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1976 No Heat

Started by cadillac91, November 03, 2018, 09:10:43 PM

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cadillac91

1976 Sedan DeVille:  Had to replace the heater core.  Now the only function that works is the automatic fan blower when the HVAC is in the off position and it is cold air.  The defroster will blow but it is cold air.  On the engine side the hose going into the heater core is hot.  The return line is cold.  When the heater core went bad I had steam coming out of everywhere.   

Cape Cod Fleetwood

I remember my '76 going through a lot of thermostats... YMMV.

\m/
Laurie
There are 2 kinds of cars in the world, Cadillac and everything else....

The Present -1970 Fleetwood Brougham

The Past -
1996 Deville Concours
1987 Sedan De Ville "Commonwealth Edition"
1981 Coupe De Ville (8-6-4)
1976 Sedan De Ville
1975 Sedan De Ville

The Daily Driver and work slave -
2008 GMC Acadia SLT *options/all

79 Eldorado

#2
You should make certain you remembered to hook up any vacuum lines and links which may have been removed/disconnected. I'm not certain what was used in '76 but there will be a temp door and it sounds like that may be in the wrong position. I also had a vacuum check relay go bad once but that resulted in seemingly no cold AC even though the AC was functioning perfectly. I would concentrate on those item you needed to remove or may have inadvertently removed during the heater core change.

You also mentioned the heater inlet is hot and the outlet is cold but how cold? You should have a vacuum operated valve in one of the lines. If it's like mine it should be normally open (open without vacuum present). If that is not operating or stuck you may have enough coolant getting past the valve to allow some small amount of flow and enough so you feel the inlet is hot but so little you essentially have no heat.

Your car is completely different than my '79 Eldorado but there is still likely a large duct bringing heat up from the box where the heater is located. In mine it's something like 6" square cross section. It slips in and tucks under a clip at the top and can be removed with 2 lower screws. If I remove that I can see the temperature mix door in front of the heater. If you have similar easy access under the dash you may be able to see what the door does in different positions.

Scott