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Question-dash air/vent fan [message #286906]

Tue, 08 September 2015 23:49

Scott Nutter

Messages: 782
Registered: January 2015
Location: Houston/San Diego

Karma: 4

Senior Member

Can any one tell me why the blower fan was designed to always be on low when the ignition is on? It's a 78 Royale. I was going through the manual and it never really explained why. Thanks, Scott.

Scott Nutter 1978 Royale Center Kitchen, Patterson 455, switch pitch tranny, 3.21 final drive, Quad bags, Dave Lenzi super duty mid axle disc brakes, tankless water heater, everything Lenzi. Alex Ferrera installed MSD Atomic EFI Houston, Texas

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Re: Question-dash air/vent fan [message #286907 is a reply to message #286906]

Wed, 09 September 2015 00:07

A Hamilto

Messages: 4508
Registered: April 2011

Karma: 39

Senior Member

Scott Nutter wrote on Tue, 08 September 2015 23:49

Can any one tell me why the blower fan was designed to always be on low when the ignition is on? It's a 78 Royale. I was going through the manual and it never really explained why. Thanks, Scott.

The positive pressure is intended to keep carbon monoxide out of the vehicle.

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Re: Question-dash air/vent fan [message #286908 is a reply to message #286906]

Wed, 09 September 2015 00:39

Scott Nutter

Messages: 782
Registered: January 2015
Location: Houston/San Diego

Karma: 4

Senior Member

It the normal thought to keep it that way? Or wire the switch to stop the fan? I am ok with both, but I don't want to over engineer either.

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Re: [GMCnet] Question-dash air/vent fan [message #286909 is a reply to message #286908]

Wed, 09 September 2015 01:10

USAussie

Messages: 15912
Registered: July 2007
Location: Sydney, Australia

Karma: 6

Senior Member

Scott,

With all due respect to the engineers at GMC I doubt very much if the blower on high speed would create a positive pressure in a GMC
let alone the low speed.

In the vernacular of the current generation the answer is; "whatever."

No disrespect meant! ;-)

Regards,
Rob M.
The Pedantic Mechanic

-----Original Message-----
From: Scott Nutter

It the normal thought to keep it that way? Or wire the switch to stop the fan? I am ok with both, but I don't want to over engineer
either.
--
Scott

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Regards, Rob M. (USAussie) The Pedantic Mechanic Sydney, Australia '75 Avion - AUS - The Blue Streak TZE365V100428 '75 Avion - USA - Double Trouble TZE365V100426

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Re: [GMCnet] Question-dash air/vent fan [message #286913 is a reply to message #286906]

Wed, 09 September 2015 05:22

Ken Henderson

Messages: 8726
Registered: March 2004
Location: Americus, GA

Karma: 9

Senior Member

Scott,

Mark's answer is correct. Just in case you, or anyone else interested,
don't know, disabling that feature is trivially simple: In the connector
at the fan speed resistor pack on the driver's side of the HVAC box beneath
the driver's hood is a yellow wire. Disconnect that wire by removing it
from the connector or bending over that pin on the resistor pack. You'll
no longer have the fan operating in the OFF position, with no other
side-effect.

Since that yellow wire is and AUX connection, it can be used as a
convenient source for add-on equipment.

HTH,

Ken H.

On Wed, Sep 9, 2015 at 12:49 AM, Scott Nutter wrote:

> Can any one tell me why the blower fan was designed to always be on low
> when the ignition is on? It's a 78 Royale. I was going through the manual
> and
> it never really explained why. Thanks, Scott.
> --
>
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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] Question-dash air/vent fan [message #286914 is a reply to message #286913]

Wed, 09 September 2015 06:06

tphipps

Messages: 3005
Registered: August 2004
Location: Spanish Fort, AL

Karma: 9

Senior Member

Ken's suggestion of using the yellow wire for 12v AUx source. I use the yellow wire to pull in one of the typical automobile relays and pass 12v directly from the battery to an AUX 12v panel. This panel provides power to both my JC4 pump and my wonderful Ken Henderson eletric windshield wipers ( plug). As long as the ignition is on, they are powered. If the JC4 is directly to constant 12v, it will run all day long when the pressure changes. That was the strange noise, several years ago.
The panel is protected by a 30 amp breaker, typical auto fashion. Panel is ATO style fuse block.

Tom, MS II

2012 Phoenix Cruiser model 2552 KA4CSG

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Re: Question-dash air/vent fan [message #286919 is a reply to message #286906]

Wed, 09 September 2015 07:10

Matt Colie

Messages: 8547
Registered: March 2007
Location: S.E. Michigan

Karma: 7

Senior Member

Scott,

The answer is actually very simple.
In the early 70's GM's HVAC engineers noted two things:
Most cars were sold with A/C.
Wind noise and water leaks at the vent wings (remember them) were a continuing issue.
They eliminated the vent wings. Owners complained about internal odors and stuffiness.
First they published a recommendation that the system be set to vent and the fan left on low. Even though the ram air ventilation could do it alone, many city drivers never got to the speeds that the ram ventilation worked well enough.
So, they reset the system so the vent was always open and the fan always ran. A no parts fix <= Cheap.

Matt

Scott Nutter wrote on Wed, 09 September 2015 00:49

Can any one tell me why the blower fan was designed to always be on low when the ignition is on? It's a 78 Royale. I was going through the manual and it never really explained why. Thanks, Scott.

Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

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Re: Question-dash air/vent fan [message #286927 is a reply to message #286906]

Wed, 09 September 2015 09:26

Chris Tyler

Messages: 458
Registered: September 2013
Location: Odessa FL

Karma: 7

Senior Member

As I recall GM promoted it as "Flow through ventilation" back in the 70s

76 Glenbrook

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Re: Question-dash air/vent fan [message #286928 is a reply to message #286906]

Wed, 09 September 2015 09:33

Scott Nutter

Messages: 782
Registered: January 2015
Location: Houston/San Diego

Karma: 4

Senior Member

Thanks everyone for the clarification. Now it's off with the yellow wire! Scott

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Re: [GMCnet] Question-dash air/vent fan [message #286931 is a reply to message #286928]

Wed, 09 September 2015 10:01

powerjon

Messages: 2446
Registered: January 2004

Karma: 5

Senior Member

Scott,
As other have said this modification has been done for years by others. Take a look at these pictures in this album.

http://www.gmcmotorhome.com/tech/heatfan/index.html

JR Wright
GMC Great Laker MHC
GMC Eastern States
GMCGL Tech Editor
GMCMHI
78 GMC Buskirk 30’ Stretch
1975 GMC Avion (Under Reconstruction)
Michigan

> On Sep 9, 2015, at 10:33 AM, Scott Nutter wrote:
>
> Thanks everyone for the clarification. Now it's off with the yellow wire! Scott
> --
> Scott Nutter
> 1978 455 Royale Center Kitchen.
> Houston, Texas
>
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GMC Eastern States
GMCMI
78 30' Buskirk Stretch
75 Avion Under Reconstruction
Michigan

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Re: Question-dash air/vent fan [message #286940 is a reply to message #286906]

Wed, 09 September 2015 11:18

JohnL455

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

Karma: 12

Senior Member

Then at a later point in time GM copied the Euro and Japan trend with the separate Rec button. Problem is lots of people don't know what that symbol means and run it in REC in winter with totally iced up windows on the Inside. Or they think it's cold out so I don't want cold outside air. So then they put it in Def which unless super cold out runs the compressor to waist fuel. Older system was much better as you could not REC in vent or heat. Vent in Rec isn't doing much venting

John Lebetski
Woodstock, IL
77 Eleganza II

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