lf, when the master vacuum switch is manually closed, the blower motor does come on, then there is a problem in the vacuum line system somewhere. Using a T, connect a vacuum gauge into one of the lines going into the master vacuum switch. It should read about 18 inches or so of vacuum, about the same as manifold vacuum, which is the source for vacuum in the system. It takes only about 3 or 4 inches of vacuum to pull the diaphragm up and close the switch. Less than that may still pull up the diaphragm but not quite get the switch closed. If vacuum in the system is that low, there is a big leak somewhere, usually caused by a vacuum motor that has gone bad; the vacuum lines themselves are usually okay. Probably the easiest thing to check (but not to replace) is the AC/heater vacuum motor on the left side of the heater core housing; it has a tan striped vacuum line running to it, see figure 3. The original vacuum motor had a metal case. The replacement units are plastic and this one has an adapter for the vacuum line. Use a vacuum pump to check if this holds air. When vacuum is applied to this the system will blow air out the dash vents; when no vacuum is applied air will blow out the floor vents. The linkage this diaphragm moves has a spring attached to it and if this unit leaks, it will default to the heater position, as well as lose a lot of vacuum. The heater will still work fine even if it leaks because in the heater mode the system doesn’t apply vacuum to this anyway, and the leak then essentially doesn't exist. But in the AC mode when vacuum is applied to it, it will lose so much vacuum that there may not be enough left to close the master vacuum switch and bring on the blower motor. If it does happen to come on it will blow cold air out the floor vents. The whole fiberglass heater core housing must be removed to replace this, as the attaching nuts are on the inside. I haven’t been able to find an exact direct replacement for this; a part will have to be matched by dimensions and mounting hole spacing, and the linkage arm may have to be modified to fit. Replacement units l have seen have an internal spring to positively return the diaphragm and so the external spring attached to the linkage is no longer needed. If, when the master switch is manually closed, cold air does blow out the dash vents then this may not be the problem.
Figure 3
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 there, 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.
Another result of the vacuum level being this low is that in the AC mode, the recirculating/fresh air door will always be in the recirculating position. Low or no vacuum will leave it in recirculating mode and it takes a strong vacuum signal to move the air door to provide fresh air. The recirculating position will cause a lot of air noise under the right side of the dash. If the system can use fresh air it is much quieter. If the system is working correctly, the only time it should recirculate air is when the AC is on, the power servo is in the maximum AC position which also puts the blower on high speed, and the dash control lever is on high. lf any one of these is not the case it should be using fresh air.