There is a vacuum switch that controls the on-off functioning of the entire system. It is supplied with vacuum from the intake manifold via a thermoswitch located on the bottom of the servo (or is it the servo bracket – memory’s going…). If the main switch does not receive vacuum, the system blower will not turn on. This is likely to be your problem.

OR you could have a large enough vacuum leak from elsewhere in the system which will not allow enough vacuum to develop to operate some/any of the vacuum switches. Whip out the vacuum pump and test every diaphragm in the system. If they are all good, check the vacuum switches directly – current should flow (continuity) when vacuum is applied..

Good Luck.

RTH