#1  
Old 10-20-2021, 03:27 PM
78PontiacB 78PontiacB is offline
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Default 78 Parisenne Brougham - New Member - Need HVAC Assistance..;)

Hello to everyone on this fine forum!
My wife and I have owned and loved Pontiac's all our life!
My first car was a '73 Grand Prix with the 455....miss that car...
Since we are older - we have come to love the ride, room and reliability of the 77-85 Parisienne models. Currently we have a 100% original 78.
I would really appreciate some direction within this forum to seek advice for:
- HVAC trouble shooting
- HVAC vacuum schematics
- dash disassembly instructions
I would appreciate explaining our symptoms on the right threads, and hoping to gain some valued advice from experienced individuals!
Thanks all in advance!

  #2  
Old 10-20-2021, 04:37 PM
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gary bennett gary bennett is offline
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Pontiac has a separate A/C manual. try and get one. I have one for 63.

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  #3  
Old 10-20-2021, 05:36 PM
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77 TRASHCAN 77 TRASHCAN is offline
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Might be good to at least know the general area that you live in. Someone here might be able to help you.
Auto AC is a lot the same, on many vehicles, obviously not all, especially later models.
I'm guessing your refrigerant is low, and system obviously not working.
IF, theres any refrigerant left in the system,it needs to be recovered in an approved machine that can filter that refrigerant, for re-use. Nitrogen can be used to pressurize the system to find the leak(s). You may be able to hear the leak, not often though. THen it turns to spraying a mixture of soapy water on ALL the fittings and connections, looking for tiny bubbles. Make the repair then check it again. You may have found one of more than one leak

Of course, if you have no experience doing this, you will need some help.

Does the car still have R12 refrigerant in it?

If it's been converted, (to R134), that would be much better...

There's much more to the diagnostic story....

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  #4  
Old 10-20-2021, 06:17 PM
78PontiacB 78PontiacB is offline
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Thanks for the kind replies.
We reside near Slave Lake, AB.
My concern is not to restore the AC on this car as it has not worked for years and was rarely used.
I am trying to restore the heat and control activation.
Here are some symptoms:
Vent selector does nothing. Whether the leaver is on vent, defrost, off or heat, it blows only through the heat vent on the floor.
Temp selector does nothing. Moving it from 65 to 85 and or back to 65 changes nothing.
I do have some heat, but ONLY at idle. Once engine is off idle, heat turns cold - with no other control changes. Once engine is at idle some heat returns.
I will be removing the vacuum actuated heat valve and running the heater hose direct - with hopes it will restore strong heat.
I would like to find schematics and information as to how to permanently adjust the air flow to DEFROST and HEAT to MAX.
I would also like some information on how to take apart the dash to get at the controls/valves which would affect the above settings manually, without vacuum or electronic assist.
I have the glove compartment out giving me some look into the valves and actuators, but would like some factory schematics to identify what I am looking at.
Hoping for your kind and continued feedback!

  #5  
Old 10-20-2021, 07:31 PM
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I don't know much about a late 70's Parisienne but might I suggest you could also try a Chevrolet web forum as I think you will find a lot of the componentry is common to both models of the same year.

Just a suggestion

Ian

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  #6  
Old 10-20-2021, 09:29 PM
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Stuart Stuart is offline
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There's a 1978 Oldsmobile chassis manual here, that includes HVAC: http://www.oldcarmanualproject.com/m...sis/index.html Full size GM cars starting in 1977 shared a lot of commonality, so it may be useful. Air conditioning is in chapter 1.

  #7  
Old 10-21-2021, 09:11 AM
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On my old 78 Impala the no heat above idle was caused by a vacuum operated shut off valve in the short heater hose on the passenger side. It would stick closed with disuse. It would would start working again when I would manually operate it a few times. It was something I learned to do every spring, and most times I was under the hood.

As for the conditioned air only coming out the floor vents, indicate you have an issue with the vacuum that operates the system. The 1st thing I would check is the connection to the vacuum globe under the hood. I have found that the nipple the vacuum hoses attach to become brittle and tend to break off. You then loose all the vacuum in the system.

Good Luck and welcome

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Old 10-21-2021, 11:47 AM
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I agree with camerjeff and think the main vacuum line inside engine compartment might be disconnected or brittle rubber line. So make sure the hvac system has air going to it from engine vacuum.

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Old 10-21-2021, 12:16 PM
78PontiacB 78PontiacB is offline
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Quote:
Originally Posted by camerjeff View Post
On my old 78 Impala the no heat above idle was caused by a vacuum operated shut off valve in the short heater hose on the passenger side. It would stick closed with disuse. It would would start working again when I would manually operate it a few times. It was something I learned to do every spring, and most times I was under the hood.

