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Defrost and Air Conditioning [message #337964]

Wed, 17 October 2018 21:49

SteveW

Messages: 538
Registered: June 2005
Location: Southern California - Ora...

Karma: 1

Senior Member

Greetings !!

We had a lively discussion at work today - about air conditioning engaging when one selects defrost on automotive HVAC systems. Dehumidifying the air being the main objective.

I agreed that "modern" cars do so and took a position that the vehicles from the 70s don't.

Might any of y'all know the specifics of when this started ?? Does it work as such in our coaches ??

There's no betting money in the mix - but I'd sure like to be correct.

Thanks in advance,

Steve W
1973 23'
Southern California

Steve W 1973 : 23' Southern California

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Re: Defrost and Air Conditioning [message #337966 is a reply to message #337964]

Wed, 17 October 2018 21:59

pjburt

Messages: 437
Registered: February 2016
Location: Fresno, California

Karma: 1

Senior Member

Yes, the air passes through the AC coils befor it goes to through the heater coils.

Jerry Burt Fresno, CA.
73 Gmc 26' Canyon Lands
Members: FMCA - GMCMI - GMCWS
A truly happy person is one who can enjoy the scenery on a detour.

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Re: Defrost and Air Conditioning [message #337967 is a reply to message #337964]

Wed, 17 October 2018 22:22

SteveW

Messages: 538
Registered: June 2005
Location: Southern California - Ora...

Karma: 1

Senior Member

Thanks Jerry -

Your are completely correct about the airflow - but does sliding the lever over to defrost cause the AC compressor to engage ??

My coach isn't at home so I can't go and immediately check.

😎

Steve W 1973 : 23' Southern California

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Re: [GMCnet] Defrost and Air Conditioning [message #337971 is a reply to message #337967]

Thu, 18 October 2018 06:47

Dave Stragand

Messages: 307
Registered: October 2017

Karma: 0

Senior Member

Yes. On GM vehicles with a/c that I have owned this was the case on all, from a 1968 Cadillac on up. The defrost position engaged the compressor circuit.

> On Oct 17, 2018, at 11:23 PM, Steve Weinstock wrote:
>
> Thanks Jerry -
>
> Your are completely correct about the airflow - but does sliding the lever over to defrost cause the AC compressor to engage ??
>
> My coach isn't at home so I can't go and immediately check.
>
> 😎
>
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1978 Transmode (403) Pittsburgh, PA

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Re: Defrost and Air Conditioning [message #337977 is a reply to message #337964]

Thu, 18 October 2018 08:52

palerider

Messages: 35
Registered: September 2013
Location: Kingsland , Texas

Karma: 1

Member

A GM rep told me years ago that they did that to keep the AC system in operational condition thru the winter weather. He said it saved a lot of AC warranty work once warmer weather started with an added benefit of dehydrated air. The rest of the manufacturers soon followed suit .....Tom

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Re: Defrost and Air Conditioning [message #337996 is a reply to message #337964]

Fri, 19 October 2018 08:27

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

It started mid 70s some time on GM. They added the ECONOMY script above the VENT and HEAT selections to show that the compressor would not run in only those two positions. All other posItions ran compressor above a sub freezing OAT

John Lebetski
Woodstock, IL
77 Eleganza II

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Re: Defrost and Air Conditioning [message #338013 is a reply to message #337964]

Sat, 20 October 2018 08:34

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

Let me qualify my statement and augment. If you opted for GM Comfotron Auto climate control the compressor ran (except below freezing). The system then blended hot and cold to achieve the desired cabin temp. My grandfather had a 67 Olds 98 LS and that was how it worked. So this preceded the manual AC running compressor on DEF by at least a decade.

John Lebetski
Woodstock, IL
77 Eleganza II

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