Until a few days ago the ATC (Automatic Temperature Control) in my low mileage 66 worked perfectly. All functions were operating as they should.
I rebuilt the carburetor and let the car idle for a while. During this I wanted all accessories to move and also turned on the heat. It came on as always and after about 30sec the blower went off again. The power servo started to move in and out all the time by itself, but the blower would stay off. The idle speed up came on and off as well depending on the placement of the power servo. After a while it stopped doing this and stayed in the fully retracted position (full heat). If I turned the temperature dial it did not change anything - but it was around freezing outside and this is what it should, do as the sensors sense the low temperatures and then send the system into full heating mode.

I checked the following things.

• Master Switch seemed to work and gets engaged by vacuum. Power is going through it as it should. Current is going in and out. (This is what I thought…)
• Blower Motor tested with a direct feed - worked
• Blower Motor Relay - tested. Put a 12 V feed to the dark blue connector and it engaged and the blower motor came on. There was no current coming through the blue wire though.
• Power Servo - cleaned all contacts and measured the resistance in all positions  - looked good to me
• Control Head in Dash - removed it and checked the blower switches - it seemed to work.

I checked the amplifier part and the big square resistor  was showing the 50 Ohms it should. Also measured the rheostat on the temperature dial, which seemed to work as well.

Full vacuum was applied to the power servo, and I also checked the vacuum and voltage at the transducer. I also checked all vacuum actuators with a vacuum gauge, but everything checked out perfectly.

I then put everything back together. Still the system did not work in the full heat position. I did a lot more tests and checked the vacuum and electrical diagrams.
I ordered the 1966 AC manual from "Cadillac Tim" which is available also on ebay for $45,- . These instructions are a must-have for your car, as everything about the AC system is explained very well by Tim. There is a great troubleshooting guide in there. Its money well spent! Tim also offers a rebuild service for many Cadillac parts and has a great selection of refurbished parts. Check out his website!

The 1966 dash control unit - here you can see the amplifier part.

After days of troubleshooting it finally dawned on me where the problem was. I had checked the master vacuum switch multiple times and when checked it always had electrical continuity. But I did not check it correctly. I only pushed the backside of the switch until I heard a click.
I should have tested it with full vacuum applied. As it turned out, if high vacuum was applied to the switch (during heating mode), the switch did no longer work correctly!

The contacts did no longer touch when full vacuum was applied to the switch…

Today I took it apart and made a video about it. As it turned out the switch contacts did not align as they should and I had to bend it a little bit to make it work again. I then put everything back together and now the AC system works perfectly again. Check out the video for all the details!

Tags:AC, Master Switch, Master Vacuum Switch, Vacuum Master Switch, 1966 Cadillac Coupe deVille, 1966 Cadillac Repair Tips, 1967 Cadillac Eldorado Repair Tip, 1967 Cadillac Repair Tip

A few years ago I sold my awesome, low mileage, all original 1966 to a friend. I always regretted that I sold it, but I did not have any other option back then.

Through a couple of car trades within my Cadillac friend community, the car became available again, and my dear friend Georg helped me to get it back.
I can't thank Georg enough - without him I would have never gotten the car back!
Today was the day to pick it up at my Georgs garage and drive it home, although I do not really have enough garage space anymore…
READ MORE!

Taking over the keys - THANKS MY FRIEND!!!!

This is how the shop messed up the ATC system of the car… I reversed everything back to original!

READ MORE...

Tags:1966, AC, 1966 Cadillac Repair Tips, 1966 Cadillac Coupe deVille, Master Switch, ATC, ACC, Master Vacuum Switch, Blower

When the 1966 Coupe deVille arrived at my garage the AC was completely inoperable. Nothing worked - the blower would not come on at all.
So I started to search for the cause of the problem.
Everything seemed to be fine on the vacuum side. When the lever on the control panel was moved all the vacuum actuated doors operated and the master vacuum switch also seemed to work.
After some searching I found out that no current was flowing through the master vacuum switch on the firewall.
So the switch was taken apart.
If you do this you have to be very careful not to damage the membrane.
The contacts inside the switch were oxidized and so no more current could flow through.
The contacts were also adjusted so that the switch could engage properly when vacuum was applied.
When everything was put back together the whole system once again worked as it should.
It works absolutely perfect now.

*UPDATE 2022* - Check out this article and video as well!
Unfortunately I forgot to take pictures of the switch rebuild but here is a video on youtube where a gentleman explains how this switch works…

A video that explains how this switch works


The original AC system of the 1966 Coupe deVille is now working perfectly again.

Thats the master vacuum switch which was defective.

The rebuilt master vacuum switch.

Tags:AC, 1966 Cadillac Repair Tips, Restoration, Video, Power Servo, Rebuild, Vacuum diagram, 1966 Cadillac Coupe deVille, Master Switch

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...

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

Tags:AC, Blower, 1967, Eldorado, Restoration, Troubleshooting, 1968, Aircondition, ACC, Vacuum hoses, 1967 Cadillac Eldorado Repair Tip, Vacuum diagram, Evaporator, Heater Core, Hot Water Valve, Vacuum Schematic, shop manual, 1967 Cadillac Repair Tip, Master Switch