HVAC blend door driving me nuts

[runderwo]

Took a few highway trips recently. AC blows ice cold on the highway. Blower is new AC Delco and it can be easily heard that it's working 100%. But, I think the blend door is screwing up and negating all of that. Without moving it from the Max A/C setting, it vacillates from blowing hard and ice cold to barely blowing lukewarm air to not blowing anything at all, even though the blower can be heard going like gangbusters. Sometimes it gets in a state where it won't start blowing Max again until the truck is parked, let sit and restarted. Also, when I hit the gas hard (intake vacuum drops), the door is affected by this and switches the blend to fresh air. Then it switches back to Max when I let off the gas and cruise. All without touching the blend switch from Max A/C.

How do I make this thing behave correctly and stop the complaints from the back seat? Is it the blend door or the HVAC control that is the problem? Also, should I be getting air blowing from the vents at the rear of the center console on a 91? How hard should it be blowing from there and on which settings?

 

[Iron Weasel]

First off, how's your vacuum reservoir and lines going to it? Are they all in decent condition? Secondly, check the vacuum line going to the blend mode actuator behind the glove box and see if it's cracked or leaking as well.

The vents at the rear of the console......I get most of the air coming out of there on the Floor setting, but at least some air comes out on all settings. Even when set on Floor, it doesn't move a whole lot of air back there.

Vents on the floor wouldn't do you much good for cold air anyway since cold air is more dense than warm air and falls to the floor anyway...so at best you'd be making the rear passengers feet cold.

 

[runderwo]

The lines going in and out of the reservoir (the black plastic blimp?) both cracked a long time ago. I replaced the one that comes from the octopus with some fuel hose. The one that goes to the HVAC control head, I used a piece of rubber vac hose that fit onto the tee coming off of there, and slipped the other end over the thin plastic line and sealed it with RTV. Could the reservoir itself be leaking?

 

[Iron Weasel]

It's possible.

The whole point of that reservoir is to maintain vacuum on the HVAC and cruise control systems when the engine load changes and the vacuum along with it. The only other thing I can think of if it's not the reservoir or a line is the blend mode actuator itself. If that's worn out it'll let the mode door move around however it wants.

 

[runderwo]

What's the best way to check the actuator? I guess the reservoir can be checked with a vacuum pump, should it hold vacuum solid or bleed slightly?

 

[Iron Weasel]

Reservoir:

Apply initial vacuum of 15 to 20".
Vacuum loss should not exceed 0.5" in 60 seconds.

For the actuator, just remove the line going to it and apply about 15" to it and see if the mode door moves. If it opens and closes while you have steady vacuum to it, then you might want to replace it.

 

[runderwo]

The vacuum reservoir tested okay, I haven't checked the door motor yet, but I have definitely narrowed down this problem to operating at low intake vacuum, such as when climbing a steep hill where the air path immediately malfunctions, or climbing a long hill on the interstate where the air path begins malfunctioning after a period of time after which presumably the stored vacuum has been depleted. So there must be a leak at the reservoir or something downstream that only manifests under these conditions.