A/C vacuum line. HVAC

Kcrist5oh · May 29, 2019

Hello, going thru this fox I picked up. My HVAC control for temp (hot to cold) works. Fan and all speeds work. However my blender does not work, no matter where the knob is positioned it is stuck on defrost. A/C has been deleted. The "A/C" line on the vacuum tree is plugged. I assume this has something to do with my problem. Where is this line (from A/C on tree)?

stanglx2002 · May 29, 2019

Yes, you are correct in that the vacuum line that is plugged is the issue. The HVAC control are vacuum-regulated, so without the vacuum it can not change to the different settings.

Kcrist5oh · May 29, 2019

So I could just run it to one of my upper intake vacuum nipples?

General karthief · May 29, 2019

Plug it into the tree in the proper place

Kcrist5oh · May 29, 2019

Where is the proper place, from the tree?

General karthief · May 29, 2019

You mean from the tree to the hvac? Or from the intake to the tree?
there should be a line from the intake to the tree, then the ac line is a small plastic tube that runs across the firewall to the right side of the engine , from the tree.

Kcrist5oh · May 29, 2019

Yes from tree to HVAC. I'll look for a line there. Thank you

Mustang5L5 · May 29, 2019

There should be a small plastic vac line that runs over to the area where the A/C accumulator is and then passes through the firewall. It's often ripped out accidentally when the EGR or air pump solenoids are deleted.

Wayne Waldrep · May 29, 2019

Right there.

NOTE:borrowed pic.

KaptKane · Jun 13, 2019

Still a newbie here (even tho I'm on my 3rd Fox Body in a year, but that's another story) and wasn't sure if this was the correct way (or place) to ask but...
My 1990 5.0 HVAC system seems to work as it should when I'm running around town ie; driving it for 45 minutes or less. When I take it on a "road trip" ie; and hour or longer (usually in AC mode since I live in FL) (running at highway speeds) it acts like the blend doors inside the heater box slowly close off the air supply to the proper ducts/vents. I can hear the fan still running on high speed and what little bit of air seeps past is cold. Doesnt matter if I cycle it thru A/C, heat, defrost etc. I've tried going full throttle...doesn't seem to change it. I've tried downshifting and letting off the gas (to increase vacuum)...doesn't change it. Idle is smooth (as can be expected). I don't think I have a vacuum leak but...
I can't get the air volume back where it should be unless I shut the car down and let it sit 30 mins or more.
Help?
Thanks in advance.

Wayne Waldrep · Jun 13, 2019

Look at the picture I posted above. That vacuum line controls 100% of the controls you are referring to. Make sure that line is connected under the hood. It goes over to the vacuum tree (marked a/c) on the firewall just above the brake booster. Then there is a multi colored set of vacuum tubes under the dash that divides the vacuum to the different places on your selector knob on the hvac controls. Those tubes (especially the black one) will slide out of the gooey clear plastic connector and will show the exact symptoms you are describing. I'll try to find a pic of that in a minute and post it. In the pic below its the lower left connector with the red, white, blue, and black hard vacuum lines.

KaptKane · Jun 15, 2019

Thanks! Is that located behind the glove box?

Wayne Waldrep · Jun 15, 2019

Sorta, yeah.

KaptKane · Jun 17, 2019

Thanks. Found it, right under the glovebox. Mine appears and feels like the lines are in good and tight. I guess I could put some silicone around them to make sure they’re sealed. Any other thoughts?

Wayne Waldrep · Jun 17, 2019

No on the silicone.

Did you check the one under the hood? Is it connected to the vacuum tree? I just read over this thread from the start. All the possible solutions are already mentioned. I don't want to muddy the waters and retype all of it. Read from the start. If that doesn't work, post exactly what you've tried. Be specific and don't leave things out. Take a pic of your car so we can see the area I posted in the first pic (between the hood hinge and the throttle body like my pic). I'm looking for that same vacuum line and how you have it connected. Also a pic of the vacuum tree on the other side firewall.

Davedacarpainter · Jun 18, 2019

One of your vacuum actuators may be going bad. They are what move the individual doors.

When I tore my box apart I had to replace one.

I found a nos one on eBay.

Mustang5L5 · Jun 18, 2019

Blend door is actually mechanically controlled by a cable. The vacuum simply changes which registers the air blows out of and open/closes the AC max door down by the passengers feet. I would recommend smoke testing the HVAC portion of the vac lines to look for a leak. There are a few check valves and such as well as vacuum actuators and a sustained leak over time can cause the doors to slowly move and require a restart to reset.

I'll admit...not an easy thing to troubleshoot.

KaptKane · Jun 21, 2019

Wayne Waldrep said:
No on the silicone.

Did you check the one under the hood? Is it connected to the vacuum tree? I just read over this thread from the start. All the possible solutions are already mentioned. I don't want to muddy the waters and retype all of it. Read from the start. If that doesn't work, post exactly what you've tried. Be specific and don't leave things out. Take a pic of your car so we can see the area I posted in the first pic (between the hood hinge and the throttle body like my pic). I'm looking for that same vacuum line and how you have it connected. Also a pic of the vacuum tree on the other side firewall.

I looked and all the lines seem OK and right where they join. Here’s the other pics.
Thanks.

Wayne Waldrep · Jun 21, 2019

Just from what I see that all looks good. Gonna have to think on this one and let the group collectively help you.

KaptKane · Jun 21, 2019

Mustang5L5 said:
Blend door is actually mechanically controlled by a cable. The vacuum simply changes which registers the air blows out of and open/closes the AC max door down by the passengers feet. I would recommend smoke testing the HVAC portion of the vac lines to look for a leak. There are a few check valves and such as well as vacuum actuators and a sustained leak over time can cause the doors to slowly move and require a restart to reset.

I'll admit...not an easy thing to troubleshoot.

Thanks for the schematic. I'm starting to think that one of the internal actuators is (going) bad. I wonder if (and where) I could attach a vacuum gauge (with a T) into the system lines and see if I can see it loosing vacuum when it starts to happen?

A/C vacuum line. HVAC

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