1969, 1970 mustang heater box repair/replace instructions (no AC)


Jan 30, 2017
So, after struggling with my heater core replacement I think I've finally landed on the perfect formula for replacing it.

After removing the upper dashboard you should have access to the heater box, take the connector off of the resistor board on the right side, remove the two heating-duct-flapper-control-rods from the heater box, there's one on the right, one on the left. open the glove box to give yourself more work space, you might need to remove the lower dashboard mounting bolts on the passenger side as well. I did.

Now, there are 6 bolts holding the box to the car's firewall, five of them are flat style, (basically a nut on top of a washer design, the OEM style combines the two into one unit with a rubber washer underneath it all) and one is a hex-head sheet metal screw (which is weirdly screwed first through a brake line clamp, through the firewall, and then into the heater box like a screw in a piece of wood) you can access them from the front of the firewall.

Once the firewall bolts are loose, you'll need to disassemble the heater box so that you can extract the pieces, two nuts hold the defroster vent to the upper dashboard mounting flange/plate) remove the nuts and slide the duct towards you . Next remove the clips that hold the two halves of the main heater box together, there should be 3 on top, 4 on the bottom, and 2 on each side, but these cars are old, missing parts aren't unheard of. now you should be able to extract all of the parts from behind the firewall and repair/replace anything you need.

(if you want to try this a different way, go for it, but this is what worked for me)

test-fit all of your new parts to make sure the box will go back together without issues

align your blower fan so that the mounting holes are all properly lined up, I would recommend using some adhesive to keep it aligned throughout this process, painter's tape should work good)

Take the back half of your heater box (the one with the firewall studs in it) and place it behind your dashboard so that the stud closest to the driver's side is pushed through the firewall and place a mounting nut loosely on the back of it, you're going to use this as a pivot point to line the other 4 studs up later in the process.

You can now re-assemble your heater box with all the shiny new parts it needed.

-keep a standard nut around that will fit your firewall studs, the factory ones aren't great for catching that first little bit of thread that's sticking out of the firewall.
-I used a few small dots of weatherstrip adhesive to keep the non-moving parts from falling out of the two halves as I was assembling it. (we want to be able to tear this apart later if needed)
-if you ordered new clips to keep the two halves of the heater box together, you might need to squish them down a bit to make them fit properly, I used a hammer to flatten mine out with a few light taps)
-a good pair of vice-grip pliers can help keep the heater-box-halves together while you attach the clips, just don't crack the box while you're clamping it
-dry graphite lubricant can help you get your new heater hoses onto your new heater core fittings

attach your heater hoses to your heater core, use some good hose clamps if you've got them.

hopefully with these tips you've re-assembled your heater box, now you just need to place your intake-to-heater-box-seal (sortof a big foam ring) on the top of the box and pivot those other 4 studs up and into place. Call a friend so that one of you can apply upward and forward pressure to the heater box from inside the car and the other can help the studs into place with a small screwdriver, you can then screw a nut onto whatever stud makes it through the firewall first.

now you can do your final reassembly of your heater ducts, hoses, control rods, and resistor board.

Refill the cooling system and check for leaks

Congratulations! you've just completed the job that basically had me tearing my hair out for a year. I hope this has been a helpful guide!
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