“Sometimes the dash control can be switched to the fog or ice positions to bring the blower motor on and then when it is switched back to auto the blower may stay on and the system seems to work fine until the car is shut off. If the vacuum leak is only moderate there may be enough vacuum to hold the master switch closed once it gets , but not enough to pull it closed in the first place. When the dash control is switched to fog or ice, it assumes the air temperature is cold and that the engine is not yet warmed up, but that the blower is needed immediately for defrosting the windshield. So it supplies vacuum to the master switch through another circuit, bypassing the temperature controlled vacuum switch on the heater core shutoff valve on the right fender well. This source of vacuum is enough to close the master switch.When the dash control is switched back to auto this vacuum source is shut off, but there may be just enough vacuum left from other circuits to hold the switch closed. This routine will work for a while but as the leak gets worse, there won’t be enough vacuum left to hold the master switch closed anymore and the blower will shut off again as soon as the system is switched back to auto.With these symptoms and behavior the problem is most likely the power servo. This is half the brains of the system; the other half is the control panel in the dash. The power servo is controlled by a varying vacuum signal from the transducer. This vacuum supply is a completely separate circuit from that which supplies vacuum to the master switch. High vacuum moves the servo to the maximum heater position, and with low vacuum, a return spring moves it to the maximum AC position. In between these two extremes, it regulates the air temperature door to blend warm air and cool air, and simultaneously regulates the blower speed. It also spins an internal rotary vacuum valve back and forth that makes various connections between the several small vacuum lines on the top. This rotary valve controls the various vacuum motors of the system that operate the AC, heater, and defrost functions. It is this rotary valve that goes bad. l have seen these things visibly warped, and very badly so. It must be caused by engine heat, because there is also one inside the car on the control panel that is moved back and forth by the dash control lever, but that one seldom seems to cause any trouble. A warped rotary valve leaks all kinds of vacuum and the first part of the system to be affected by low vacuum is the master switch; there’s not enough vacuum there to close it and turn on the blower. To verify this is the problem, take some spare vacuum line, some scraps of 1/8 inch pressure line (like for air shocks), and a vacuum T and manually make the connections that the rotary valve should make in the maximum AC position. The connections that the rotary valve makes in various positions are shown in the factory shop manual. Disconnect the black connector with the striped vacuum lines from the power servo. Connect the yellow, red, and purple lines together with a T; also connect the tan and blue lines together and connect the orange and green lines together. If this rotary valve is the one and only culprit the AC will now work beautifully and the blower motor will come on every time, like magic. With the dash control on high the system should be recirculating air and there will be a lot of air noise coming from under the right side of the dash; when the dash control is moved to auto the air door should move to provide fresh air and the system will become much quieter. Of course with these manual connections the system is not fully automatic and the heater won’t work right at all, however this is enough to diagnose the problem.“
So this rotary valve needed to be repaired...
I removed it from the power servo and disassembled everything.
The removed power servo
after opening the cover, you can see the rotary valve in the lower center.
remove one screw and a clip and the rotary valve comes out.
The rotary valve consists of two halves which make the connections - as you can see it was very dirty and rough - and slightly warped.
I started to wet sand both halves with 800 grit sandpaper.
When everything was flat and shiny again I used some 2000 grit sandpaper for a perfect finish.
Looks shiny and flat again
Make sure that none of the ports are clogged - I used compressed air to clean everything.
When you put the two halves together lubricate with WD40 to make sure that when you put them back into the power servo that they are properly held together with the underlying spring.
After I reinstalled them into my cars - almost everything worked as it should. In the 67 Eldorado I still had the problem that in the “AUTO“ position the fan would always be slow - so I knew that the control head in the dash had a problem with the “LOW - AUTO“ switch as well.
In the 67 DeVille the “VENT“ and “LOW“ position still did not work at all...
So I switched the control panels between the two cars and suddenly everything in the 67 Eldorado worked perfectly. I never had such a perfectly working system before. Its like it just left the factory!
In the DeVille I still had no “VENT“ and “LOW“ settings working... So out came the control panels again... (its a pain in the a*** to remove and install them...)
As it turned out the “LOW-AUTO“ switch on one panel was stuck in the “LOW“ position. You can easily remove it and readjust it to work again - sorry I forgot to take pictures. Make sure that all switches on the control head are adjusted properly or they wont work as they should!
So I knew that both control heads were working correctly by installing it back to the 67 Eldorado and the other panel back to the 67 DeVille.
While the 67 Eldorado was perfectly fixed now with everything working as it should - the 67 DeVille was still giving me the problem with a non working “VENT“ position...
I studied the shop manual for hours and could not find what the problem could be. I knew that the power servo was working perfectly now and I knew that the control head was in perfectly working order as well. I verified this once again by switching these parts between the cars once again and everything worked as it should in the Eldorado but not in the DeVille.. I´m really good in switching these parts between my cars now ;-)
I started to look for electric connection problems and finally found out that no electricity was coming out of one of the three power servo connectors, but I did not know why...
After I removed the power servo once again to re-check it again it struck me - one of the pins was slightly bent and the wire connector did not make proper contact. This problem was not visible with the power servo installed in the car. It took me 2 long nights to find this little flaw... I just bent it straight and everything works as it should on both of my 67 Cadillacs. Now I have to recharge the systems and I should have perfectly working AC again... I will also install new dryers and a re-calibrated R134 POA valve before the recharge.
One the one hand the 1967 ACC is very complicated but once you know how it works its pretty easy to fix. The rotary valve in the power servo is pretty easy to refurbish, the dash servo valve is riveted together unfortunately, so its not such an easy fix, but NOS valves are still available at “Classic Auto Air“