kentuckyKITT

iTrader: (4)

Forgive me if this has been covered, because I could not find it after about an hour reading through threads. On my 90 Formula with A/C, it was stuck blowing air through the defroster since I've owned it. I suspected the diaphragm was leaking. I got another HVAC unit for it and finally got around to replacing it today. After swapping faces/***** to the better unit, I noticed something different. The one in my 90 has two vacuum selectors. The typical large one on the passenger side and another smaller one mounted horizontally on the driver's side. What the heck? What's the smaller one do with two vacuum lines? As a test, I installed the new unit that does not have the 2nd diaphragm and turned the car on. At first it was still blowing through the defroster, but then I plugged the smaller vacuum line connector with my fingers and it worked properly through all the settings. Okay, so I got small piece of hard plastic tubing, heated and bent it in a U-shape and used it to seal the smaller one since it won't connect on this other unit. As far as I can tell, it works the way it's supposed to with that smaller one by-passed. So, what does that do?

I could not install the smaller diaphragm on this other unit since the metal slider underneath doesn't have the groove for it. I'm a bit puzzled that there are differences. Never noticed it before. So, tell me what's not going to work now because I can't tell. What am I missing?

John in RI

iTrader: (167)

Later cars needed a second vacuum source to control the Coolant bypass. Your replacement HVAC head is from an 86(7?) or earlier Thirdgen. I think the coolant bypass was on 87+ cars.

Drew

iTrader: (58)

kentuckyKITT

iTrader: (4)

So, if I'm understanding this correctly, it controls the plastic heater control valve in the engine bay where all the heater hoses connect, which means if bypassed as I have it, means coolant will flow through the heater core at all times like my 82 does? Hmmm, well fudge. Guess, I'll be swapping the large diaphragm to my unit then and hope like hell that the smaller diaphragm was not the source of the leak, otherwise I'm back to square one again. I have another unit around here somewhere that leaks, hopefully it has the smaller diaphragm as well in case I need another one.

Drew

iTrader: (58)

Should be able to just use a bolt & nut to replace the broken clip that normally holds the valve together.

Drew

iTrader: (58)

Here's what I mean... The small valve is in the first pic, and you can see the plastic clip that holds the valve together. In the second pic of the larger control valve, you can see how one side of the clip's prongs is busted and missing. When that happens, you can either replace the clip (good luck sourcing a replacement) or you can remove the clip, and run a screw and nut through it to hold the valve together.

There is a thread around here somewhere showing how to do it. I'd pull up google and type in something like "hvac valve fix" and site:thirdgen.org. Should be pretty easy to find.