I had replaced the heater core recently as it was leaking and it was disconnected when I got the car. As I did not know when the cooling system was flushed for the last time I wanted to flush everything before I reconnected the heater core.
I first flushed everything with a cooling system flush chemical followed by fresh water and then I put in all new coolant (I used Prestone) and connected the heater core to the system again. Everything came out very clean, so I think that a good flush was already done a short time before I got the car.
It took a while until I got the air out of the system and the new heater core did not work at first. But after about 30 minutes of letting the car idle at about 2.000 rpm, the air was out and the AC finally blew out warm air.
Its great to have heat in the car again.

When I reconnected the heater valve to the heater core I needed a short piece of heater hose. I found perfect reproductions at Fusick - they look exactly like the original GM heater hoses with 5 ribs and are available by the foot. They are not exactly cheap, but perfect replacements if original appearance is important for you. They are also available in the correct size - 5/8 and 3/4. The smaller diameter goes from the heater core to the hot water valve.
By the way - Fusick has exceptional customer service , which impressed me very much. I will definitely buy from them in the future again.

Fusick offers a perfect reproduction of the original ribbed GM heater hoses. The original heater hoses had 5 ribs just like these super nice reproductions.
I re-connected the new heater core to the cooling system and used correct reproduction heater hoses to connect the original heater valve which is still working perfectly. The hose clamps are not the correct ones yet. I think there should be tower style hose clamps which I do not have at the moment. There is still an original one on the bottom of the valve and also one on the other heater hose, thats why I´m very sure that the other two clamps should be tower style as well. I have not found a perfect reproduction clamp yet, but I´m still looking…



When I got my 74 Cadillac Coupe DeVille the heater core was disconnected. Unfortunately the previous owner could not tell me why, as it was already disconnected when he got the car.
There could have been 3 possible scenarios why it was disconnected:

  1. The heater core is leaking
  2. The heater core is blocked
  3. The heat could not be turned off in the heating system due to a problem in the ATC

Most probably the core is leaking on such old cars. So I had ordered a new core a while ago, but I never really wanted to tackle this job as it needs quite a bit of effort to get it done and it is not a very funny job. It's not that technically challenging but very tedious. I finally bit the bullet and decided to finally do it.

You can normally detect a defective heater core by fog build up inside the car and by the smell of coolant.

I studied the shop manual and started to tear the dash apart to get access to the heater box which houses the heater core.
I found out that the shop manual lists a lot of necessary work and does not show all the screws that you have to remove to get the box out. Some of the screws are very well hidden and finding them was the most frustrating part of this job.

Here is how to remove the heater core: (text in black is from the shop manual - my remarks are in green)
First you have to remove the Instrument Panel Pad

Removal of Instrument Panel Pad

  1. Disconnect negative battery cable.
  2. Remove three climate control outlet grilles - right, left and right center (see picture below on how it is done with a special tool - I just did it with two flat screwdrivers)
  3. Working through outlet openings, remove 3 fasteners securing pad to instrument panel support.
  4. Remove screws securing pad to instrument panel horizontal support.
  5. Pull pad outward and disconnect electrical connector from windshield wiper switch.
  6. Remove pad.
(Note: To facilitate removal or installation, place shift lever in LO range and on cars equipped with tilt wheel, place wheel in lowest position.)

Heater Case Removal

  1. Drain radiator. (not necessary if you just clamp the hoses off, but flushing the system never can hurt)
  2. Remove hoses from heater core nipples and plug nipples to prevent spilling coolant remaining in core. (Thats indeed extremely important as you have to move the heater case around and it would definitely spill any remains of coolant on your carpet. Also make sure that there are no traces of leaked coolant underneath the carpet from the bad core.)
  3. Remove instrument panel pad. (As described in the chapter above.)
  4. Remove center A/C outlet support and connector from position between cowl and horizontal support, Fig. 1-36. (Scroll down to see the illustration, You basically only have to unhook the rubber part of the outlet and push it downwards.)
  5. Remove left A/C outlet hose from A/C distributor. (There are some steel clips inside the tube where it is mounted to the plastic distributor. You have to reach in and unclip them to be able to remove the hose without using a lot of force and maybe damaging it. See the picture where I put my arm into the opening below.)
  6. Remove center support and associated braces as shown in Fig. 1-36. ( I can´t see any reason why this should be done - I removed only one of the braces, and it was completely unnecessary. You only have to remove the round bent brace on the bottom of the heater case as its in the way to get it out - see the picture where I am unscrewing it below.)
  7. Remove two screws securing A/C distributor to heater case and remove distributor, Fig. 1-36. (If you did not remove the braces mentioned above like I did, you can´t get this part out. I just put it to the side where the glove box normally is, which you will also have to remove anyways - see later.)
  8. Remove one screw securing defroster nozzle and remove nozzle. (You have to remove the screw - but leave the nozzle in place. There is no whatsoever reason to take it out. And its also impossible to get out if you did not remove ALL the braces and brackets.)
  9. Remove glove compartment liner. (Its self explanatory - just remove all the philips screws you see inside.)
  10. Remove vacuum and electrical connectors from programmer. (You can do this now through the glove box opening.)
  11. Disconnect vacuum hose from recirc door actuator (Orange - near the right door kick panel.) , defroster door actuator (You can see it on the left side of the heater box with a blue and a yellow hose going into it.), mode door actuator (There is no way you can remove these hoses now, as they are on the back of the heater case. The upper mode door is green and the lower one is tan. You can really only remove them once the entire box comes out.) and position vacuum harness out of way, Fig. 1-37.
  12. Disconnect aspirator hose from in car sensor. (The thick black hose on top of the heater box)
  13. Remove three nuts and one screw securing heater case to cowl on engine side of cowl, Fig. 1-8. (It was the most tricky part for me to find these screws. See my picture below where I have marked the location of the screws as seen from the engine bay.)
  14. Remove heater case from position under instrument panel .(See picture how you have to position it to get it out - remove the two vacuum hoses mentioned before on the back now. Be careful - they are attached with metal clips)
  15. Replace heater core. (Only 2 screws - self explanatory - also re-use the rubber seals from your old core around the water nipples.)

