Doing a motor swap, and of course the hole in my passenger side exhaust manifold of the original motor toasted the hvac harness. None of the wires were damaged, but one of the vacuum is completely melted to the point i touched it after unwrapping tape and it fell off in my hands. Now i've pretty much figured out that it's the long hose for my hvac settings cause after that incident i was stuck with defrost all summer, but whats the shorter vacuum line that runs into the engine bay and has a 90* bend to the adapter end? Probably a simple answer but i can't find it in my service manual. Thanks!

 

Two lines go into the firewall.

1) Comes from the tree on the intake manifold - basically there's a line off the tree - a check valve is in place with 3 ports on the valve - one goes to vacuum ball under the driver headlight - this "ball" holds vacuum for the controls, one goes to the tree, and the third goes into firewall for the controls - the check valve allows the ball to hold vacuum without it escaping back through the system.

2) The other line runs OUT from the A/C controls to the heater diverter valve - this sits on the pass side, has coolant lines to it, and when controls set to HOT, the vacuum opens the valve and allows hot coolant to circulate through the haeter core, when on COLD, the valve closes, and the hot coolant is not allowed to run through the heater core.

Hope that helps!

 

So shall i assume since i'll be running complete 4th gen hoses and cooling system, that I no longer have use for the second vacuum line that you say lets coolant flow into the heater core? I'll just need to hook up the one line to the intake for a vacuum source to the controls to fix my "stuck on defrost" mode and get all the other modes back? Thanks!

 

Also, if logic holds true, i should be able to cut out the burnt part of the vacuum hose, and hook up my brake bleader vacuum hand pump and as long as i can keep the vacuum test all my functions of my hvac before dropping the motor in?

Also, to what i said before, does that second hose need vacuum to move the door from cold to hot as mentioned above? So in theory i would need both hoses, one for functions and one to move the heater core door back and forth? Right?

 

I don't remember there being 2 hoses, unless you're referring to the fact that there is a vacuum reservoir T'ed into in the system.

Most 3rd gens use a T after the check valve. One side of the T goes under the dash to move the HVAC servos, the other side goes to the vacuum reservoir (round plastic tyhing in the front of driver's side inner fender). Since the reservoir and the HVAC controls are both after the check valve they're are essentially on one line, just that the vacuum supply source taps into the middle of that line (at the T) instead of at one or the other end of it. Goes like this, if I recall correctly:

Engine --> vacuum hose --> check valve --> short hose --> T with lines running in opposite directions off of it (one to the vacuum reservoir, the other up under the dash along with the main wiring harness)

Many other GM cars have 2 connections at the vacuum reservoir itself. One is the vacuum supply side from the engine, the other side goes up under the dash to run the HVAC flapper doors.

Early carbureted 3rd gens have a system like the second example, but the reservoir is a plastic "mailbox" shaped thing located near the power brake booster.

Functionally, they all work identically. But the vacuum line routing is different.

 

Naw, i have two hoses going through my firewall on the passenger side right above the transmission tunnel. It may be an LO3 thing, but not sure what the 2nd hose does as i know that one of them controls HVAC. Curious what that second hose controls, if it just controls a solenoid in the engine bay to allow coolant into the heater core like mentioned above and nothing else, or if it's a second seperate vacuum line to control the hot/cold flapper door.

Is it possible that one hose goes in and controls things, and then sends a second hose back out that routes to the vacuum ball?

 

No one of them hoses goes to the heat diverter valve. It goes straight across n plugs in. Dont try to understand it, just hook it up n let it do its job. I almost got into a fight with my friend over it because he didnt believe it had anything to do with the heater. Anyways just wanted to 2nd camaronewbie since there was some doubt