Question from Bret (1973):
I noticed that a hot air rear defrost was optional in 73. Was this system electrically or engine coolant heated?
I am certain it was a coolant heated system. Separate heater core and blower mounted near the spare tire under the package shelf.
I was not aware of a heated rear defrost. Not doubting anything just news to me. My 72 parts car had the rear defogger fan but it is just a fan that blows air from the heated interior to the glass through a little squirrel cage fan mounted below the package tray. My 57 had an identical unit so I suspect they were used for many years.
The same goes for my 1974... it just blows air from inside the car at the window. It works well once the car is warm.
I have seen this option in a 67. There is a heater core and blower mounted under the rear shelf. It is a real heater core with hoses and coolant not electric. There is one air inlet and one outlet to defrost in the shelf and two air ducts run under the back seat to two outlets on the floor under the seat cushion.
My 1967 also has this option...indeed, it is a wonderful thing.
My '68 LeBaron had this option. It involves some pretty long heater hoses. My '68 Newport also has this. It's nice to get the rear heat from under the seats. Warms the rear floor area faster than just the front heater would.
Question from Neil (1981):
Do all '81 - '83 Imperials have a wheezy demister or is something wrong?
The FSM talks about a small amount of air going to the screen (windshield) in demist, in my cars' case this is a bit of an understatement as I get
toasty toes but only a trickle up to the screen, which is ok on a long drive but not enough to clear a frosty screen on my short (5 miles) trip to work.
Only the export models have the wheezy demister option.
The FSM has an extensive section [Chapter 24] on diagnosing the vacuum control system. There is a heat-defrost door which directs the air to the appropriate outlet determined by the vacuum switch. The settings are "off," "A/C Max," "A/C/Vent," "bi-level," "heat," and "defrost." [according to the FSM] [No "demister."]
The answer to your question is that something's wrong. You probably have either a vacuum switch problem or vacuum actuator or door problem, probably the heat-defrost door. The FSM has a flow chart at page 24-23 in addition to the detailed troubleshooting.
Don't know where you read about "a small amount of air going to the screen (windshield) | in demist." What I read [24-19] is, "Interior windshield fogging can be quickly cleared by operating in Defrost mode."
All mopars used what is called a vacuum motor to actuate the flaps that direct the air up and down in the heater box. You have a vacuum line off or the diaphragm(motor) rubber is full of holes, or the actual linkage from motor to door has fallen off. Chrysler used vacuum controls way back. My earliest example was a 62 Imperial. Check it out.
My 1983 has no problem demisting/defogging the windshield. Push the defrost button in (on far right), turn the fan on to low or medium, and the windshield is clear in minutes. The fan seems a bit noisier in defrost mode than either A/C or heat, but it works nicely.
By rights, when the defrost mode is chosen, the bulk of the air should be directed to the windshield. In the heat mode the bulk of the air should go to the floor with a small amount to the windshield.
There is a note in the 1981 service manual that states pulling out the DEF button provides defrost mode with the refrigeration off.
Adding to Bill's comment, the push buttons have to be pressed in first to activate the function desired, (including AC, Bi-Level etc.) and then pulled out to their "click" stop to operate the system in automatic temperature control mode without the compressor running.
In the case of trying to get the mist off the inside of the glass, however, I advise leaving the refrigeration "on" because the dehumidifying effect of the AC is a major contributor to clearing the windows. Since the little compressor used on these cars uses only a minuscule amount of power from the engine, I usually leave mine on all the time, even in the winter, as the dehumidifying effect makes the car comfortable under any conditions, even in a cold rain (and I hope we'll see some of that this winter!).
My '83 is good, but I've seen the problem w/A-body mopars where the tubes directing the air to the windshield crack and then the air just dumps out on the floor.
"I advise leaving the refrigeration "on" because the dehumidifying effect of the AC is a major contributor to clearing the windows."
I heard this was a good practice because running the a/c in winter occaisionally oils the seals, compressor etc. to keep it in good working order. That way you don't have to remember to run it once in awhile or sit in 20 degree weather with the a/c on.