Author Topic: 1966 HVAC question - heat won't come on without first going to defrost  (Read 779 times)

Offline scotth3886

  • Posts: 738
  • CLC Number: 32524
  • Name: Scott Harris
I have a 66 Fleetwood with ATC and did manage to get the A/C working really well.  However, winter is getting here and I have a heating question.  I go to cars & coffee every Saturday morning and now that it's in the 20s, I thought I'd better ask this question.

I know that the heat doesn't begin to blow until coolant temp is 120 degrees.  Even when above that temp, mine won't turn on until I move the control lever to defrost and then back to one of the two heat settings.  Then it works fine until the next time I have to start the car then I have to it over again.  The temp seems to vary like it should when I turn the dial down from 85.  At 75 or so, it starts blowing cooler air out the dash vents so at least that works.  Turn it down to 65 and its full A/C. 

Not a big issue, but I like everything to work correctly.  One other thing is I can't tell much difference between the low and high-speed fan setting so something seems to be not quite right here. 
« Last Edit: November 28, 2021, 01:26:49 PM by scotth3886 »

Offline cadillactim

  • Cadillac Tim
  • Posts: 799
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #1 on: November 28, 2021, 08:55:51 PM »
Very common issue. The problem is vacuum leaks. In the settings other than Defrost there is too much vacuum leaking off to overcome the spring pressure in the master vacuum switch. Once that spring pressure has been overcome there is barely enough vacuum to keep it closed when the lever is moved back to Automatic. But once the vacuum leaks more (like under acceleration), the switch shuts back off.

Most common area of vacuum leaks are the rotary valves in the power servo. But there could be other leaks elsewhere.

Tim
Tim Groves

Offline scotth3886

  • Posts: 738
  • CLC Number: 32524
  • Name: Scott Harris
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #2 on: November 29, 2021, 02:05:21 AM »
Very common issue. The problem is vacuum leaks. In the settings other than Defrost there is too much vacuum leaking off to overcome the spring pressure in the master vacuum switch. Once that spring pressure has been overcome there is barely enough vacuum to keep it closed when the lever is moved back to Automatic. But once the vacuum leaks more (like under acceleration), the switch shuts back off.

Most common area of vacuum leaks are the rotary valves in the power servo. But there could be other leaks elsewhere.

Tim

And I thought I had all of this fixed, but I haven't replaced the power servo yet.  I was going to wait until spring to do that.

IIRC, you rebuild power servos, correct?  Problem is that I have to wait for a day warm enough for these nearly 80 yr old hands to do the work.  When it finally got cold here it has stayed cold.
So it can take me a few to get the core back to you. 
« Last Edit: November 29, 2021, 02:10:14 AM by scotth3886 »

Offline Larry Savino

  • Posts: 28
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #3 on: November 29, 2021, 06:01:56 AM »
Although I cant answer your question.and I don't have a Classic Cadillac yet (I am searching for a 1965 thru 1968 convertible) I did find a guy who sells manuals year specific to diagnose these complicated HVAC systems . I think I paid 49.00 plus shipping I can post he contact info if you would like

good Luck I had a 67 D
eville in the 90's and I was always chasing gremlins out of the HVAC system

Offline cadillactim

  • Cadillac Tim
  • Posts: 799
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #4 on: November 29, 2021, 07:15:35 AM »
Yes, I sell rebuilt power servos. You can reach me directly at 864-348-7732, or email me at   tagroves@wctel.net    for more information.

Tim
Tim Groves

Offline scotth3886

  • Posts: 738
  • CLC Number: 32524
  • Name: Scott Harris
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #5 on: November 29, 2021, 07:55:31 AM »
Although I cant answer your question.and I don't have a Classic Cadillac yet (I am searching for a 1965 thru 1968 convertible) I did find a guy who sells manuals year specific to diagnose these complicated HVAC systems . I think I paid 49.00 plus shipping I can post he contact info if you would like

good Luck I had a 67 D
eville in the 90's and I was always chasing gremlins out of the HVAC system

You did indeed.  He's Cadillac Tim who is posting in this thread.

