1977 TransAm F Series heater issue - Second Generation Pontiac Firebird (1970 - 1981) - Firebird Nation

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1977 TransAm F Series heater issue


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10 replies to this topic

#1 mobess1954

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Posted A week ago

Recently changed-out heater core in 77 Pontiac TransAm. All is good except  have  high heat all the time. Took the driver-side seat and console out today to check function of heater/AC controls. Car has AC but is inoperable. Sliding the heater controller on the dash left/right appears to open/shut the inside heater door in the heater case assy. All pneumatic tubing is terminated properly -- positioning the heater/AC selector from  OFF/NORM/BILEVEL/VENT/HEATER/DEFROST appears to change setting of outside heater case door. Heater door control latch is OK.

 

After 10-15 minutes run time have high heat at the heat box even with heater selector in OFF position or in any position.

 

Any comments or direction to relevant tech info on this topic would be welcome.

 



#2 ZumpTA

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Posted A week ago

If I'm understanding correctly, there is a valve underhood that prevents/allows coolant flow into the heater core.  It may be stuck open, worn out, or it's vacuum line has issue.


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#3 b_hill_86

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Posted A week ago

Those valves were only on early cars I believe. Not on a 77.

When you say high heat I assume you mean hottest air and not necessarily high blower setting. My guess would be the door connected to the cable which is actuated by the HOT/COLD slider isnt adjusted correctly and/or is not closing completely when moved to cold. Thats where I would start.

The vacuum actuated blend doors which are controlled by the upper selector (OFF thru DEF) only effect the recirculate door, and otherwise direct the air to the floor/defrost or vent.

Also, its just worth mentioning the fan blower is always running, even in the off position so if your HOT/COLD door isnt closing all the way it will draw air through the hot heater core.

#4 NOT A TA

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Posted A week ago

Vacuum operated shut off valve when AC is turned on was only on early 2nd gens as Brian mentioned.

 

Often the plastic screw down tab on the blend door cable breaks off where the cable mounts to the heater box and although the door still moves and seems operational it doesn't close completely.

 

Also (since you're poking around under the dash looking at the heater box pieces) don't be alarmed when you realize there's a gap between the heater box and the windshield defrost ducting, it's supposed to be like that.

 

You can put a manual shut off valve in the heater hose. Open for cool months and keep closed during summer.

 

And as Brian also mentioned the fan never stops completely, you can't shut it off. It will always run on slow speed as part of the Astro Flow Through Ventilation system.


Edited by NOT A TA, A week ago.

John Paige

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#5 mobess1954

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Posted A week ago

The nylon tab is not broken. Made slight adjustment to the blend door cable at the cable adjuster – just 2-3 revs. That was enough to get the blend door fully closed with temp lever setting on COLD – now no more heat coming thru. See attached photo.

 

I probably disturbed the cable tension during the process of getting the heater case in and out for core change out. 

 

Thanks Tony/John/Brian for the prompt replies and very helpful analysis.


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#6 b_hill_86

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Posted A week ago

Good deal glad you got it figured out.

#7 NOT A TA

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Posted A week ago

The nylon tab is not broken. Made slight adjustment to the blend door cable at the cable adjuster – just 2-3 revs. That was enough to get the blend door fully closed with temp lever setting on COLD – now no more heat coming thru. See attached photo.

 

I probably disturbed the cable tension during the process of getting the heater case in and out for core change out. 

 

Thanks Tony/John/Brian for the prompt replies and very helpful analysis.

Excellent!  Love it when folks let us know the outcome.


John Paige

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#8 mobess1954

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Posted A week ago

I have a photo showing all this -- but can't figure out how to attach?



#9 Injuneer

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Posted A week ago

Right below "reply to this topic" text box, click on "More Reply Options" (below right corner)

 

"Attach Files" will appear right below text box (left corner)

 

Click "Choose File"

 

Select a photo from your device

 

Click "Attach This File".


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#10 mobess1954

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Posted A week ago

Attached photo shows top side of the heater case with blend door cable & adjuster.

 

Attached Files



#11 JJBroc

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Posted A day ago

Hey Mobess,
How did you do with the heater core removal??
Is your car all together. I am putting mine back on the road after years of sitting. Contemplating changing the core and hoses just to prevent issues.
Lower hose under the hood look like a pain to get to??
My car is all together.
Had to vacuum out a mouse nest. So I have the heater duct work out so I am half way there under the dash.




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