This was intended as part of the Air Conditioning installation walk through, but after starting the rebuild, I realized including this would make the walk through too long.  The reasons I chose to install factory air vs. aftermarket are discussed in that walk through.

Many factory a/c parts are not available new through vendors.  I purchased the heater box off E-bay and the blower / evaporator housing from a forum member.  Since these are not available new, you can expect to have some restoration work on your hands.  My heater box inspection revealed many cracks, especially around the bolt and screw holes.  Several screw holes were badly shattered or cracked, raising concerns about getting a good seat with the screw.  A major area of restoration for the box was the outlet flap.  The flap seal was missing and I would need to improvise a repair.  Normally this flap cannot be removed, but in this case, there were two very large cracks all the way through the outlet duct that allowed me to get the flap out.

I purchased the blower and evaporator housing understanding that some fiberglass work was necessary there.  The blower half contained a very large crack and the evaporator half has a couple of holes and several cracks in it.  All in all, I have my work cut out for me.  Since I plan on painting these boxes, I'm not concerned about matching the original SMC.  Before starting this project, there were a few things I considered:

THERMAL PROTECTION:  Several posts I've read bemoaning the factory a/c performance indicate that evaporator housing heat soak is a major culprit of decreased performance.  To counter this, I'll cover the evaporator and blower housing with the Hy-Tech Metal Shield #1267 thermal paint I used on the firewall in the engine bay project.

INCREASED BLOWER PERFORMANCE:  I've read that replacing the stock C3 blower wheel with one from a 1986 or later C4 increases the airflow.  The fan is longer, so a spacer will have to fabricated  Apparently it's the pitch of the blades and not the additional length that generates the extra breeze.

DAMAGED SCREW HOLES:  Many of the screw holes, especially on the heater box, had been over tightened, resulting in cracks and shattering inside the hole.  Since fiberglass is difficult to drill and tap because its not flexible, I decided to drill all the holes out 1/4" and epoxy in some nut inserts.

​Modifications:  Two

  • Install nut inserts in place of tapped holes in fiberglass case (Westward 5NNK7 Rivet nut)
  • Stainless steel hardware to prevent rusting

Upgrades:  Two

  • Install blower from C4 (AC Delco 15-8418)
  • Cover blower motor and evaporator housings with thermal paint (Hy-Tech Metal Shield #1267)​​​

​​​​​​​​​​​​Heater Box

​​I disassembled the heater box down to the metal frame shown in the first slide.  The remaining slides show the steps for flap removal / installation.  I blasted the pot metal frame with fine glass bead.  The air dam and diverter flap had minor surface rust, and the rubber diverter seal was in great shape.  I taped these off and applied two coats of Zero-rust.  

At some point the mounting bolts had been over tightened, cracking the case and deforming the frame.  I used a bench vise, body blocks, and body hammer to straighten it as best I could.  Pot metal is soft, so no heat was needed.  Slide three shows the finished frame.

The last six show how to install the diverter flap. 

  • Put flap tang through opening as shown
  • Rotate flap clockwise to bring tang through the hole 
  • Push flap forward at bottom and position flap in v-shaped channel
  • Pull flap back to drop alignment pin into hole in v-shaped channel
  • Ensure tang shaft and alignment pin are in final position
  • Install retaining clip

To repair the case, I used a dremel 1/16" spherical engraving bit to mark the cracks (slide 1).  Adding a cone and cylinder bit, I removed shattered and broken material.  The spherical bit was useful for removing cracks by tracing along them until the crack could no longer be seen under bright light. The cone and cylinder bits were useful where multiple cracks met and there was additional shattered material.  This tedious work took several hours to complete.  The third and fourth slides show what remained of two screw holes after removing the ruined material with the cylinder bit.  The final slide shows one of the holes I drilled out for the insert.

The heater box was deformed from over-tightening.  With a heat gun set at 550 degrees, I softened the SMC and clamped the housing against the straightened metal case. Once cool, it retained the new shape better than I expected.  Click on the arrow and check it out for yourself.

Though the pictures at right are not very good, they manage to show the extensive damage at the heater box outlet.  There were two large breaks and a missing section where the hot air control box attaches.  This damage was a blessing in disguise as it allowed removal of the outlet air diverter for seal fabrication.  

For seal fabrication, I explored several options ranging from foam to soft silicone.  After weighing the cost of these materials against their properties, I selected high quality neoprene due to its reasonable price, durability, and operational temperature.  A Grainger tech confirmed that high quality neoprene was suitable for the application and recommended adhesive backed neoprene.  He stated the adhesive would bond to metal, hold up to the heat, and be stronger than the neoprene once it set.  This info saved me the trouble of finding an adhesive.

