AC issues

[berfinroy]

I've recently been struggling to replace the temp control on my 72 4.5 and thought I'd pass on what I've learned. Firstly, you probably don't need to replace it. If your compressor wont stay on pretty much all the time as it is supposed to in the most clockwise position (which was my problem), it is adjustable via a screw located behind the fiber panel on one end of the control. You MAY be able to access this screw by removing the small panel under the dash below the 2 AC controls, disconnecting the wires to the temp control, unscrewing the bezel on the front of the panel and moving the control around to get the fiber panel off. If not, removing the lower dash entirely is your only option. Here's how that is done.
Drop the under dash covers. Remove the tunnel cover--2 screws, one behind the gas pedal and one under the carpet on the upper passenger foot well. Remove the 2 screws under the right side lower dash section. There are 3 screws to the right of the steering column; one up underneath just to the left of the controls, one under the glove box up above the plastic part of the underdash and one on the outside wall of the foot well. Next you have to remove the glovebox in order to gain access to the capillary tube from the temperature control. 4 push retainers on the bottom and 2 on the top. You must push the plastic center shaft all the way through to get them to release. They are reusable...if you can find all the little plastic shafts that you pushed through! Then ther's the light; pull down from the right side. And the latch itself held in with 2 screws. The glovebox is pretty flexible and you can giggle it out. Now pull the capillary tube out of the evaporator housing (it goes in about 4 inches). now you can remove the lower dash in its entirety, although you may have to do a little tugging to break the time induced bond between the foam formed lower dash piece and the bright trim piece just above it.
The capillary tube is supposed to be cover by a plastic tube as it passes through the air delivery plenum. Mine was not, which means somebody messed with it before my time. I think that tube insulates the capillary tube from being excessively influence by the cold air in the plenum; its the part that sticks into the evaporator housing that is the primary temperature sensor. You my also find that a formed foam wedge on the right side of the exposed underdash under the aluminum panel is deteriorated or missing altogether. Its purpose is to keep cold air from escaping out of the plenum. Fashion something new and replace it! Your cold air flow out both the center vents and the dash vents will be noticeably improved.
The fan reostat is much easier to replace/repair. That can be removed by just removing the small underdash panel. If it is erratic, it probably just needs cleaning a little tweeking and a smidge of electrical lube in the moving contact parts.
Lots of words, I know. Hope this helps somebody.