I've been learning a lot more about HVAC controls lately than I ever thought I wanted
to know. When the contol panel on my '85 T-Top car fell apart, I started looking around
for a replacement. Those that have done this know it's not always a simple task.
Here is a list of the differences, as much as I know right now. I have grouped them
into four basic versions, though there could be more that I'm not aware of. Note that
these are all for cars with factory air conditioning. The panels for cars with heaters,
or dealer-installed air conditioning, are completely different.
I would appreciate if anyone can help fill in the blanks.
First version, 1978-1980
The most visible difference with these units are the two illumination bulb holes between
the fan switch and rear defog or de-ice switch, if equipped. The upper of these two bulb
holes is for the de-ice or defog indicator, which is separate from the switch. The lower
of these holes illuminates the fan switch. The de-ice/defog switch lettering did not get
any backlighting. The de-ice switch is a center-detent momentary contact switch, that
connects to a timed relay above the glovebox, which supplies current to the grid wires
on the inside of the back glass. The de-ice switch may also be labeled R.DEF. The defog
switch, if equipped, is a 3-position Off-Low-High switch. The rear defog option consists
of a blower unit with an electric heating element, that mounts in the rear package tray.
This version of the control panel also has an additional vacuum control that the '83 and
later panels don't have, that cuts vacuum to the heater core restrictor door when the
temp slider is all the way at the "cold" setting. The selector, temp, and fan switch levers
are about 1/8" longer than the earlier years. The later fan switch will fit, but the knob
won't stick out as far as the others. Backlighting on this panel is blue.
Second version, 1981-1982 (?)
These have a single illumination bulb hole between the blower and defrost switches.
The rear defrost indicator is relocated to the tip of the defrost switch knob. The defrost
switch is larger, and now contains the timing function. The separate timed relay over the
glovebox is eliminated. The rear defog blower option is also eliminated. Pictograms appear
for the fan and defrost legends. The selector, temp, and fan switch levers are shortened
about 1/8". The earlier fan switch will fit, but the knob will stick out farther than the rest
of the levers. As near as I can tell, the heater core restrictor door function was eliminated
in the HVAC box for 1982, replaced by a redesigned blend door, but the control panel still
had the vacuum ports for it. Backlighting is still blue.
Third version, 1983 (?) -mid 1985
Looks identical to the second version, except the backlighting is red. Second and third
version faceplates can be swapped to change the backlight color, and also to add/delete
the rear defrost switch. (Involves re-melting or gluing the posts.) The heater restrictor
door function is eliminated from the control panel.
Final version, mid 1985-1986
"Heat" and "Hi-Lo" functions are replaced with "Mix" and "Floor", which apart from having
the positions swapped, and names changed, have slightly different functions. "Heat and
"Floor" do exactly the same thing. "Mix" sends air to the floor and defrost vents, while
"Hi-Lo" sends air to the floor and A/C register vents. "Mix" also engages the compressor
clutch, while "Hi-Lo" does not.
I have also discovered the heater box end of the heater cable changed sometime
between 1984 and 1985. The early cable has the same style housing clips at both
ends. The later one has a round housing clip at the air box end.
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