How does the Ventilation System work?

I can't speak specifically for the F-150, but I doubt it's any different than my F-250. There are three air direction doors that are actuated by vacuum motors. The heater control knob turns a somewhat complex vacuum valve that determines when a door motor gets vacuum or not. If there is no vacuum to a door motor, a spring draws it back to it's default position. There are two electrical components to the control switch also. One determines if the a/c pump is active, and one determines if the fan is on or off.

I don't remember the exact order of air direction. But the upshot is, the default door positions (no vacuum applied) is recirculated air to the defrosters.

So, the short of it is, what you are experiencing is a loss of vacuum to the HVAC system. Somewhere you have enough of a vacuum leak to bleed off enough vacuum to not allow it to hold the doors in the desired positions. The engine makes the vacuum, and it makes less vacuum the more the throttle valve is opened, and more vacuum at higher RPM's. This explains why there is a relation to engine load and throttle position.

The only way to fix this is to start following vacuum lines from the source until you find the leak. I doubt it's a leak in any of the big lines, because you'd lose enough vacuum there to prevent the HVAC from working at all and you'd lose vacuum to the brake booster as well. It's most likely somewhere in the smaller lines.

I've had this happen twice on my '93, the problem both times has been a cracked/split vacuum line under the dash. What usually happens is if I'm going up a long uphill grade with the A/C on recirc after a couple seconds of having the engine at full throttle or close to it (mine's a

300, so this happens rather often) the air delivery will shift up to the defrost vents.

nate

nate

Here are some quotes from the 1996 F-150 service CD that might help.

"Vacuum Control Motors The vacuum control motors (18A318) for the PANEL/DEFROST and MIX damper doors are attached to the underside of the heater air plenum chamber (18471). The previous illustration titled «Outside Air/Recirculated Air Door Vacuum Motor Installation» shows these vacuum control motors disassembled from the heater air plenum chamber.

Windshield Defroster Door

Removal

1. Remove the vacuum hose from the vacuum control motor.
2. Remove the two screws that attach the vacuum control motor to the heater air plenum chamber.
3. Rotate the assembly so the slot in the bracket is parallel with the tee-shaped end of the door crank arm. Carefully disengage motor link from pin on crank.

Installation

1. Follow removal procedures in reverse order.
2. Verify that the system functions properly.

Heater and A/C Inlet Duct Door

Removal

1. Remove the heater outlet floor duct (18C433).
2. Disconnect the two vacuum hoses from the vacuum control motor.
3. Remove the two screws which secure the vacuum control motor and bracket assembly to the heater air plenum chamber.
4. Using a small screwdriver, depress the tang on the side of the door operating lever and pull the arm out of the lever.

Installation

1. Follow removal procedures in reverse order, sliding motor arm into door lever until locking tang engages.
2. Verify that the system functions properly."

From the picture, it looks like the blend door vacuum motor is on the fan housing.

"Function Selector Switch

Removal

1. Remove A/C control.
2. Remove the screw that secures the function selector switch to the heater control assembly and remove switch.
3. Pull the vacuum assembly back.

Installation

1. Follow removal procedures in reverse order.
2. Start the engine to provide vacuum. Then, move the function selector knob to each of its operating positions to verify that vacuum is being distributed properly through the function selector switch to the applicable vacuum control motor (18A318)."

I sent two PDF files to the Yahoo email address (hopefully it's a real one) with the troubleshooting steps and the vacuum motor diagrams. Maybe they'll be close enough to the '97 to be of use.

SC Tom

As Nate said, failing vacuum lines are a frequent cause. One of the worst on Earlier F150's and other Ford vehicles is the white tube at the right rear of the engine compartment. It is directly exposed to a rather harsh environment and the material detriorates becoming porous. It looks good but may have a brittle porous section at some poine. It litterally lwaks right through the tube. This tube controls the outside air door but dumps vacuum in the entire system if it fails. Th easisst repair is to cut back the protective sheathing and replace the section to the solenoid with vacuum hose.

Lugnut

That certainly describes my symptoms -- and in a lot less words than I used.

What you seem to be saying is that if the system ever does anything right, then the complexities of the door vacuums and electronics are not the problem. And since my system does work correctly until a certain point in the throttle plate and load of the engine is acheived, then a vacuum loss to the heater box is suspect. I get it.

I noticed some cracks on the large line that goes to the brake booster, I'll check those first. Thank you.

The adress is a good one. I'll go check it. Thanks.

I'll have a look at the vac line to the brake booster. I'm not having any problems with the brakes, but there are cracks that could cause the vacuum to be decreased elsewhere in the system.

I own a 1993 F150 XL, 4.9L 6 cylinder 5 speed manual, with PS/PB/AC and a 1994 F150 5.0L V-8 AT/PS/PB/AC, XLT 302. both are 2WD.

The loss of vacuum due to the main vacuum feed tube that is near the evaporator and AC filter Dryer, right (passenger) side of engine, toward the firewall, causes the entire vent control system to fully relax, and go to "defrost with heater door open" position, where all the vacuum motors are relaxed to their static position.

Check for broken, disconnected, leaking vacuum line from the engine manifold vacuum ports, out to the embedded harness, and to the vacuum reservoir.

I replaced the porous and split hard plastic vacuum lines on my trucks with neoprene plastic lines (used on newer vehicles) from salvaged pieces from the junk yard vehicles, with 1" splices of very good grades of rubber hose on each end, from the auto parts stores.

Hope this helps you!

A quick check (and not much action as of yet) indicates that the main vac hose to the brake booster is badly cracked, and I see no reason to think the other hoses are going to be in much better condition. At the very least, I have leakage in the brake booster hose, but I'm pretty sure there will be others. I haven't looked at the passenger side of the engine bay, but when I go in for the brake booster, I'll be looking at the others.

Whod'a thunk rubber would only last 13 years? Sheesh ...

Okay, I lied. Sorry. I said that the line to the brake booster was cracked, this is false. There is a gizmo right next to the brake booster that appears to lead to the gas tank that had a cracked line between it and the manifold. I replaced the line, but still have vac issues.

I can only identify one other line from the manifold, it is on the back and I can't see where it goes, but it heads in the general direction of the heater box.

What size is the line to the heater box that will control the mixing doors? The line I see looks to be a 3/8 (10 mm) hose. I don't want to reach in and mess with it and then have it break and I can't see where it goes. If I break th eline, the truck is down until I get it repaired, and I can't do that. It's better to have the life support system on the fritz than to have the whole truck out of service.

One of you guys sent me a troubleshooting guide that helps, but I just don't know where the lines are that I need to be looking at. I used to fix this stuff when I was a kid, but they've added so much crap under the hood that I just don't know my way around as well as I should. If I had a parts breakdown with the hose identified, I could replace it. I just can't see what I'm looking at and don't know where it should be.

I found vacuum hose diagrams for my 1992 Ranger (a year or two ago) on the Autozone website. They (the diagrams) helped me figure out where a hose that fell off belonged. It looks like you have to register with them to get to that information now. May be worth a try for you.

Glen

I looked through all the categories on that CD I have, and I'll be darned if I can find a routing chart or pictures anywhere. I'll try again when my eyes recover. Looking for something on it is like trying to trace any one piece of tubing under the hood of any newer vehicle :-(

Don't know if any of these I found through Google will help.