The A/C switch behind teh control panel.
 

seem to control both vacum tubes and wiring for the heating? Any idea what its called?

Re: The A/C switch behind teh control panel.
 

I think it's called a vacuum switch, I had to replace one on my 83 K5 back in the 80's.

Re: The A/C switch behind teh control panel.
 

Thank you. Can anyone tell me which years used an identical switch to a 75blazer? I can't think that something like that was changed too often.

Re: The A/C switch behind teh control panel.
 

bump to the top - I have not been able to find anything myself for a 75 - LMC has teh switch for 83 - 91 but zip for older models.

Re: The A/C switch behind teh control panel.
 

I'll jump out there with a big guess, I was hoping someone who knew would respond. I've read here that some of the earlier round eye Blazers and Burbs had heat/AC fans that ran at a low speed, even with the fan switch all the way down - there is no OFF position. Someone else here even said it's like that so you won't get exhaust in the passenger area, that it's there to create positive air pressure inside the truck so that any exhaust fumes in the low-pressure area at the back of the truck won't be drawn inside the cab around any air leaks that might be present in the tail gate area.

Someone else here, member motornut who has two round eye K5's, posted in a "favorite budget mod" thread Here: Your Favorite Budget MODs ($50) that he likes to snip the low wire on the switch so the fan will have on off position. See post 14. John (motornut) does not say what color wire it is, and he does not post near as much as he used to but he does still log on pretty regular. If someone here doesn't respond what wire it is, might send John a message. He won't bite.;) HTH