Interior HVAC Plenum Part 1

I have two HVAC plenums to go, I finished the heater core plenum and decided to tackle the interior plenum next. These are all pretty roached so there is a high probability that yours will be in better shape. I guess these are more worst case scenario situations.

I picked one out of three to redo. They are all about in the same condition.

The other two will be part donors if need be.

SITREP: The foam is either missing, or falling apart. The rubber on the fresh air door is starting to delaminate from the steel door. The steel evaporator inset is rusting, the plastic case has cracks, but the air door servos are still good, so that's something.

First step is removing the main foam section to expose the rivets to drill out in order to separate the steel surround plate.

Then drill out seemingly a million other rivets to take the whole thing apart. Ford packed parts of this with Automotive Bedding Compound and it's a pain to remove and clean. It's just slightly less annoying to deal with than butyl.

This is after walnut blasting. The plastic is in decent shape, and I decided to fix the broken spots and just clear coat this one inside and out. Since this plastic plenum is on the inside of the cabin it sees less heat and hydrocarbon exposure so it isn't degraded as the engine bay side plenums.

Plus I did find a little bit of body seam sealer in here, which of course is Asbestos in a rubber binder from Ford. So it's a good thing to have that removed.

After countless hours of blasting, repairing and painting, here's the parts ready for reassembly.

Here's where I am going to do things a bit different than factory. I am going to use this very sticky insulating tape on parts of the steel evaporator inset.

The reason for this is this steel inset piece sees the cool air from the evaporator and on a muggy hot day this will sweat like mad in the plastic plenum. Now the unique design of the plenum exposes the back side of this steel inset to the defrost (windscreen passage) and heat (floor) passage. If this starts condensing water from the interior it will start raining water out of the heater vents from the plenum.

One problem I have noted with these air con systems is the evaporator temperature is controlled stritctly by the thermostat and not refrigerant pressure as a more modern vehicle would be. That means if the thermostat has a set point lower than freezing it will take the evaporator down that cold. In my '68 XL on full cold the air coming out of the dash vents is so cold it hurts your hands. Add a muggy day and it would be raining condensate from the back side of this metal insert and all over the carpet by your feet. In order to mitigate that, if I insulate the back and side, that should preclude that possibility.

This is the idea to start.

To get a better idea of what I'm talking about, that metal insert goes straight in. The upper air channel is the defrost to the windscreen section and the lower portion is the floor heat air channel. That metal insert divides three distinct air channels (heat/defrost/air con to centre dash vents). It's clever in an ignorant way. This is because whilst is a good use of limited area under the dash, the whole system is a bit retarded because you cannot get dehumidified hot air to the windscreen where it would really help reduce fogging than regular heated outside air, which of course is what you get with these 65-68 full size systems.

I am going to use a host of this generic aftermarket EPDM weatherstripping to put this back together.

Instead of packing the underside with gobs of nasty Automotive Bedding Compound I am going to use weather strip to seal it off.

Here's a better shot where you can see the lower heat passage formed by the metal insert and the upper defrost passage formed. Not less forgetting the obvious air con dash passage formed in the centre. This is a test fit.

Before you can install the big metal evaporator insert this side heat vent must be installed. Now this was all originally riveted together. I am choosing to use stainless fasteners rather that rivets for two really good reasons. First the plastic is aged and getting brittle and any unnecessary crimping force can split/crack it. Secondly Ford used Rosette style rivets because they spread out the load and do not crimp as hard. Those rivets are very hard to find and expensive when you do find them. The stainless hardware I used is expensive but easy to source and I can control how much clamping force to use.

I decided to acorn nuts because I still have loads of custom wiring to do in this confined dash space and one of pet peeves is sharp items cutting my hands. Using regular nuts or KL nuts would probably lead to that. Plus I think the acorn nut looks nicer.

Now this can get installed.

Continued in next post.