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Discussion Starter · #1 · (Edited)
You start up your pride and joy one crisp fall day and find that even though your defrosters are on high, the windshield gets fogged up. There is also a sweet smell in the air and you know it is not your cologne. There may even be a green puddle forming on the passenger's side floorboard. These are all good signs that the heater core has a leak. Luckily new ones are cheap and fairly easy to come by. I will detail some of the finer points of heater core replacement.

Because the heater box has some electrical connections, it is best to remove the negative battery terminal prior to starting this project.

The heater core is hidden inside the box-like structure found under the right side of the dash. In order to replace it, the whole box needs to be dropped out from under the dash. Here is what we are dealing with:



In addition to a new heater core, expect to replace the heater hoses as they are not readily accessible otherwise. Examine the defroster plenum, defroster hoses and vents as they will be fairly easy to replace at this time too. You may also want to get a heater box insulation kit as most of the original foam insulation is in very poor shape after forty plus years of service.

In order to drop the heater box out from under the dash, partially drain the radiator. It should only need to be drained about half way down, just enough so that the main level is below where the bottom heater hose connects to the water pump:



The heater hoses on the water pump and the intake manifold will need to be removed. Mine are practically hidden in this photo:



Here both hoses have been disconnected from the engine:



Be sure to remove the hose clamps as they will be in the way during a following step. If you are planning on replacing both hoses (again, highly recommended), you can save yourself a little aggravation by taking a razor knife and cutting the hoses at the firewall. Once the hoses have been removed and drained as much as possible, place a couple of old towels on the floor boards to protect the carpet from the antifreeze/rust mixture that remains in the heater core.

Look under the dash and unplug the electrical connector found on the front of the heater box, just to the left of the interior light:



Note the orientation of the wires before removal. Here are the spade connectors that the plugs are attached to:



The above wiring block is normally red in color, but this one had been painted black, probably when the dash was repainted a long time ago.

It is now time to remove a few fasteners. The first bolt that I remove is found up under the dash and it is a support for the heater box (circled in red):



Next remove the 4 nuts found in the engine compartment, but first you must disconnect the 2 (or 3 wires) that the power the fan motor, as seen on the right in the picture below:



There are 4 nuts surrounding the heater motor, a couple are seen in the above photo. These nuts are all that hold the heater box securely to the firewall. Remove all 4 nuts, preferably with a helper so the heater box doesn’t fall to the floor. The heater box still cannot be removed from the car as the defroster/heater cables are still attached:



Those cables will need to be removed. Normally there are little metal hats on top of where the cable is wound around the mechanism, but mine are long gone. Be sure to mark the cables in order to help with reassembly.

Next disconnect the defroster hoses from the defroster plenum. Normally there are clamps that need to be removed:



Disconnect the defroster plenum cable. This allow you to remove the plenum with the heater box assembly:



Now you can remove the heater box assembly and take it to a workbench for further disassembly. The heater hoses need threaded through the holes in the firewall.



Remove the hoses from the heater core. Then remove the 4 screws that hold the defroster plenum to the heater box assembly:



It is now time to dissemble the heater box itself. There are several metal spring clips that hold the 2 halves of the box together. I use a screwdriver to remove them, but you must be very careful as the heater box is fiberglass and it is easily broken. The clipping surface can be damaged so the clips will not longer fit. Remove all the clips around the perimeter of the heater box:





Separate the 2 halves and then you will see the heater core. It will have a retainer around it:







Pull the new heater core out of the box and compare to the old one.



Installation will be the reverse of disassembly. Install the heater core into the heater box. Install the core retainer. Then reassemble the heater box using the spring clips. Reattach the defroster plenum (this is a good time to replace that cardboard thing with one of the improved plastic ones.)

Install the heater hoses on the heater core and tighten the clamps:




Note how I oriented the heads of the hose clamps. This makes it easier in the future to loosen the clamps without taking the whole assembly out of the car.

Thread the hoses through the firewall. Install and adjust the 3 heater/defroster cables. Have a helper hold the heater box into position so that you can install the 4 nuts around the heater fan. Connect the wires at the heater fan. From inside the car, install the support bolt, the heater box wiring plugs, and the defroster hoses.

Next reconnect the heater hoses to the engine. Pour the antifreeze back into the radiator and put the battery cable back on the batter. Next start the engine and let it get up to temp. Check for leaks and top off the radiator again as needed.

The above procedure should help familiarize you with the installation of a heater core. This procedure will also be good on other similar small Fords (such as Falcons and Comets) and for other years, assuming that the car does not have in-dash air conditioning.
 

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I bought my car back when I was in high school, and didn't have any money...

Sometimes, when the engine rpm's would 'zing', the heater core would find a weak link and start peeing all over the carpet. I tried soldering it a few times, and re-installing... but it wouldn't take too many more 'zings' to open up a new hole. I eventually broke down and bought a new one. :)

I also eventually bought a replacement heater/flapper box, as the original is cardboard paper, and mine was shot. The replacement was plastic. (Although, the outlets where the hoses fit were too small, so the duct hoses didn't fit very well. Hopefully, they've addressed the issue in the last 20 years. lol)

When I would do the job, I would just remove those clips, and split the thing with it still in the car. It's been a LONG time since I've had it apart, so I don't know which one of us is/was doing it the 'hard' way, or if there's really much of a difference. LOL!

