1966 Mustang Project: Heater Case Rebuild

After more than 30 years of service, your heater case is probably home to several pounds of dirt, leaves, twigs, dead bugs, and more fun stuff. The original foam seals are always long gone, allowing air to pretty much flow where it wants to.

Removing the case from your Mustang isn’t a hard job. Four mounting nuts on the firewall and a large screw above the fresh air door will get the job done. Don’t forget to disconnect or cut the heater hoses. Underdash air systems make the job harder but not impossible. Once the case is dropped down, you can carefully pull the cable retainers off.

Remove the set screw and clip to allow the cable to come free of the temperature, defrost, and fresh air doors. We decided to replace everything for a fresh start, but it’s possible to clean these cables up and reuse them.

With the heater case now out of the car and on a workbench, remove the four retaining nuts and carefully pry the blower motor assembly from the back of the case.

The squirrel cage is removed from the motor for detailing by loosening the set screw on the side of the motor shaft and carefully prying the cage off the shaft.

Our blower motor was the original one with the correct wiring plugs (notice the build date ink stamp). The motor was tested with a 12-volt source and worked fine, it simply needed a major cleaning and painting.

To get into the case, you will have to carefully remove the case retaining clips found around the circumference of the case halves. Different methods work for different people, but most have good luck with a screwdriver applied as shown to “roll” it off the case lip.

Inside, we found an original heater core with the date stamp and paint blotch on the end, along with dried seals and a bunch of trash. Remove the core and this metal brace. The fresh air door can be removed by removing the two screws that retain it to the actuating rod.

On the other half of the heater case is the temperature-blend door assembly. Remove this from the case by locating and removing the two retaining screws on the backside of the case.

Once the case halves have been gutted of all brackets and doors, wash the case halves with warm soapy water to remove any dirt and grunge from years of hiding under your dash. Our case halves were in perfect condition, but reproduction cases are now available if yours is severely damaged.

Our case had some paint overspray from someone’s quick dash painting job. Lacquer thinner and the careful application of a rag cleaned the case back to its bare fiberglass look. We masked off the mounting and cable brackets and refinished them with some Eastwood Spray Gray.

For the added detail touch, we handpainted all of the bracket rivets with Testors Bright Silver. The fresh air door is painted semigloss black, including the blower motor housing and other metal parts.

The original blower motor had ancient undercoating oversprayed on it and took some gentle but lengthy cleaning to come clean. The bare motor was then lightly scuffed and refinished with semigloss black.

All of our case clips were in good condition and none were missing. CJ has replacement clips if you’re missing some. We quickly cleaned our clips, then hit them with a coat of the semigloss black as well.

Most of our interior metal items had enough corrosion on them to make us worry. The only way we knew to fix the problem and our worrying was by sanding off as much crud as we could get to, then refinish the metal parts with Eastwood’s Corroless paint for rusty metal. No more worries.

The foam kit from CJ has all of the internal foam pieces to seal everything inside the heater case for all ’65-’68 models. Here, we have just applied the foam seal to the inside of the fresh air door.

Putting all the seals in the correct place on the temperature-blend door assembly can be tricky. Take your time and double-check positioning before applying your adhesive. Speaking of adhesive, we simply used trim adhesive in a spray can, but you can use weatherstrip adhesive.

This metal brace gets a seal on it as well to cushion the heater core and seal it properly.

Our new heater core is carefully placed into the firewall half of the case. You will notice the temperature-blend door assembly has already been reinstalled at this point.

Once the blower motor has dried sufficiently, you can reassemble it to the backing plate and the squirrel cage using the new rubber and foam seals provided in the kit.

Before reassembling the two case halves, inspect the case seam for old sealer and scrape it out of the seam. We applied a small bead of butyl sealer to the seam, then pressed the two case halves together. To aid in installing the case clips, a gentle squeeze of the case halves with a pair of pliers will help.

The new defroster plenums are made from plastic and not the original cardboard (if yours was even still in place). The plastic doesn’t look 100 percent correct, but it works better and will last forever. Besides, by the time you install your carpet and possibly A/C, you’ll never see it.

Reattach the blower motor assembly to the rear of the heater case assembly and you’re almost home.

Finish off the case restoration with the cowl seal that is included in the foam seal kit. Now you can set aside the case for a few minutes while we handle the control mechanism.

We checked our blower motor switch (and the resistor too) with an ohm meter; the switch checked out fine. We simply cleaned it with contact cleaner and rewrapped the original pigtail with the correct fiber tape.

We ordered all-new chrome heater control knobs and a faceplate for the hardtop. Before we installed our fresh chrome, we took a section of superfine steel wool and polished the control levers, since they were quite crummy; they are visible exiting the control faceplate.

Our controls were cleaned with contact cleaner and were in perfect working order. The controls were then reassembled with the new faceplate and knob kit for a Grade A, first-class look.

New cables make the heater controls work as smooth as day one. We opted to mount the control cables to the control assembly and the defroster, temperature, and fresh air doors on our workbench so that we could adjust the cables without the confines of the dash. Once you have the doors controlled, properly mark the cable with tape at the heater case end of the cable and remove them for installation.

Installing the firewall blanket is a tough job to perform by yourself. Get a helper to aid in the installation of the blanket. The original retaining clips are these long metal pins, which were still in our car. If you can locate some of these, use them for correctness.

Whenever I rebuild a heater case or replace the heater core, I insist on new hoses. These white stripe concours hoses will look great in our engine compartment. The correct tower clamps are being used here.

Again, a helper is a great friend in getting the heater case lined up with the firewall opening, sliding the heater hoses through, and getting the retaining nuts started. You can go it alone, but make sure you have a section of two-by-four or some other item to hold the case in place while you install the retaining nuts.

The controls with attached cables are slid into their home in our bare dash. Carefully guide the cables into their proper locations, ensuring they clear the wiper mechanism and anything else along their path.

Remember the cable adjustments you made earlier? Now all you have to do is slip the cable into place, line up the tape with your retaining clip (or other landmark), and secure the cable. Now there is no fumbling around under the dash to make adjustments. Don’t forget to reinstall the cable retainers to prevent the cables from popping off.

It’s mid-August and you’re thinking, Why in the world is this guy rebuilding a heater box? It may be 96 degrees outside right now (actually I wrote this in April, and it was a nice, partly cloudy day and 67 degrees out), but I rebuilt this heater case in the middle of last winter. Let’s just call this preventive maintenance for the upcoming winter months for those of you with running driver Mustangs.

For me, though, it’s a different story. With Project ’66 now painted, suspended, and braked, we can begin work on installing the fresh, black Pony interior (hands down, the favorite part of a restoration). Before we can start laying door-to-door carpet, though, we have to get our firewall in order. This means rebuilding the heater case, new insulation, refurbishing the heater controls, and more. Thus, I froze my butt off (though my fellow staffers claim to not notice the difference) in my garage for three nights, rebuilding the heater case assembly.

For our heater case rebuild, we called on the fine folks at CJ Pony Parts for everything we would need to spiff-up our heater and have it pumping out volumes of 289ci heated air. As you will see, all of our electrical components were in good working condition, but if you need a blower switch or a blower motor, the crew at CJ has ’em on the shelf ready to go. We also enlisted the help of Ssnake-Oyl in insulating our ’66 hardtop from road noise and the elements.

While you will see most of Ssnake-Oyl’s kit in an upcoming issue, we installed the firewall pad now, since it has to go on before the heater case.