When driving today in my 1978 98 with Tempmatic I tried switching from Heater to Vent, at first it wouldn't switch at all, then the second time there was some air out of the vent but most of it still from the heater vent below the dash. I'm pretty sure it wasn't supposed to do that so I checked the chassis service manual, but two completely different answers. The heater diagnosis chart says it is, the AC diagnosis chart says it's not. While I'm pretty sure there's either a loose connection on the controller or a blown diaphram I still found it odd. Below are pics that I took from the manual.




Are AC and non AC car controllers really that different? I'm not really understanding why it is this way.

 

Yes, they are very different. I'm not familiar with '78 but earlier cars even had different vent configurations based on whether or not the car had A/C. Totally different controllers, totally different wiring, vacuum tubes, different hardware/tubes, everything.