One thing that can be done to avoid 'core busting' pressure spikes, is to put a restrictor in the core's inlet hose. Ford installed them in some cars right from the factory. These can be as simple as a 1/4" drive socket that fits the ID of the heater hose, and it can be held in place by a hose clamp. The restrictor allows plenty of coolant to pass through for heating, but guards against the pressure spikes brought on by high rpm. OH, the inlet hose is the one coming out of the intake manifold. A restrictor on the OUTLET hose would NOT be a good thing. :)

Nice article, Dennis!

Good Luck!
 

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Wonderful write up! I did this last year and what a difference. I replaced the noisy original motor, replaced the inner seals, replaced the plenum, and make sure there were no air leaks. I also took the time to adjust the cables just right and the result is plenty of hot air even on very cold days. Enjoy!

David
 

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dennis,
Your removal of the clips that hold the halves together can be safely done by not prying the clip off and breaking off the bit of fiberglass that the clip catches on (ask me how I know) but inserting the flat screwdriver blade in the curved clip edge and prying it open and rotating it off with its tension off the box. There is a tool for this but....you know.

cushman
 

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Discussion Starter · #6 ·
Yes Cushman, I do use your method. Your discription of clip removal is much better than mine.

Thanks!!! :)
 

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I'm in High School and just bought my 65 mustang about a week ago and have been passionately working on it every day since. These instructions with the pictures are awesome. I used your instructions and replaced my heater core with ease!
Thank you
 

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Discussion Starter · #8 ·
Welcome to FM, T-camp. I got my first 66' while in high school a long, long time ago and have enjoyed these cars ever since.

I am glad that you found the instructions useful. These really are easy cars to work on--all it takes is a little basic knowledge.

I really appreciate your feedback and good luck with your project.
 

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Thanks for the tech article. Dad told me it helped him to figure out how the heater system in the 66 he's putting together for me, went back together.
 

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Discussion Starter · #11 · (Edited)
Thanks for the tech article. Dad told me it helped him to figure out how the heater system in the 66 he's putting together for me, went back together.
Thank You and I am glad that you dad found the article useful. :)

If you don't QUIT doing this stuff Dennis, there will be no more room for US to put up Tech Articles! You've already covered all the easy stuff!!!! :D

Nice stuff. Few mistakes, good Article!
Oh FE, I have done a couple of hard articles too . . . . . ;)

My intent of these articles is to help instill confidence into those who are faced with similar obstacles when working on our Mustangs. I may not do it the best way, but I won't throw anything out here that I haven't done myself. For that reason I would like to clarify or correct the mistakes that you have found. Care to share so that we can improve the accuracy of the article(s)?
 

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They say a picture is worth a thousand words. In this case, your 20 pictures saved me many hours of frustration. I searched high and low before finding these instructions. I was very uneasy about starting the process with the limited information I had found. Once I read this, I was able to jump into it and get the job going. The box is completely out and I'm now just waiting for my new core to be delivered from VA Mustang. While my particular car has AC and the fit is just a bit tighter, all worked out well. You mention your goal is to give some confidence to a weekend mechanic so he'll try this stuff on his own. Your goal was achieved :)

Thank you!!!
 

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Wow Dennis, you really helped some people out. I think it's great that they found your article probably by doing an internet search. I just wish you had a 68 with air!! I pulled mine apart last fall and am dreading putting it back together. The unit with air is quite a bit different. I'm thinking of simplifying. I have 3 or 4 65-66 heater boxes and wished they would work.
 

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Discussion Starter · #14 ·
They say a picture is worth a thousand words. In this case, your 20 pictures saved me many hours of frustration. I searched high and low before finding these instructions. I was very uneasy about starting the process with the limited information I had found. Once I read this, I was able to jump into it and get the job going. The box is completely out and I'm now just waiting for my new core to be delivered from VA Mustang. While my particular car has AC and the fit is just a bit tighter, all worked out well. You mention your goal is to give some confidence to a weekend mechanic so he'll try this stuff on his own. Your goal was achieved :)

Thank you!!!
You are certainly welcome Trader. By the way, welcome to FM.

I am glad that I could help make you feel comfortable working on this project. :D
 

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These instructions with the pictures are great. I used them and removed/repaired & replaced my heater core with no issues.
I did however undo the two bolts that attach my A/C unit so I could move it foward slightly and ease the removal of the heater box.
Thank you,
Alistair Wilson.
New Zealand<!-- / message -->
 

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from that pic, it looks like the upper hose on the heater box goes to the bottom on the water pump, and the bottom hose goes to the intake. Is this correct. or does it matter?
 

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Discussion Starter · #17 ·
from that pic, it looks like the upper hose on the heater box goes to the bottom on the water pump, and the bottom hose goes to the intake. Is this correct. or does it matter?
Correct.

Top of heater core goes to the water pump.
Bottom of heater core comes from the intake.

The heater will work either way, although this the recommended way.
 

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Discussion Starter · #18 ·
These instructions with the pictures are great. I used them and removed/repaired & replaced my heater core with no issues.
I did however undo the two bolts that attach my A/C unit so I could move it foward slightly and ease the removal of the heater box.
Thank you,
Alistair Wilson.
New Zealand<!-- / message -->
Thank You for your comments. Glad that you added the input on the A/C unit.

I suppose that the full console that is sometimes found on non-AC cars would also need to be removed.
 

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Dennis, We own a fully refurbished 64 1/2 Mustang that we bought about 10 years ago. Yesterday we noticed that we were leaking anifreeze on the passenger side. Is the only reason for this a bad heater core-we never use the heat as it is a convertible and we only take it out in nice weather . My husband replaced the one on our 73 Grand Torino Sport and he had to drop the dash. Your instructions look like this is a much easier job to do.
Thanks,
JudyandBob
 
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