Found a lot of similar threads, but none that discuss my question directly.

Car is my 1989 Iroc. I have TWO vacuum lines coming out of the firewall, all taped up with the rest of the HVAC harness. One goes to the back of the HVAC vent mode controller, right? I DO want this to function.

The OTHER...and I could be wrong here, but is it connected to the diverter valve? .....and if I no longer HAVE the diverter valve connected....would that potentially cause a vacuum leak??? Since I don't know...I'm not going to over think this. That's why I'm posting up.

Any thoughts?

 

For the record, it's a weekend, nice weather car, and I haven't had heat in many years. I DO however like to have the fan/vent options since I drive in summer a lot.

 

Ok, one goes straight to the diverter valve. The other is tee'd off to the back pass side of plenum and runs to cruise unit on drivers fender and again tee'd to the vacuum ball in the fender. If no diverter valve, you would need to cap that line or you'll have the leak. HTH!

 

OK, well I haven't had cruise control OR heat in a gazillion years. .....am I correct in that one of them controls the vent/mode? ....I mean, all I want is to be able to turn on the fan and get some air movement inside the car. Do i even NEED vent/mode??? I know the blower works. ....right now, the only vacuum lines are MAP, AFPR, PCV, and EVAP. ....I mean it's pretty stripped down.

 

I'm pretty sure you will need one of them to go to the back of the heater controls head. Default mode is defrost as you are aware. So, without vacuum, it will come out the defrost. Other than that one line, that's all you should need.

 

OK, I can track down that one line to control the vent mode....

....now the OTHER line, which as of now is just hanging out unconnected, and uncapped....do I need to do anything with it, or does it need to be capped? If it's somehow connected to the first line, and that first line is connected to vacuum, then leaving the second line uncapped would be a leak, right? ....sorry for the questions, I just can't picture the schematic of how it's all connected.

 

If I'm not mistaken, if you put the panel lever in the HEATER mode, you will be sending vacuum from the back of the control unit to the diverter. So in essence, you would have a leak at the end of the hose. So I would plug it.

 

OK, that's exactly what I was thinking. ...that makes sense.

 

You can run manifold vacuum directly to the HVAC controls but when the engine is under load the air will divert to the defroster vents.

The vacuum line needs to be tee'ed off the line that runs between the one-way valve and the reservoir ball.

 

Guys, thanks for the info.

Paulo, that's interesting, and it makes sense since vacuum changes under load. Those "other" parts of the vacuum system are long gone. My car was never intended to be an original/survivor car, but I admit, I junked a bunch of stuff way back before I came to appreciate all the little amenities GM bestowed upon our thirdgens. That being said, 99% of the time I drive the car, it's sunny, and between 50 and 85 degrees. She lives a pampered life for sure!

...one day I may look to restore/replace some of the functionality, but for now, I'll make do!