The following outlines an option for replacing the heater A/C box on 69 and 70 Mustangs and Cougars.
My 1970 Cougar original Heater A/C box had the typical broken drain connection, a broken off support at the outlet end, many wide cracks, many more fine cracks and a lot of distortion with the rest being very brittle. Repair with epoxy and fiberglass was going to be a major undertaking and good results were not assured. The heater core hold down bracket and temperature blend door baffle was severely corroded, the heater core restrictor door was missing and its shaft too corroded to re-use. A heater core leak and a suspect evaporator drove the need to remove the box.
Some research suggested that the available 67-68 Heater A/C box repro from APC (All Classic Parts part number FM-EH027) could be converted. Initial comparison of the old and new cases prior to purchase was promising. The 67-68 repro heater A/C box comes with damper doors, shafts, actuator mounts, some supports, and the heater core hold-down bracket/temp blend door baffles. Closer examination after purchase and disassembly still shows them the same, although it has not yet been re-installed due to other on-going work. In summary the plastic/FRP box is the same, much of the rest is not, but is transferable from the old case.
• The basic plastic/FRP box
• The heater core hold-down bracket and heater temperature blend door baffle. These were in very bad condition on the old box and were not available even used.
• The damper doors
o Heater restrictor door (heater door)
o Temperature blend door (evaporator door) w/ manual cable)
o Outside air / recirculation air door (air inlet) The WCCC website shows different part numbers for the original doors but the one from the new box and the original did not show a significant difference.
• The center upper support bracket (bolts on the outside)
• All three shafts (ends and/or clock orientation differences)
• Actuator mounts (both the brackets and the locations)
• Outlet end support (none on the 67/68)
• Upper support at inlet end (same location but the new metal bracket is about a ½” short to the mounting hole)
• No vacuum switch mounting pad
• The temperature blend door manual adjust cable attachment bracket is in a different location
• The AC evaporator for 69/70 has different inlet and outlet connections requiring a larger opening in the side of the box
• The holes for the plastic shaft bushings are smaller and had to be enlarged for the older bushings. There were some clearance issues (noted below)
• The inlet on the repro is a full circle whereas the original is trimmed flat on the outside edge (the Cougar appears to have enough space for the full round but it could be trimmed or the inlet pieces transferred).
• Salvage all three shafts, the two actuator brackets, the outlet end support bracket, and the inlet end support bracket from the old box. The shafts are available used (from WCCC).
• Be careful removing the parts from old box. It will be needed to locate and map the required hole locations on the new box.
• Clean and paint old metal parts that will be reused as desired
• A seal kit will be needed for a few parts (the repro box comes with the damper door seals and connection gaskets but no insulator pads for the heater core sides or other system gaskets)
• Test vacuum motors/actuators, vacuum and de-icing switches. Pressure test heater core (~15psi) and evaporator if not known to be sound. Replace if in question, pulling this assembly is a major task.
• Remove all of the hardware from the 67/68 box. Save the backing plates from the inside of the box (some will be reusable). Note the front to back orientation of the damper/doors. The fresh air/ recirc. door has an off center shaft from the face and should be reinstalled the same way to get tight closure. The temperature blend door has one corner trimmed to clear one of the screws.
• Map out the rivet hole/ bracket locations (creating paper templates helped) from the old box, transfer the locations to the new box, and drill the holes.
• Enlarge the evaporator connection hole to match the old case
• Determine and acquire the screws or rivets to be used (I used #8-32 SS pan head screws with nylon insert stop nuts, screw heads to the inside).
• Fabricate new backing plates (for inside reinforcement of the mounts) for the Vacuum switch, and others that may be needed (at least an actuator and a mount).
• Drill new holes and assemble.
• I had to shave about 1/8” from one of the original heater core end insulation cover to fit into the new case. The old case was significantly bulged in that area (don’t know why).
• Had to notch the end of the heater restrictor door to allow the larger plastic shaft bushing to seat full in the case. This provided a better seat for the bushing and better clearance between the actuator arm and the manual temp control cable
• There was a little interference between the heater core hold-down bracket and the larger plastic shaft bushing on the temperature blend door. A little metal removal with a Dremel took care of that.
• The split line between the casing halves is slightly different at the inlet end and might be more prone to leaking condensate, if any level of liquid accumulated. I used a bit of flexible strip caulk at that end
• Take a lot of pictures before and during the disassembly of the old box and bag and tag parts as they come off
• Support the plastic box sides carefully when driving the shaft out of the dampers to prevent damage (the old box is needed to map new mounting hole locations so don’t let it get broken up).
• Drill out or grind heads on the existing rivets carefully to avoid damage to both boxes
• This is a good opportunity to clean up all of the old parts and paint them to minimize future corrosion
• West Coast Classic Cougar’s (WCCC) video on heater boxes (esp. on removing the clips that connect the box halves and other removal and parts considerations)
• WCCC used parts listings for shafts and other heater A/C parts
• 1970 Ford Car Shop Manual Section 34
For just over three hundred dollars you get a new plastic/FRP box, new heater core hold-down brackets / blend door baffles and new damper doors. The rest must be reused from the old box or bought. There is a fair amount of work in transferring all the other components but there is even pulling and reinstalling the box, so it pays to get it right.