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#4490261 - 08/16/17 03:15 PM Air out of dash and feet only
dlundblad Offline


Registered: 09/30/13
Posts: 10002
Loc: Indiana
On my Chevy blazer.

It looks like it's all vacuum actuated. I believe it may be the AC/ heater control switch. Even with the dial turned to off, a bit of air still comes in at the feet when the vehicle is cruising at highway speeds. I just need to take the trim on the dash out so I can get to it. The lines are small so checking it may be difficult.

Rock Auto has the part for $140 shipped. Anyone have any cheaper alternatives? There's always junkyards, but how do I know the one I pull isn't faulty either?
_________________________
03 Jeep WJ 4.0 199k Castrol Edge 10w40 HM Fram XG16
02 Volvo S60 2.4T 177k M1 0w40 Mahle OX149D
97 Chevy Blazer 4.3 145k Rotella T5 10w30 Fram TG3980


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#4490264 - 08/16/17 03:19 PM Re: Air out of dash and feet only [Re: dlundblad]
JMJNet Offline


Registered: 09/06/07
Posts: 931
Loc: TX, USA
Sometimes it is the motor.

The gear inside is cracked.
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#4490266 - 08/16/17 03:21 PM Re: Air out of dash and feet only [Re: dlundblad]
Padawan Offline


Registered: 08/24/08
Posts: 333
Loc: NJ
Does this happen even when the system is set to recirculate? Did it only recently start?

All of our vehicles in recent memory have allowed some air in through the vents at speed unless set to recirculate.

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#4490271 - 08/16/17 03:28 PM Re: Air out of dash and feet only [Re: Padawan]
dlundblad Offline


Registered: 09/30/13
Posts: 10002
Loc: Indiana
No re circulation.
_________________________
03 Jeep WJ 4.0 199k Castrol Edge 10w40 HM Fram XG16
02 Volvo S60 2.4T 177k M1 0w40 Mahle OX149D
97 Chevy Blazer 4.3 145k Rotella T5 10w30 Fram TG3980


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#4490272 - 08/16/17 03:29 PM Re: Air out of dash and feet only [Re: JMJNet]
dlundblad Offline


Registered: 09/30/13
Posts: 10002
Loc: Indiana
Originally Posted By: JMJNet
Sometimes it is the motor.

The gear inside is cracked.


Pardon my ignorance, but what motor? smile
_________________________
03 Jeep WJ 4.0 199k Castrol Edge 10w40 HM Fram XG16
02 Volvo S60 2.4T 177k M1 0w40 Mahle OX149D
97 Chevy Blazer 4.3 145k Rotella T5 10w30 Fram TG3980


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#4490273 - 08/16/17 03:29 PM Re: Air out of dash and feet only [Re: dlundblad]
John_VT Offline


Registered: 07/08/17
Posts: 43
Loc: Vermont
Won't go to defrost? I've had the rod pop off on my suburban.
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#4490274 - 08/16/17 03:31 PM Re: Air out of dash and feet only [Re: John_VT]
dlundblad Offline


Registered: 09/30/13
Posts: 10002
Loc: Indiana
Originally Posted By: John_VT
Won't go to defrost? I've had the rod pop off on my suburban.


I said dash, but I should have said defrost.

From what I've read, going to defrost and feet is a safety feature so you can still drive (and see) when whatever vacuum component fails.
_________________________
03 Jeep WJ 4.0 199k Castrol Edge 10w40 HM Fram XG16
02 Volvo S60 2.4T 177k M1 0w40 Mahle OX149D
97 Chevy Blazer 4.3 145k Rotella T5 10w30 Fram TG3980


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#4490376 - 08/16/17 05:37 PM Re: Air out of dash and feet only [Re: dlundblad]
mk378 Offline


Registered: 09/27/15
Posts: 1414
Loc: USA
It is almost certainly electric. By 1997 vacuum controls were obsolete.

If you turn the key on and don't start (so there's no engine noise) and push buttons on the control panel, do you hear gears grinding?

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#4490441 - 08/16/17 06:50 PM Re: Air out of dash and feet only [Re: dlundblad]
NHGUY Offline


Registered: 10/09/11
Posts: 5041
Loc: USA
Google "Mode door actuator".

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#4490496 - 08/16/17 07:55 PM Re: Air out of dash and feet only [Re: dlundblad]
Warstud Offline


Registered: 09/02/05
Posts: 5390
Loc: MIchigan
Originally Posted By: dlundblad
Even with the dial turned to off, a bit of air still comes in at the feet when the vehicle is cruising at highway speeds.

That's normal for GM vehicles.

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#4490515 - 08/16/17 08:09 PM Re: Air out of dash and feet only [Re: Warstud]
dishdude Offline


Registered: 11/14/08
Posts: 8730
Loc: Phoenix
Originally Posted By: Warstud
Originally Posted By: dlundblad
Even with the dial turned to off, a bit of air still comes in at the feet when the vehicle is cruising at highway speeds.

