Packard Forum Index » V-8 (1955-56)

I was driving my car the other day at night and the windshield started to fog up. I selected the controls on the dashboard for defroster but nothing happened. Luckily i had a big roll of paper towels which I was able to reach and wipe the windshield off. I obviously cant live with that situation forever. I recently had the car serviced at Ross Miller's garage and he said (after putting the car back together) that the heater was working which it was. I am wondering if I should replace the whole heater box under the dashboard to fix this problem. If so does anyone have one in good shape?I believe the proper name for this unit is the plenum chamber.

As the heater functions OK, chances are the problem is likely either with the cable controls for the defroster mode, or the ductwork that directs the airflow to the defroster outlets. A part of the ductwork is a pair of cardboard elbow and these haven't always survived in functional condition. A quick look under the dash should identify the problem.

In addition to the cardboard ducts O_D mentions there is a cardboard like flapper valve or air diverter inside the plenum. When the cardboard ducts are inspected also check to verify the small crank arm visible toward the right side of the exposed front surface of the plenum is moving when the knob is moved. If it is and ducts are OK then most likely the flapper material has come loose from the crank shaft and is not moving properly. The flap has to move down and cover the heater side before air can flow thru the defrost side.

If things look like they are working you can remove your plenum and take the cardboard back off to see what is happening or if there is a problem see if it can be repaired. Believe the back is stapled onto the plenum so you could take it off and fasten back with other means. You can also buy a new plenum from Quietrides if you don't want to mess with fixing yours. I would call and verify the flapper assy is present and operational in the new plenum because I believe when Cli55er was working with them on doing the repro it was a question if it would be included or if some of the old pieces would need to be moved.

Here is an illustration of the repro and also a bit from the service manual on the function of the flapper.

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Well hopefully on Monday which is supposed to be dry (it has been very wet here as of Late) I will pull the car out of the garage and remove the plenum from the car and open it up from the back to see whats going on inside. I think the flapper is damaged or missing since when i pull the control from defrost to heater there is almost no tension at all to it. That being the case i will fabricate a new flapper out of plastic or cardboard or plastic and plywood.

Well I was lucky the minute I stuck my head under the dashboard (nearly getting it wedged in there!!!!!!!) I saw the problem the clamp that held the cable to the plenum had come loose so there was no tension on the cable when switching it from defroster to heat. All i did was affix the cable to the clamp tighten the screw and I was good to go took me like 5 min. By the way thank you HH56 your diagram of the plenum showed me exactly where the problem was!