Defrost and Air Conditioning

[SteveW]

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

 

[jerry burt]

Yes, the air passes through the AC coils befor it goes to through the heater coils.
--
Patti & Jerry Burt Fresno, CA.
73 Gmc 26' Canyon Lands -
77 Palm Beach
Members: FMCA - GMCMI - GMCWS - 49ers
A truly happy person is one who can enjoy the scenery on a detour.

 

[SteveW]

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.

 

[dave stragand1]

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.

>
> 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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[john r. lebetski]

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

 

[john r. lebetski]

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