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…
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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!

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