...What exactly controls the different air/vent settings on these cars? I did a heater core replacement a weekend or two ago, and had forgotten I had this problem before I started. So of course it's still here now. I haven't had a chance to look at it yet. But as stated in the topic all the air blows out the defrost vents, no matter what setting I have the controls on. The slider for temperature control still works fine, I thought to test that while I had everything apart. But I forgot to look at the prob w/ the other controls. I assume there is some ducting and a flap or something that routes the air somewhere down there around the heater core. I'll check it out later today or next weekend I guess, just wanted an idea of what to look at.

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Ray87Z
-Vortec headed 350.
86 IROC w/ a cammed 305 TPI.
Formerly Ray86IROC.
www.inter-scape.com/Ray

 

In the engine compartment, usually going into the heater/AC box, should be a vac. hose.

This should have constant vac. going into the car.

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ASE Mechanic/Machinist/Smog Tech

1999 NBM Trans Am
1986 Chevy 3/4 ton pick up
1981 corvette
1995 Kawi ZX6R

R.I.P. #3
My favorite quote about D.E. "He can see air"

 

"Defrost Only" is a default mode. That way if something happens to the vacuum source, you can't sue GM for crashing your car becuase your windshield fogged up, no lie.

You need to inspect the vacuum line from its source at the intake manifold, down to the vacuum reserve canister, and up to the fire wall. If all looks good, pull the hush panel down on the passenger side and make sure you are getting vacuum at the pink vacuum line under the dash. More then likely, you have a broken or rotted out vacuum line, or a "T" or checkvalve is busted.

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If you live in Southeastern US, check us out!
South East Thirdgen


ASE Master Tech + L1
Savannah, GA

'87 Trans Am
S/D TPI retrofit including functional PassKey,
22# injectors,
Whatever chip I feel like burning,
JET AFPR, Ported Plenum,
TB Coolant Bypass, Custom Cold Air,
SSM SFC, KYB Shocks, Boxed LCAs, Wonder Bar,
8mm Accel wires,
Flowmaster Exhaust,
16" GTA rims,
Corvette Servo,
-->14.97 @ 94.9 MPH<--

'97 Bonneville SSE

 

i've saw several thirdgens have a leak in the "tree" on the back side of the circular **** on the right side of the control panel.

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ICON Motorsports

1st & 3rd

 

I fought the problem for several years without success. One day while speaking with the counter man at my local dealership, I mentioned the problem. His fix? Remove the vac harness where it connects to the AC/Heater Control head and swab Vaseline on the black plastic 'tubes' that slip into the harness. Reconnect. I'm smiling because the whole procedure took 30 minutes and cost $1.00. Yep, it worked.

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