Home » Public Forums » GMCnet » A/C cycles cold then warm, then cold again
| A/C cycles cold then warm, then cold again [message #92912] |
Thu, 22 July 2010 20:49  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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My A/C blows cold, then warm then back to cold again.
At first I thought it would go cold when then engine was under more load, going up a hill etc, and there was a vacuum switch cutting off the compressor so save engine HP. But I looked at the wiring diagrams and I don't see such a switch.
I also noticed the longer the drive the duty cycle of cold air seem to decrease to the point were it wasn't really blowing any cold air to speak of.
Am I low on A/C charge? My experience has been when a leak develops it just doesn't deliver any cold. It has always been like this since I have had it, 3 summer now, so I don't think I have a leaky system.
This is the first trip I've had during warm weather where I haven't had a major malfunction so I'm getting down to the little things now.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: A/C cycles cold then warm, then cold again [message #92917 is a reply to message #92912] |
Thu, 22 July 2010 21:17  |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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| RF_Burns wrote on Thu, 22 July 2010 19:49 |
My A/C blows cold, then warm then back to cold again.
At first I thought it would go cold when then engine was under more load, going up a hill etc, and there was a vacuum switch cutting off the compressor so save engine HP. But I looked at the wiring diagrams and I don't see such a switch.
I also noticed the longer the drive the duty cycle of cold air seem to decrease to the point were it wasn't really blowing any cold air to speak of.
Am I low on A/C charge? My experience has been when a leak develops it just doesn't deliver any cold. It has always been like this since I have had it, 3 summer now, so I don't think I have a leaky system.
This is the first trip I've had during warm weather where I haven't had a major malfunction so I'm getting down to the little things now.
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There is a thermostat mounted in front of the evaporator that is supposed to prevent icing.There are several possibilities--it may have come loose from the evaporator, it may be out of adjustment, it may be starting to ice or the system may be low on refrigerant which can aggravate icing. You can temporarily bypass the thermostat to see if that's it:
http://www.gmcmhphotos.com/photos/showphoto.php?photo=17174&cat=3351
Bob de Kruyff
78 Eleganza
Chandler, AZ
[Updated on: Thu, 22 July 2010 21:17] Report message to a moderator |
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| Re: A/C cycles cold then warm, then cold again [message #92931 is a reply to message #92912] |
Thu, 22 July 2010 22:34 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
Karma: 0
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mine does that too. i tried the jumper but it iced up and the air flow dropped to almost nothing. i think my problem may be low air flow or low charge. i removed the front cover of the box and the evaporator is clean and the thermostat is properly attached.
i know i have a slow leak.
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: A/C cycles cold then warm, then cold again [message #92935 is a reply to message #92931] |
Thu, 22 July 2010 23:17 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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| fred v wrote on Thu, 22 July 2010 21:34 |
mine does that too. i tried the jumper but it iced up and the air flow dropped to almost nothing. i think my problem may be low air flow or low charge. i removed the front cover of the box and the evaporator is clean and the thermostat is properly attached.
i know i have a slow leak.
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If it's low on charge, it will ice up as you describe.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] A/C cycles cold then warm, then cold again [message #92936 is a reply to message #92931] |
Thu, 22 July 2010 23:18 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Fred,
When you pulled the HVAC cover, did you also pull the evaporator out? Or
at least pull the resistor pack out and examine inside the cabinet behind
the evaporator a little? That's where a lot of trash accumulates -- I was
shocked when I pulled the evaporator to find so much blockage there.
Ken H.
On Thu, Jul 22, 2010 at 11:34 PM, fred veenschoten <fredntoni@cox.net>wrote:
>
>
> mine does that too. i tried the jumper but it iced up and the air flow
> dropped to almost nothing. i think my problem may be low air flow or low
> charge. i removed the front cover of the box and the evaporator is clean and
> the thermostat is properly attached.
>
> i know i have a slow leak.
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] A/C cycles cold then warm, then cold again [message #92953 is a reply to message #92936] |
Fri, 23 July 2010 07:20 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
Karma: 0
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Senior Member |
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| Ken Henderson wrote on Thu, 22 July 2010 23:18 |
Fred,
When you pulled the HVAC cover, did you also pull the evaporator out? Or
at least pull the resistor pack out and examine inside the cabinet behind
the evaporator a little? That's where a lot of trash accumulates -- I was
shocked when I pulled the evaporator to find so much blockage there.