Offline scotth3886

  • Posts: 738
  • CLC Number: 32524
  • Name: Scott Harris
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #6 on: November 29, 2021, 07:57:08 AM »
Yes, I sell rebuilt power servos. You can reach me directly at 864-348-7732, or email me at   tagroves@wctel.net    for more information.

Tim

Tim, thanks.  I'll give you a call. 

Offline JV3RI

  • Posts: 5
  • CLC Number: 24496
  • Name: JimV
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #7 on: November 29, 2021, 07:37:12 PM »
and my other project ...... finding and replacing every inch of vacuum hose in my 66 Brougham.  Got the manual from Cadillac Tim - thanks Tim. Got all the hoses changed - even the hoses and accuators behind the kick panels, under the carpet, in the door panels, under the dash, under the hood. That got the power door locks working again. It also helped smooth out the idle.  Everything was working and the idle and acceleration were great. Then, just a few days later - with all that brand new rubber hose everywhere - the engine starts running extremely rough. So rough that the restoration shop refuses to start it again for fear of really damaging something. All this vacuum work was done this year - and last year we hauled the engine out and did a full top to bottom rebuild.  Not sure what to do next - call the Exorcist ??

Offline cadillactim

  • Cadillac Tim
  • Posts: 799
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #8 on: November 29, 2021, 08:23:36 PM »
Pinch off the vacuum lines coming from the intake manifold, including the one to the brake booster. If engine smooths out, then trace vacuum leak. If engine still runs rough, spray some carb cleaner around the intake and see if idle improves. If it does you may have a cracked intake manifold.

Tim
Tim Groves

Offline scotth3886

  • Posts: 738
  • CLC Number: 32524
  • Name: Scott Harris
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #9 on: November 29, 2021, 09:20:51 PM »
Pinch off the vacuum lines coming from the intake manifold, including the one to the brake booster. If engine smooths out, then trace vacuum leak. If engine still runs rough, spray some carb cleaner around the intake and see if idle improves. If it does you may have a cracked intake manifold.

Tim

Not sure if you're replying to me or JV, but I'll reply anyways.  I've replaced the intake twice.  The last time I used the block off gasket from Olsens and that stopped the issue.  Although it does make it a little more challenging to get going on a 20-degree morning without lengthy idling to warm things up a little.  Idles pretty well overall.  Not perfect, but better than it has since I've owned the car. 
« Last Edit: November 29, 2021, 09:23:33 PM by scotth3886 »

Offline scotth3886

  • Posts: 738
  • CLC Number: 32524
  • Name: Scott Harris
Re: 1966 HVAC question - heat won't come on without first going to defrost
« Reply #10 on: November 30, 2021, 02:45:41 AM »
and my other project ...... finding and replacing every inch of vacuum hose in my 66 Brougham.  Got the manual from Cadillac Tim - thanks Tim. Got all the hoses changed - even the hoses and accuators behind the kick panels, under the carpet, in the door panels, under the dash, under the hood. That got the power door locks working again. It also helped smooth out the idle.  Everything was working and the idle and acceleration were great. Then, just a few days later - with all that brand new rubber hose everywhere - the engine starts running extremely rough. So rough that the restoration shop refuses to start it again for fear of really damaging something. All this vacuum work was done this year - and last year we hauled the engine out and did a full top to bottom rebuild.  Not sure what to do next - call the Exorcist ??

You or your mechanic didn't happen to have the intake off, did you?  Earlier in my ownership with my 66 Brougham my mechanic did this by accident and got the order of shims, spacers, etc between carb and intake backwards and my car ran awful.  I mean really awful enough that I thought the engine was going to have to come out.  I didn't yet know that the spacer, shims and everything had to be installed a certain way.   Thanks to this forum I soon found out. 

 

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