That's normal for GM vehicles.


Yes. Off on these vehicles defaults to floor and fresh air allowing air to flow through the system without the fan to prevent the air in the vehicle from getting stale.
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#4490580 - 08/16/17 09:17 PM Re: Air out of dash and feet only [Re: dlundblad]
clinebarger Offline


Registered: 12/19/13
Posts: 1902
Loc: Fort Worth, Texas
The lower Mode Door has "Minimum Flow Holes" that allows air flow from the outside with the system off, The Temperature Blend Door will still regulate the air temp.

All Air Mode doors are Vacuum operated.
The Temperature Blend Door is control by a electric actuator.

If you lose Mode functions.....Check the Vacuum line from the engine to the Vacuum Storage Canister, Check Valve, And the Vacuum line between the canister & firewall. The rubber lines under the hood will rot right off these trucks.
The interior vacuum harness is very reliable as are the mode actuators & switching valve.

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2006 Chevy 2500HD LBZ/Allison 1000
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#4490597 - 08/16/17 09:48 PM Re: Air out of dash and feet only [Re: mk378]
clinebarger Offline


Registered: 12/19/13
Posts: 1902
Loc: Fort Worth, Texas
Originally Posted By: mk378
It is almost certainly electric. By 1997 vacuum controls were obsolete.

If you turn the key on and don't start (so there's no engine noise) and push buttons on the control panel, do you hear gears grinding?


It really depends on what vehicle your reffering to....

My 2001 Camaro is Vacuum controlled Mode Doors, And Cable controlled Temp Door....Probably the most reliable HVAC control set-up.
A lot of the Subcompact cars still use cables to control all the door functions besides the Recirculation Door which is a simple Bi-Directional electric motor.
The OP's Blazer has Vacuum controlled Mode Doors, And Electronic Step Motor controlled Temp Door.

The 100% Electronic Step Motor controlled systems are the most common today & in my opinion is the least reliable control system.....But folks got to have their fully automatic dual zone temp control!!
_________________________
2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/Auto

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#4491187 - 08/17/17 03:33 PM Re: Air out of dash and feet only [Re: clinebarger]
dlundblad Offline


Registered: 09/30/13
Posts: 10002
Loc: Indiana
Originally Posted By: clinebarger
The lower Mode Door has "Minimum Flow Holes" that allows air flow from the outside with the system off, The Temperature Blend Door will still regulate the air temp.

All Air Mode doors are Vacuum operated.
The Temperature Blend Door is control by a electric actuator.

If you lose Mode functions.....Check the Vacuum line from the engine to the Vacuum Storage Canister, Check Valve, And the Vacuum line between the canister & firewall. The rubber lines under the hood will rot right off these trucks.
The interior vacuum harness is very reliable as are the mode actuators & switching valve.



Good to know the interior harnesses are very reliable. Looks very tedious to work on.

The check valve is another possibility. I'll recheck my hoses, but I replaced them hoping they were the problem.
_________________________
03 Jeep WJ 4.0 199k Castrol Edge 10w40 HM Fram XG16
02 Volvo S60 2.4T 177k M1 0w40 Mahle OX149D
97 Chevy Blazer 4.3 145k Rotella T5 10w30 Fram TG3980


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#4491193 - 08/17/17 03:40 PM Re: Air out of dash and feet only [Re: clinebarger]
Silverado12 Offline


Registered: 02/26/13
Posts: 3111
Loc: Central Virginia
Originally Posted By: clinebarger
Originally Posted By: mk378
It is almost certainly electric. By 1997 vacuum controls were obsolete.

If you turn the key on and don't start (so there's no engine noise) and push buttons on the control panel, do you hear gears grinding?


It really depends on what vehicle your reffering to....

My 2001 Camaro is Vacuum controlled Mode Doors, And Cable controlled Temp Door....Probably the most reliable HVAC control set-up.
A lot of the Subcompact cars still use cables to control all the door functions besides the Recirculation Door which is a simple Bi-Directional electric motor.
The OP's Blazer has Vacuum controlled Mode Doors, And Electronic Step Motor controlled Temp Door.

The 100% Electronic Step Motor controlled systems are the most common today & in my opinion is the least reliable control system.....But folks got to have their fully automatic dual zone temp control!!


Totally agree. I've replaced a blend door on my 2010 Mustang. Not only is it a PITA but one would think they should last longer than they do.
_________________________
12 Chevy Silverado ext. cab LS, 2WD 4.3 auto
16 Challenger R/T Shaker 5.7 manual B5 blue
07 Harley Dyna Street Bob 96 c.i. 6 sp.
00 Mustang GT auto

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