I've had some problems with my 85 El Camino with a 305 running rough. I found that my EGR valve isn't working. It get it's vacuum via a thermal vacuum switch between the carb and the EGR. There is a 2 pin connector that has 12 volts and what I assume is a ground wire. I can see vacuum changes at the carb, so I think the TVS is bad. My question is how does this thermal switch work? How does it sense when to open and allow vacuum to activate the EGR valve.