joeys grandpa

Is there a "polarity" on vacuum switches? I am referring to the ones that screw into the water jacket and have 2 ports, one to the vacuum source and the other to the the purge valve for the carbon canister in this case. On the 81 928, the thermal vacuum switch is there to prevent the the purge valve from opening until the engine is warmed up. One port of the switch has a white dot on it. Anyway I am not sure if it matters where the vacuum source goes. My theory is that the German engineers would worry that the vacuum might add a bias to when the switch opens and closes.
Thanks
Harold