Ken H.
On Thu, Jul 22, 2010 at 11:34 PM, fred veenschoten <fredntoni@cox.net>wrote:
>
>
> mine does that too. i tried the jumper but it iced up and the air flow
> dropped to almost nothing. i think my problem may be low air flow or low
> charge. i removed the front cover of the box and the evaporator is clean and
> the thermostat is properly attached.
>
> i know i have a slow leak.
>
>
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yes. i did pull the evap. all the way out. the back side has a thin layer of oily dust on about 1/4 of the surface. i think my air flow problems could be from those doors not sealing all the way or ducts loose behind the dash.
but that is all cab air flow. the evap. sees air direct from the blower asi understand it from your research.
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: [GMCnet] A/C cycles cold then warm, then cold again [message #92956 is a reply to message #92912] |
Fri, 23 July 2010 07:54 |
Richard Denney
Messages: 920
Registered: April 2010
Karma: 9
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On Thu, Jul 22, 2010 at 9:49 PM, Bruce Hislop <bruce@perthcomm.com> wrote:
>
> My A/C blows cold, then warm then back to cold again.
>
Mine behaves that way, too, when I'm low on charge. The coil ices, and the
thermostat shuts down the compressor to let it thaw, but low air flow
through the iced coil allows the air to be warmed up before it gets to the
vent.
I finally charged mine properly, by adjusting the idle speed to 1500 and
then charging the system to the proper low-side pressure (22 psi in the case
of HC12a). I had not been meticulous with the engine speed when checking it
before.
Rick "whose AC blows consistently cold and doesn't freeze when sufficiently
charged" Denney
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| Re: [GMCnet] A/C cycles cold then warm, then cold again [message #92973 is a reply to message #92956] |
Fri, 23 July 2010 11:58 |
Neil
Messages: 271
Registered: July 2007
Location: Los Angeles and Magalia, ...
Karma: 1
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snip "The coil ices, and the thermostat shuts down the compressor to let it thaw, but low air flow through the iced coil allows the air to be warmed up before it gets to the
vent."
I have what is probably a stupid question.
Whatis the bad thing that happens if you jumper the sensor and icing occurs? Does cooling stop? Things break? Air can't get through the iced coil?
Becuase my a/c was doing that on the last trip and all cold all the time sounds great. I see a jumper or a by-pass switch in the future...
Neil
76 Eleganza now sold
Los Angeles
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| Re: [GMCnet] A/C cycles cold then warm, then cold again [message #92975 is a reply to message #92973] |
Fri, 23 July 2010 12:16 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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If you live in a humid area and you jumper the thermostat, there's a good chance it will ice and block the airflow. The fan will be trying to run against a dead head. The compressor will continue to run, but there won't be any heat transfer. I doubt it will hurt the system and I don't think the head pressure will be a problem, so long and short, I don't think it will cause anything to fail.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] A/C cycles cold then warm, then cold again [message #92984 is a reply to message #92980] |
Fri, 23 July 2010 14:10 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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BUT, without the thermostat, the entire evaporator will probably wind up in
the center of a big hard block of ice that won't melt for a long, hot time,
during which there will be NO air flow.
With the thermostat shutting down the compressor periodically when a little
ice forms, the air can continue to flow, melting the ice while continuing to
cool you off at least a little.
In other words, while the condition you have with the thermostat cycling
between warm and cool flow, without it, you'll probably soon have no flow.
Best to get the charge back up to normal and leave the thermostat alone.
Ken H.
On Fri, Jul 23, 2010 at 2:41 PM, Neil Martin <nmartin@hfbllp.com> wrote:
>
>
> Thank you Bob.
> --
> Neil
> 76 Eleganza
> Los Angeles
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List Information and Subscription Options:
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] A/C cycles cold then warm, then cold again [message #92987 is a reply to message #92984] |
Fri, 23 July 2010 14:43 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""BUT, without the thermostat, the entire evaporator will probably wind up in
the center of a big hard block of ice that won't melt for a long, hot time,
during which there will be NO air flow.""
When I still lived in humid Michigan, I had both the overhead as well as the chassis AC ice up and it does take forever to defrost--not a fun thing. Luckily it isn't much of a problem in AZ although the monsoon season and high humidity is upon us. It's interesting that in this high heat, even 30% humidity feels like a killer!
Bob de Kruyff
78 Eleganza
Chandler, AZ
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