Neoprene is available in hardness durometers.  I assumed the missing seal was similar to the existing one, and based on available hardness descriptions, selected durometer 40 neoprene.  Using the other seal as a guide, I estimated two 1/8" thick pieces adhered together would be close.  Putting this together with the flap size, I ordered one square foot of 1/8" adhesive backed, high-quality, durometer 40 neoprene ($25 including shipping).  In retrospect, 1/16" would probably work fine, and would've cut better.

Based on fit checks, I estimated that a good seal would extend 1/2" beyond the diverter's edge.  The first photo shows the pattern I drew on the adhesive backing.  The center line was traced out using the diverter, the outer line was measured 1/2" out from that, and the inner line was free hand based on observation.  Photo two is the fit check to the flap, and photo three is the fit check to the heater box.  I had to trim a couple times before I was satisfied there was no contact with the outlet walls.

A couple things:

  • You cannot cut 40 durometer neoprene with a razor knife.  It stretches and compresses too much.  I used a scissors, which worked OK, but didn't leave a clean edge, as you can see in the last picture.
  • When you put the halves together, peel the backing away a little at a time on the second piece, and work it into place.  There is some time to pull it up and repositon it before the adhesive sets.​​

To repair the case, I only needed to fill the ground out areas.  There were no missing pieces except for the small corner previously mentioned.  I mixed a structural putty of epoxy, colloidal silica, and 1/32" milled fiberglass; adding silica and fiberglass equally until it was a little thicker than syrup.  Using aluminum tape for backing where necessary, I filled the cracks and holes with epoxy.  Once cured, I removed the tape and sanded to shape.

I selected 8 - 32 nut inserts because they were slightly less than 0.24 outer diameter and a little under half an inch long (slide 1).  With a 0.25" bit tape marked at 0.5", I drilled out the screw holes.  After a fit check (slide 2) I put some 1" long screws in the inserts for manipulation and to keep epoxy out of the threads.  I abraded each insert with 80 grit and covered it with epoxy.  I also filled cracks and gaps around the holes before putting the insert in place.   Once the epoxy cured, I removed the screws and sanded it down.  The third slide shows four inserts ready for sanding

Fixing the cracks at the outlet posed a challenge because the flap had to be in place during the repair.  The large crack was repaired like the others.  For the missing section, I ground away the ruined material and made a bevel that I sanded with 80 grit paper.  I used aluminum tape for backing material and built the area up with a thick application of putty.  Once it dried, I used the pink grinding stones to shape it (slides 3 & 4).

After all repair work, I painted the case with rattle can primer and three coats of satin black.  I powder coated the vacuum switch bracket and heater core hardware to protect it.  I replaced the vacuum switch and heater core, which showed signs of leakage.  I fastened it together with stainless steel allan head button screws 3/8 inch long.  You can see a couple in slide 2.  I did not install the seal kit at this time, but plan to cover that later during installation.  To the right are the assembly pictures.  Overall I was pleased with how it turned out.

SUMMARY:  This was about a 7 out of 10.  There was a lot of grinding, cutting, filling, and shaping; and fabrication of the door seal was some extra work many folks won't have to do.  It was tedious and time consuming, but the end results was worth it.  This would have been even harder had I tried for a factory look, which would've required matching the SMC color.

Hot Air Diverter

The hot air diverter is connected to the heater box outlet by the two holes on the right.  It is a vacuum actuated assembly that diverts vent air through either the defroster or floor vent.  Excuse the lack of pictures, but my camera was destroyed before I could get them all loaded into the computer.

This simple device usually requires little other than a cleaning and functional check.  The vent assembly consists of two halves glued together.  It is attached to the vent box by a spring clip (barely seen in the picture) and a screw on the opposite side.  To separate the two, remove the screw and pull the vent down and to the left.

The vent box is made from a light, fragile material resembling a fibrous plastic.  It consists of two halves glued together to house a hard plastic flap.  Mine had some stains and damaged areas where carpet was embedded; and the glue seam had separated along significant sections of the two halves.  I removed as much material as I could and smoothed it out with a razor blade.  Before I could separate the vent box halves, I had to remove the actuator and hardware.

The actuator is shown after testing and clean-up.

I was able to separate the two halves by gently inserting a small screw driver between the seam and carefully twisting it.  This stuff is fragile, so I was careful not to force anything.  ​​After cleaning out the seam, I used a two part epoxy to put the halves back together.  The upper half of the vent box is shown.

Final assembly consists of reinstalling the actuator and hardware, and attaching the lower vent assembly.  The picture shows the screw used to hold the vent assembly in place.

SUMMARY This was a very straight forward 3 out of 10.  The only potential tricky part is separating the vent box halves, and that's only necessary if yours has separated.  Otherwise, this is a clean and check job.

This walk through continues on the next page with the Blower Motor Rebuild.

Evaporator / Heater Box

Progress  so Far