Question about Chevrolet S 10

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Air only coming out of the defroster on 1991 Chevy S10 , but not switching over to the vents. Traced a good vacuum into and out of the tee going into the firewall. Wanted to know if I could be supplied with a vacuum diagram that controls the vent doors.

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  • yadayada
    yadayada May 25, 2011

    Replace the vacuum mode control switch.


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Sounds like door is jammed by debris(PENCIL/PEN ,PEN CAP,ETC) -might need to disassemble dash and heater box- if have good vac pressure at controls and they are working right....

Posted on May 25, 2011

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1 Answer

Air just comes out of vents, won't switch to floor or defrost, 2001 chevy blazer

-Sounds like it control by vacuum. Check vacuum line. It might be
loose or bend.

Good luck

mai Lor

Oct 07, 2013 | Chevrolet Blazer Cars & Trucks

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Air conditioner will only blow air out the defroster vents.

Defrost is the default when the controller is not receiving vacuum from the intake manifold. Look for the small vacuum line going from the engine intake manifold to the firewall and trace it to the control switch. If the vacuum line looks OK, try to pull the vacuum connection from the control switch. Then start the car and see if there is vacuum coming to the connector from the line that you traced from the engine. If no vacuum, the line has a leak--retrace to the manifold. If there is vacuum, your switch is probably leaking. You can buy a new vacuum switch from Ford or some parts stores. More questions, please let me know.

Sep 09, 2010 | 2004 Ford Crown Victoria

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My 96 dodge ram 4x4 has a vacuum leak somewhere the air blows from the vents until i accelorate then it shifts to the winshield defroster vents once i let off the gas it switches back to the vents at...

there are two small black vacuum lines near the A/C accumulator. These Tee into a single line that goes to the intake plenum. If you install a vacuum check valve in the one that comes from the intake it will solve your problems. Just did it and fixed mine that was doing the exact same thing......Good Luck

Aug 07, 2010 | Dodge Ram 1500 Cars & Trucks

1 Answer


yes vacuum lines can cause this when system looses vacuum it defaults to defrost there is a vacuum line going thru firewall usually near blower motor it has direct manifold vacuum on it check it follow it back to source make sure not cracked split or fallen off

Jul 25, 2010 | 1991 Buick Riviera

2 Answers

91 ciera only does defrost, no vents or AC

i believe this is a vacuum operated system. if there is a problem with the vacuum supply to the control unit, it will automatically default to defrost. i would start by checking the vacuum source lines. good luck!

Mar 22, 2010 | 1991 Oldsmobile Cutlass Ciera

1 Answer

I have a 91 s10 Blazer 4.3 4x4. I need to find out where the vacuum lines go from the switch on the transfer case to the engine go. When I got the Blazer the vacuum lines were gone.

You should go to a salvage yard and get the plug that plugs into the top of the transfer case.It is a 3 line plug and it is a molded rubber plug.The lines go to the actuator.Which is under the battery tray.The other goes to engine vacuum.The other tees off to the vent pipe on the rear of the transfer case and atmosphere.That`s why I suggested the salvage yard.Hope this helps.Good luck.

Dec 20, 2009 | 1991 Chevrolet S-10 Blazer

1 Answer

1998 Chevy S10 pickup air condition compres is on instead of heat

First be sure that you do not have the controls set to defrost the windows since on just about all vehicles nowadays with the controls set that way the air conditioner will be turned on...even if it is not so warm outside. sure that you do not have any of the controls set to defrost and that the heater is turned all the way on.

It sounds like though that the control for the vents is not operating properly. This is a combination of mechanical movements of the vent flaps and small "air motors" that operate the flaps. These air units are controlled by air vacuum taken from the engine compartment through tiny hoses. If the hoses get cracks, holes or come loose from their fittings then your vents will not move correctly so it is a matter of looking and tracing each one of the hoses to make sure they are fastened correctly, have not come loose and are not cracked or leaking. The problem can even be in the engine compartment where the hose(s) connect to the engine vacuum it will take lots of looking carefully with flashlights etc....good luck.

Aug 21, 2009 | 1998 Chevrolet S-10 Pickup

4 Answers

Heating / air conditioning control stuck to front defrost vents

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

Jun 19, 2009 | 1999 Dodge Durango

1 Answer


when your car is in defrost it has an ac cycling it removes the moisture then sends the air over the heater core depending on how warm you set it you can feel warm or cold but the ac is always cycling...ridding the need a good ac tech it will be okay...its just expensive going into the heater core if needed...!

May 07, 2009 | 2000 Chevrolet Astro

1 Answer

The blend doors on a chevy s10 are not working. air is coming out of the defrosters and the floor vents

i had the same problem on my 95 s10. there is most likely a vacuum line that came loose under the hood near the firewall on the driver side. that solved my problem.

Aug 10, 2008 | 1995 Chevrolet S-10 CA Edition

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