 X-Canyonlands 26ft
> Original 455 exc for timing chain,
> Rockwell intake, valve covers. 141k miles.
> Winter Springs FL
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
If I attempt to idle my coach 1500 RPM in gear with the brake on, it will quickly overheat. Hate to think what it's doing to the transmission. I
don't think I'd set it up that far. All the carbureted air conditioned stuff I had in the late 60s and 70s had a solenoid connected to the compressor
clutch which bumped the throttle a bit.

--johnny
--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased
 
The torque conver will take the higher RPM and will not tost the
transmission.
Your Service manual spells it out.

On Fri, Jun 15, 2018 at 8:12 AM, Chris Tyler
wrote:

> https://www.ebay.com/itm/Idle-Stop-Solenoid-Rochester-
> Quadrajet-Carburetor-Select-GMC-Applications-/331272757811
>
> one of these should do the trick. I recall some Caddys had a two stage one
> that closed the trotttle entirely to prevent dieseling.
>
> --
> 76 Glenbrook
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
I see idle solenoids available for 1975 Quadrajets that might work. Worth a shot anyway.
--
Mike & Apryl Ballard
Fair Play, Missouri
1975 Glenbrook
 
GM did exactly this on some 70s cars. I have a 76 Grand LeMans with Pontiac 350/2. There is an idle stop solenoid that increases idle speed about 50
RPM when AC is requested. With the added A6 load the result is no RPM LOSS. Keep in mind that 76 was the last year before CCOT compressor cycling
became the new standard. The solenoid needs enough oomph to bump the throttle each cycle start. When the ECMs started, AC request was through the
computer and the AC clutch relay. The IAC (or carb solenoid) was given a signal to increase RPM and the clutch was delayed .4 second to allow engine
RPM to rise as clutch was engaging
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
However this was used more on "anemic" 4 and 6 cyl models. With the 455 you should be able to set a good idle speed and be fine. I prefer the higher
end of the spec range for better cooling and oil throw from the crank up to pistons and walls to lube and cool.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
I think you're right. Our old Buick didn't have one. some of the others did. Set the idle correctly with the A/C off, then engage the compressor
and solenoid, and adjust the plunger bolt for the same idle as before. The dealer installation in a bud's Dad's '58 Chev had a pull knob to increase
the idle. Most were misadjusted to the point you could use them as a no - cancel cruise control.

--johnny

--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased
 
I may be mistaken but I think "some" cars from the thirties still had pull-rod throttles. Maybe called hand-feed as opposed to foot-feed which many old-timers called the accelerator pedal!

Mac Macdonald in OKC

Sent from my iPhone

I think you're right. Our old Buick didn't have one. some of the others did. Set the idle correctly with the A/C off, then engage the compressor
and solenoid, and adjust the plunger bolt for the same idle as before. The dealer installation in a bud's Dad's '58 Chev had a pull knob to increase
the idle. Most were misadjusted to the point you could use them as a no - cancel cruise control.

--johnny

--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased

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