I was having intermittent problems w/ the heat not working. I discovered that if I disconnected the vaccuum line from the heater control valve, the heat would kick back on. So I replaced the selector for the heater controls inside the car, but it didn't fix the problem. I used an old part I had that I thought was good... is this a common problem? How much of a chance is there that both of them would be bad? Is there anything else that has any control over the vaccuum going to the heater control valve? It's getting vaccuum even when it shouldn't, so that's why it works when it's disconnected.

 

There are 2 kinds of heater control valves: the kind that are normally open and close when vacuum is applied, and the kind that are normally closed and open when vacuum is applied. They are exactly the same, except completely different.

I'd suggest checking your vacuum line, and seeing when it has vacuum (that is, does it have vacuum when it's calling for heat, or when it's calling for the heat to be shut off), and make sure your valve matches your control's logic.

 

The line has vaccuum all the time, regardless of the position of the heater control switch. This causes the heater control valve to close(?), preventing warm antifreeze to get to the heater core. Unplugging the line removes the vaccuum from the valve, allowing the heater to work normally. I currently have the line disconnected from the valve and plugged w/ a bolt.

If the logic of the replacement control is the opposite, would I get heat if I turned the temp selector all the way to cold (thereby shutting off the vaccuum)?

 

Well no, because then, your little blend door (that the cable controls) would be over at the full cold setting, i.e. no heat.

Sounds like maybe the vacuum valve on your control head is farkled.

 

That's what I thought, so I replaced the original. But it seems unlikely that it would fail in a manner that causes vaccuum to ALWAYS be present. It is possible though...

And from what u'r saying, it sounds like the replacement part (which I believe came from my sister's '86 IROC) is set up for the 'opposite' system. However, I should still be able to shut down the vaccuum to the valve if I put it on the cold setting -- I still won't have heat cuz the blend door will be shut, but the vaccuum to the heater control valve should be gone. I'll have to double-check that...

 

You might want to look at it and see if it's moving like it's supposed to; and not broken itself.

Just the fact that you put another used one on there, is no guarantee that it's good. Definitely check it and see if it has vacuum when it's supposed to, and doesn't when it's not.