As for the conditioned air only coming out the floor vents, indicate you have an issue with the vacuum that operates the system. The 1st thing I would check is the connection to the vacuum globe under the hood. I have found that the nipple the vacuum hoses attach to become brittle and tend to break off. You then loose all the vacuum in the system.

Good Luck and welcome
Thank you. Indeed there is a vacuum operated heat bypass valve on one of the hoses going to the heater core. I am going to remove that and plumb the hose directly. Hopefully that will restore FULL heat....!

  #10  
Old 10-21-2021, 12:18 PM
78PontiacB 78PontiacB is offline
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Quote:
Originally Posted by ta6point6 View Post
I agree with camerjeff and think the main vacuum line inside engine compartment might be disconnected or brittle rubber line. So make sure the hvac system has air going to it from engine vacuum.
Forgive me - however what would this Main Vacuum line look like, and Where on the engine would it be attached? Near the or on the carb? On the manifold perhaps? Where on the engine does this Main Vacuum Line get it's vacuum from? Many Thanks!!!

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Old 10-21-2021, 04:07 PM
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77 TRASHCAN 77 TRASHCAN is offline
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Look on top of the engine and on the carb, for a vacuum hose connected somehow to the fire wall area.
What engine?
You should be able to see where most of the vacuum hoses are. The air cleaner needs to be off, to do this. If it’s a 400’or 350, there’s a multi hose fitting that screws into the back of the carb base plate (lower part of rear of carb). The vac hose should some how make it’s way onto this fitting/connector.

Look all your vacuum hoses over, to insure they are OK.

I usually use a mirror to look under the dash, there’s no real easy way to poke around under there.

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1977 Black Trans Am 180 HP Auto, essentially base model T/A.
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Last edited by 77 TRASHCAN; 10-21-2021 at 04:16 PM.
  #12  
Old 10-22-2021, 10:45 AM
78PontiacB 78PontiacB is offline
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Will be removing/bypassin the heat bypass valve and will inspect vacuum hoses off carb and manifold and try and locate the vacuum canister this morning. Will report back this afternoon. Thx all!

  #13  
Old 10-22-2021, 11:21 AM
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I’ve never seen a 1978 AC manual
The big green 1978 repair manual has a whole AC section.

  #14  
Old 10-22-2021, 02:41 PM
78PontiacB 78PontiacB is offline
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Well I removed the heat bypass valve. Both heater hoses to the heater core are hot, still no heat.

I did find the broken vacuum line on the vacuum reserve tank and spliced it into a vacuum line on the bottom of the carb. This worked in restoring my dash controls, it now changes to Vent, Heater, Defrost etc....that function of the control has been restored.

But oddly no real heat is made even with the control level all the way to the 85F setting.. Just a luke warm at best. And again when the engine is off idle and or driving, the luke warm heat goes cold?!?!? How is that possible?

That is the most strange thing? What is the world is going on...? I have all controls back except to heat....could the heater core be plugged suddenly?? If it was then why would a little heat go cold off idle when the car is driving down the road...?

I'm stumped....

Many kind thanks!

  #15  
Old 10-22-2021, 04:25 PM
jerry455 jerry455 is offline
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The few times I have seen no heat at idle was when a car was low on coolant. It happened to my buddies M Van frequently. They had those darn plastic intake gaskets.

  #16  
Old 10-22-2021, 04:47 PM
78PontiacB 78PontiacB is offline
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PROBLEM SOLVED!

First of all - so many thanks to all of the replies, but one guy knew exactly what my problem was: so a big Shout Out of THANKS to member camerjeff!!

Earlier in this thread he experienced the exact same problem: no heat off idle in his 78 Impala. It was indeed the interior vacuum operated shut off valve sticking - to be the major problem. I have the glove compartment off and I can see when the temperature lever is moved it connects to a linkage at the bottom which is attached to a black vacuum operated valve. This valve linkage also has a spring return it appears....it seems to be on a track which is indeed dusty and dirty. It looks like it should be able to travel about 1.5-2 inches, yet is stuck in the middle...and so after some WD40 and pushing the valve in as far as possible, Heat is 100%.

So this HVAC system has two heater core vacuum valve bypasses, one right in the manifold behind the alternator on the heater hose (vacuum operated) and a second vacuum bypass valve inside, low down behind the glove compartment. This valve was also stuck and the track was filthy and dirty being completely exposed....

As well finding the broken vacuum line off of the vacuum reserve tank and plumbing it directly into a main vacuum line from the back bottom of the 4BBL carb - all is working perfectly.

Many many thanks to all the members on this fine forum!

  #17  
Old 10-23-2021, 02:00 PM
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Awesome! Now get that AC working.

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