I've restored vaccum to the 2 small brittle lines above the A/C condeser and still do'n have directional flow. I mostly get air out the vents, a little leaks by to deforst and a little to the floor. I'm about to start tearing into this, but I hate to do extra work. Where should I start looking?

 

gotjeepzj,

The vacuum originates at the intake manifold [multi-port outlet, sometimes referred to as a tree]. A single vacuum hose is routed to a T-shaped plastic piece, which is a one-way check valve. One line goes to a plastic ball-shaped canister located on the inner fender liner, which is a reserve for vacuum. The other line, from the "T" goes to the area which you have described "above the A/C condensor". I believe one line is black with a white stripe (?). Anyway, the line goes through the firewall and down behind and to the right side of the glovebox. If you lay across the seat and place your head under the glovebox looking up, you will see two lines routed to a clear plastic [triangular shaped] junction which is also a quick disconnect. This is located by the passenger side kick panel. One of these lines [same color as described previously] goes under the dash to a vacuum control valve/switch. This switch is what directs the vacuum to the appropriate servo motor to direct the air flow depending on what setting has been selected [vent/heat/A/C/def]. The second hose which you have found and also passing through the quick disconnect is a line, again controlled by the vacuum control valve/switch, and is connected to the shiney metal servo motor which can be seen under the hood on the plastic housing where the two hoses pass through the firewall. That servo, I believe, controls the door which allows fresh air to enter the system.

What is happening when you describe that air comes through the floor and defrost is that the system requires a constant vacuum to control and maintain desired settings. The engineers designed a "fail-safe" feature where if vacuum is lost, the default is to the defrost mode. This is for safety, as defrost is deemed to be the most critical need.

Often, what happens is that either the plastic hoses become brittle and leak or the reservoir [ball-shaped canister] cracks. With the engine running at slow speeds, there is enough vacuum present to still operate the ventilation system. However, when the truck is pushed to climb a hill, accelerate, etc., the air flow which has been coming out of the dash vents suddenly decreases and begins to blow out the defrost vents, only to resume normal function once the engine load is decreased. This is what the ball-shaped reservoir is for... when the engine vacuum is low [while under open throttle], the reservior holds the settings since the previously mentioned T-shaped one way valve prevents the loss of vacuum in the lines.

I am realizing this has become quite lengthy [my apologies to everyone, especially the moderators]. I will stop and if any of this is beneficial, let me know and if the need arises, I can add a little more.

 

Wow, good info. So one of the two little lines should have a white stripe and this feed the hvac system. The other little hose goes to the metal servo. I have the T with the vac check valve and the vac resovoir.

Let me clear something. The air blows out the vents all the time. Only a little leaks by to the defrost. This is regardless of driving conditions or setting. I was just looking at where to start.

 

My response will be brief, as I have to go and will not be able to check the forum until next week.

You have more than a minor leak, which can be overcome when the engine is running. Usually, the plastic lines fail first in the underhood area [due to heat]. Look at all lines and replace [inexpensive] if at all questionable. Replace the plastic with rubber. If a splice must be done, the smallest vacuum hosing at Auto Zone works [I believe it is 3/64"]. Wet the plastic line and insert carefully. The reservoir and check valve as well as the tubing can best be tested using a hand held vacuum pump [can be rented at some parts suppliers]. One can be purchased for approximately $25-40. Test each section by pulling a vacuum and see if it holds. The servos can also be checked this way.

Have you replaced a stereo unit recently? I ask because my son's 1984 was losing vacuum. While under the dash [I removed the stereo to see better] I found that while his recently purchased stereo unit was being installed [professionally] one of the vacuum lines had been pulled from the servo unit near the top of the dash. By the way, that servo unit has (2) vacuum lines, as it is used to "mix" the defrost and heat. Once the line was reattached, everything worked correctly.

Check everything under the hood first and good luck!

 

OK, here's where I'm at. The metal servo for fresh air is connected to a vac tree on the intake. This needs to be connected to the all black plastic line going into the firewall, correct?

I found the other black w/ white stripe line going into the firewall. This should be connected to a T. What I thought was a check valve, a T with a round bottom that says "FORD" on top and "VAC" on bottom, is not a check valve.

I'm about to go to o'reiley's and try and sort this out.

I haven't bother to check behind the dash yet, since I know under the hood is wrong. I wan to correct that first.

 

Ok this is where I'm at now.

Vac feed line coming from junction block on firewall. Attaches to check valve that feeds both the reservoir and the black/white plastic line going through the firewall. Solid black plastic line that goes through the firewall feeds the shiny metal servo that should control the fresh air or cabin air.

If this is correct then my issue must be behind the dash somewhere. After hooking everything back up, I still don't have directional control.

 

If I have a ball vac reservoir then what is the coffee can reservoir for?

 

Emissions & engine controls.

 

So then the can isn't needed?

 

gotjeepzj,

On 11/5/09, when I mentioned the two lines going through the firewall and the black one with the white stripe (?), I was going from memory. I just now looked at my truck [1986] and I was wrong. The vacuum originates at the tree on the manifold and through a rubber hose goes to the junction on the firewall. From the junction, through another rubber hose, it goes to the bottom of the "T" shaped plastic piece. The side of the "T" facing the front of the truck goes through a rubber hose to the ball shaped reservoir. The firewall side of the "T" goes through a plastic line which is black in color through the firewall to the quick disconnect and control valve which I previously described. The plastic line which is black with the white stripe returns from under the dash to the servo motor which controls the fresh air. Sorry for the confusion.

In regard to your more recent question about the coffee can reservoir... I don't remember seeing both types of reservoirs being used at the same time. My 1978 F100 had the coffee can, but my son's 1984 and my 1986 have the black plastic ball reservoir. Perhaps it was based on year model?

Let me share one more bit of experience which might be of assistance. Since rereading your posts, I see where your "multi-vent" discharge is constant. That is different from what I recently experienced. Your condition was happening on my son's 1984 and I took it to a mechanic, so I will not be able to exactly describe details, but I know what was done... there is a mixing door located in the housing by the heater core [others may want to correct me if I am wrong, thank you] which I believe is controlled via the metal cable which is connected to the dash lever used to select cold-hot. The mixing door has a plastic hinge which at some time fails. The door then will not seal or direct as designed. What was done to repair this was to purchase a brass piano-hinge from Lowes. It was relatively inexpensive, but the shortest one was something like 24" and all you need is a few inches [hack saw to shorten]. Brass was chosen to prevent rusting due to moisture. The new hinge was connected using rivets. The air flow was corrected and has not been a problem for over two years. Maybe this is what is going on?

 

I'll swap them and see if there's a difference. I'm not sure why I have both the ball and the can. Unless the can leaked and instead of removing it they just added the ball.

 

I see both the can & ball in the junkyards all the time... All engines get the can, the ball comes
when you also get AC. Look at the vacuum hose routing diagram on your radiator support
under the hood.

 

GotJeep, I have an '85 with the same issue. Its not a vac problem, it is the door. I manually pushed mine to defrost for the winter but whenever you move the controls it returns to coming out the vents. I've heard of the direction door repair before but no details about what disassembly is involve to get to it. Please post how you got to it and how you fixed it. Would appreicate it.

 

Jeep, any luck on your defroster?

 

Got tied up with swapping my intake. I'm going to take a look at it tomorrow.