Climate Control article by Lynn Nicholls from http://www.modifiedcadillac.org/articles/Climate1.html
The climate control system in 1968 Cadillacs has some chronic problems that l will address. Virtually the same system is also used in 1967 with only very minor differences from 1968, but other years are completely different. The most common problems are: the blower motor won’t come on at all, or the blower motor won’t come on in air conditioning mode even though it may still work fine in heater mode, or cold air conditioned air blows out the floor vents. l have seen many cars in wrecking yards with a toggle switch stabbed through the lower dash wired directly to the blower motor to make it come on, which looks absolutely horrible. It is a brute force solution when the correct fix is unknown.
These climate control systems look like a nightmare but it’s not necessary to know what every single vacuum line is for to fix the system. Having a factory shop manual tremendously helps diagnosis. Fortunately all the special diagnostic equipment it describes isn’t necessary, although a vacuum pump with an attached gauge is very useful. The system can be broken down into two main parts, the first of which I will call the Freon system, which includes the compressor, evaporator, condenser, drier, and POA valve, and other related parts. The second I will call the automatic control system, which includes vacuum motors, vacuum lines and other automatic control devices. Assuming the blower motor will come on, if the system puts out cold air then the Freon system is working. All the problems mentioned above are in the automatic control system.
Incidentally, if the system is low on Freon to the point where it no longer blows cold air the system needs to be shut off, or at the very least the compressor needs to be unplugged. In 1968, the system doesn't have a thermal protection switch for the compressor in the event that pressure in the system gets too low to carry oil, and if the compressor is allowed to keep running it will be ruined. The thermal protection switch started being used just a few years later.
If the blower motor won’t come on, the first thing to do is, with the engine running and the system on auto, push upwards on the underside of the master vacuum switch on the top of the firewall shown in figure 1, until a click is heard. If the vacuum line system is working properly, it may already be collapsed all the way. Once the master switch is collapsed all the way, it closes the circuit to the blower relay and if at this point the blower motor does not come on then there is an electrical problem somewhere. The most common cause of this is a poor ground connection on the blower relay. A ring terminal held against the relay case by a screw running into a fiberglass housing is not a good ground. It worked fine for a while when the cars were new, but when the metal gets dirty this is an easy source for trouble. Some replacement relays have a ground terminal next to the other three terminals that go into the plastic wire connector. In this case take an extra wire with a spade terminal on one end, connect it to the relay ground terminal, and ground the other end. In my case the relay didn’t have a separate ground terminal, so l scratched an area of the case clean and soldered another wire to it and grounded that, as shown in figure 2. l left the ring terminal where it was even though it’s now redundant.
Figure 1 (The "tugboat")
Figure 2
