temp control 89 5th ave

Monkeyed · Sep 7, 2014

I have little to no control over the hvac temp. the a/c works, the buttons all seem to function properly, I put a thermometer in one of my vents, and the temps are random. usually low, but sometimes 120*+. the Haynes manual wasn't too helpful. It shows a "typical" later model hvac unit with a blend door, and a temperature control cable next to the fan. I got under the dash and didn't see anything like that. I could see one servo with vacuum lines on it closer to the top behind the glove box, but I'm not sure if that's it or not. the slider doesn't feel like it has a cable on it, but I don't remember from when I had it out to change the bulb in it..

does anyone know if the temp control is vacuum operated, is there a door, or a valve?

Monkeyed · Sep 8, 2014

no one?

Jack Meoff · Sep 8, 2014

I was hoping you'd get an expert to chime in but I guess you're stuck with the driveway mechanic.....let me dig into my factory service manual when I get home and I'll see what I can find out.....never torn one apart myself.

Jack Meoff · Sep 8, 2014

Do these help at all?

Jack Meoff · Sep 8, 2014

According to my 86 FSM there is a cable controlled blend door for the heat.
As seen in the pic....

MiradaMegacab · Sep 8, 2014

No cable is an ATC unit(Automatic Temp Control) IIRC

Monkeyed · Sep 9, 2014

I see a water actuator in the second pic. the reason I don't know it it's a cable/door problem, is it should at least pull the door one way, then do nothing, or be sluggish going back the other way. right now it's hot or cold, and very slow to switch (if it does at all) with the slider.

Monkeyed · Sep 9, 2014

unless, the cable was broken, or not connected, but then it should stay at one tempature permanently.. did I mention, I really don't want to have to take the dash out.

Jack Meoff · Sep 9, 2014

I'm no expert on this but I'm thinking it's one of the vacuum lines.
That means PITA........

Monkeyed · Sep 9, 2014

Captain Caravelle said:
I'm no expert on this but I'm thinking it's one of the vacuum lines.
That means PITA........

I'll take the control unit back out, maybe something got unplugged when I changed the bulb

Monkeyed · Sep 10, 2014

MiradaMegacab said:
No cable is an ATC unit(Automatic Temp Control) IIRC

My slider has numbers on it, and when you take the unit out it looks like there's a variable resistor on it.

Jack Meoff · Sep 10, 2014

So then it looks like this?

Monkeyed · Sep 10, 2014

even has ATC stamped right on it..

the resistor tests ok, 5ooish ohms, to about 12-1500ohms at the other end, the vacuum line going to the heater hose only has vacuum with the off button pushed, any other position and it fooks off to space, or wherever it is vacuum goes...

Jack Meoff · Sep 10, 2014

Wish I could be more helpful but I don't know this area that well.
What I have learned is supposedly the blend door is electric.
And from my own experience with my Fifth if I switch between heat and defrost there's a delay and then I hear the windshield vents open up.
So I'm figuring that's where the vacuum lines come into play.

Sorry man....sort of a grey area for me......

If you're getting random heat is it possible your blend door is sticking?

Monkeyed · Sep 10, 2014

Captain Caravelle said:
Wish I could be more helpful but I don't know this area that well.
What I have learned is supposedly the blend door is electric.
And from my own experience with my Fifth if I switch between heat and defrost there's a delay and then I hear the windshield vents open up.
So I'm figuring that's where the vacuum lines come into play.

Sorry man....sort of a grey area for me......

If you're getting random heat is it possible your blend door is sticking?

it switches back and forth between vent/def/floor ok, it's the temperature that isn't working right. not finding much information about the operation either, there's a vacuum operated valve inline with the heater core, hmmm, maybe that is suppossed to only be opened or closed.. a sticking door would make the most sense...