Air blowing out wrong vents

[Wade Nelson]

Has anyone ever dug in and diagnosed or debugged a HVAC system?

There appear to be 3 vacuum "motors" that make the whole thing work, and it appears one of mine isn't working.

Cold air (AC) seems to come out the defrost vents rather than the vents it's supposed to. Heat and defrost seem ok, so I'm thinking it's the vent door actuator.

The floor door actuator SEEMS to be working ok. Really can't tell if the bypass door is working, effect is too small.

FSM is of limited help.

Other than broken hoses / hoses fallen off barbs, any common problems / failures with this system I should look for?

[Kurbycar32]

You dont have what model car your working on but since its AC the vent system is mostly the same. I have a 260z with a Factory service manual and the AC section of that book actually has extremely detailed information on troubleshooting air flow. To confirm i downloaded the 260z manual from here: http://xenons30.com/reference.html and opened the AC section. Figures AC 7-9 show how the valves should be opened for vent, AC, and heat operation. From your description i would assume that your mode door is either stuck open; because of the vacuum mode door actuator valve not working or the seals around the mode door are gone and leaking a bunch of airflow. Page AC-47 has a flowchart on diagnosing this type of problem but is mostly what i just listed.

[Walter Moore]

His profile lists the car as a ZX. (From memory a 79 I think...)

I have heard that the automatic climate control version of the ZX can fail in a way that causes the airflow to be directed to random vents, and that it is difficult to fix. But that is just hearsay, so if someone knows the real story please chime in.

[Wade Nelson]

1980 ZX. I just drove it out to San Diego and back (2000 miles total) and here's what I found.

On AC settings, cold air comes out NOWHERE. It's "as if" the thing has blocked ALL the exits.

The VENT setting in the middle DOES send air out the AC vents, as does bi-level.

Heat comes out the bottom, as planned, but not nearly as much DEFROST comes out the defrsot vents as it should.

The little open/close switch under the ignition switch --- can't really tell what the heck it's doing, if anything.

I'm convinced two of the hoses got swapped, but I'm just not sure which ones. I've got the FSM and trying to construct a truth-table to deduce which two are reversed BEFORE I tear the dash out...again.

My best guess --- right now --- is the fresh air vent line got swapped with the vent door up top, as shown in the FSM.

Upside down under the dash is my favorite working position.

NOT!

[Wade Nelson]

Okay, I got this thing fixed. None of my vacuum lines were cracked or off their barbs, all the HVAC door motors were working, hot/cold cable adjusted properly and so on. That only left one thing. The control head.

The way it works is it has two disks that rotate relative to each other. The bottom one has 8 hose nipples on it. The top one is toothed, like a gear.

Apparently the upper disk has a variety of passageways in it, and rotating the selector knob lines them up with various nipples on the bottom disk.

Don't ask me how it got that way (It seems virtually impossible) but the disks were mis-aligned. I put the selector on VENT, where air was blowing out the vents I wanted for AC, pryed the toothed lever up so it wouldn't rotate the top disk, and rotated the selector lever to the coldest (furthest left) AC position and set it back down. I then tried it in all the other positions, and air came out the right vents, including defrost.

I simply cannot imagine how this disk got mis-rotated, but it did. Someone installed a new control head and didn't line it up?
It wasn't from hitting a pothole, it took considerable effort to pry the lever up above the top disk.

Now, here are TWO things I realized that will HELP you diagnose YOUR system when the air comes out the wrong vents.

When the selector is on VENT, ALL of the vacuum ports should be connected to vacuum. Every vacuum motor should be actuating.

When the selector is on RECIRC (far right, past defrost) NONE of the vacuum ports should have vacuum.


I still have ONE issue, which is the AC quits coming out the right vents when I floor it and lose vacuum. My guess is I'm missing a one-way valve up at the vacuum reservoir, or else it's stuck open. Alternately, a small but continuous vacuum leak somewhere could cause the reservoir to be depleted. This one should be easy to fix, can use a smoke machine if necessary.

HTH

[Kurbycar32]

Is this the disk thing your talking about? I put the yellow arrow over it

[Wade Nelson]

Yes. That's it viewed from the bottom, seeing the barbs the hoses go on. Mine had more barbs, is that perhaps one for the automatic climate control?