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 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:

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!

Tags:AC, Blower, 1967, Eldorado, Restoration, Troubleshooting, 1968, ACC, Vacuum hoses, Vaccum diagram, Vacuum Schematic, shop manual