You can then put everything back together which should be much faster, but before you could also remove the clock, which is very easily accessible now and clean and lubricate it (the shop manual recommends to do this every 2 years). You also have access to all the dash lights, tell tale and warning lights and to the speakers (which are shot on my car due to a defect in the amplifier in my radio) while everything is apart. While I had the heater box out of the car I also cleaned the inside of the programmer and re-lubricated it with white lithium grease. I used electronic cleaner to clean the electric contacts inside the programmer while it was open, as most probably the contacts will have some oxidation on your car as well. While the heater box is out you can also lubricate the various doors and clean out all the dust which has collected inside the box over the last decades.

The dash removed. The radio is also out for repair.
There are metal clips inside holding the left A/C hose in place. You have to reach in and unclip them.
You will have to remove this support bracket.
Recirculation door - un-plug the orange vacuum hose.
You have to remove these 4 screws/nuts on the firewall. You can access number 4 from underneath the car only. Number 3 is hidden underneath the cables.
The number 4 screw on the cowl is holding a cable clip in place and the other end is the right bottom screw of the heater box - you need a second person with a 7/8 wrench to release it from the side under the car while you unscrew it from inside the car with a 3/8.
Once all the screws are off you can carefully remove the box. You have to slip it to the passenger side to get it out.
The old core in the heater case.
The 1974 Cadillac heater case and heater core. You can see the two mode doors here and on the right is the recirculation door. The thick hose on top is the aspirator hose going to the temperature sensor in the dash.
This is where the old core had developed a leak.
The old core still in place. Note the rubber seals on the tubes of the heater core. You have to move them over to the new core.
The new heater core in place. I got mine from Its very similar - the only difference is how the two tubes are braced to the core, but that is no problem at all.
Out of curiosity I checked where the old core was leaking, and the leak was pretty obvious as you can see.
I just used two flat screwdrivers instead of this special tool.


The 67 Eldorado´s Automatic Climate Control System is back in working order

Today I managed to solve the issue with the partly non working Automatic Climate Control of my 1967 Eldorado. As described in the entries below, the blower would only come on in full heat modes when the car was accelerating. The AC of the Eldorado is very similar to the one from the DeVille btw.
I suspected a vacuum leak somewhere in the system. Troubleshooting began with going through all the vacuum hoses. I replaced some which were a little worn out at the connectors. I also cut off the ends of most hoses, as they were all becoming a little loose during the last 40 years. I then plugged them in again.
Not much did change though.
When I came to the hose of the hot water valve and the delay relay, I had the first success. At least the blower motor would stay on in all “Auto” modes, even when the car was idling - but would still go off in “Fog” and “Ice”. I then suspected a leak in the power servo or in the dash controller. So I installed the power servo from my 67 DeVille where everything works perfectly, to be able to rule out any problems. Nothing changed though - the blower would still come on and off depending on the throttle position.

I then soon found the culprit of the whole issue - it was the master vacuum switch - which seems to have a problem with the internal electric switch. As I had a spare one around I installed it - and voilá - everything worked as it should. It always looked good when I checked it before.
I´m very happy that everything is back in working order. The only thing which I still have to repair is the fast idle diaphragm which completely dried out and is no longer functioning. Fortunately I do have a spare 429 engine in the garage from where I can grab and use this part - so I will repair it soon.
I then need a new condenser which is already on the way across the Atlantic coming from Old Auto Air.
By the way - Cadillactim has an excellent trouble shooting guide for the ACC on his website.

The evaporator had to be replaced as well along with the heater core. This is a pretty big task as you can see on the pictures below. You even have to remove the hood to get access to the whole unit. The fiberglass AC enclosure had to be repaired at my car as well as the previous owner tried to repair the evaporator and missed a screw and made a tear into the enclosure. The restoration shop repaired this professionally and it looks like new now.

The best troubleshooting guide for 1967 and 1968 Cadillac´s Automatic Climate Control Systems I got today from Gary Sisk - owner of a 1968 Cadillac.
This is a very easy to follow explanation which will save you a lot of time and you do not even need any special tools.
It was written by Lynn Nicholls and can be found on Stampies Cadillac page - here is the direct link to it.

Below you can find some pictures of my system:

The whole AC unit has to be removed to get access to the evaporator and heater core... The slight surface rust that showed up where the gaskets were, was removed as well of course...
The big hole on the left is where the blower motor is installed.

This is where the AC box usually is installed

You can see how much has be removed just to get to the evaporator and heater core...

I discovered the first issue at the hot water valve
The hoses around the time relay delay were leaking a little bit
For testing purposes I hung a spare master vacuum switch into the car - this solved the problem
The old fast idle diaphragm was leaking and brittle and was replaced with a better one...
1967 Cadillac Automatic Climate Control Vacuum diagram
The 1967 Cadillac Automatic Climate Control Vacuum Schematic from the shop manual -
1968 is almost identical BTW... Click on the image for a larger view!

Here is a video about the Master Switch that a gentleman posted on youtube