.....where it wants to come out.

My truck is a '92 F250 SC, 5.8 EFI, E4OD. With the A/C on and air coming out of the dashboard vents, when I speed up to cruising speeds, the air will begin coming out of the defrost vents instead. This usually happens first at about 60 mph and after levelling out at 70 - 75 mph, it will usually switch back to the dashboard vents. Does anyone have any idea what is causing this and how to fix it? Is it a vacuum leak somewhere or is it more likely related to the switch? Thanks for any input.

James Beaman
Houston, TX

 

Sounds like its switching to its no-vacuum default mode, which is Defrost. When you accelerate at higher speeds/loads, intake vac drops, and goes back up some as you level out at speed (because you are easing off the gas, closing the throttle plate some). So the controls are reacting to a loss/low vacuum.
Could be a:
Vacuum leak at hose connection,
or a one-way check valve to the controls that is leaking back vac towards engine,
or a worn-out vac selector switch in dash (don't want that one, $$),
or a vac motor that gets used in the control position you want, that has a leaky diaphram in it.
One of those hand-squeeze type of vac pumps and a gauge would be a real help in troubleshooting it.

 

Go to your engine compartment look in the upper left corner of the engine compartment locate a vaccum pod mounted on the top of the blower moter housing. This is the actuater for the vent that opens and colses for the defrost and the vent.
Check the hose connected to it for cracks holes. If no cracks are found disconect the hose and vaccum test the pod. If you dont have a vaccum tester you can do it the redneck way. After wiping off the hose, suck on it and watch to see if the pod opens if you cant keep it open while sucking on it or it doesnt open at all its bad.
If it does open and stays open while sucking on it then ofcourse the problem isnt in the pod.
It could be that the vaccum canister is bad it looks like a coffee can you cant miss it.
Good luck.

PS. Please repost when you figure out the problem. So many people post their problems but never post the answers to their problem.

 

Check also to see if your vacuum canister is still holding vac. I've seen them rust along the seam if it is the Campbell's soup can style. I had that problem on my other vehicle, and I had to add the vac canister back to it (after engine swap) and put a check valve on it since the cam I was running produced lower vacuum.

 

Thanks for the help on this. I intend to keep the list informed, especially since I don't know much about troubleshooting vacuum systems.

I went ahead and bought a vacuum gauge but it does not include a pump bulb. I also checked the vacuum hoses and all that I can see appear to be in good shape. Any suggestions on how to use this gauge? I assume I should hook it inline with the hoses in different areas and start the engine but I don't know how to tell if there is a leak. What pressure should I be looking for? Also, where is the soup can vacuum canister located? Thanks for the continued input.

James Beaman
Friendswood, TX

 

OK, I was able to make some preliminary checks of the vacuum system last night and can report some findings. First, the vacuum line going from the intake plenum to the check valve maintains a steady vacuum of 18 in. Hg at idle. Second, with the A/C set to the dashboard vents, when the vacuum line from the check valve to the intake is disconnected, the acuator will gradually open back up to the defrost mode. Is this normal or should that vacuum be maintained? Third, there does not appear to be any leakage of vacuum through the check valve back to the intake. I checked this by disconnecting the line after a vacuum was pulled and quickly slipping the gauge hose on the check valve. No vacuum was observed as the actuator opened. Any suggestions on what to try next?

I also tried it the "redneck way" and sucked on the vacuum line to the vent actuator and could not get it to move. My suspicion is that there is a leak inside there somewhere as there are no visible defects in the part of the vacuum line I can see. Is there any way to easily get to the other end of that vacuum line where it goes into the actuator? If I could do that, at least maybe I could isolate the line and determine its condition.

James Beaman

 

I can't exactly remember what it looks like under the hood of your year truck, but on my 88 the can is on the passenger side fenderwell. Yours may not even have one. If not, get one from a junk car or get an aftermarket one, I know a lot of high performance suppliers sell them. To hook it up, just put a tee in where your vac line for your heater system starts at the manifold, or anywhere inline of it.
As far as the vents going back, yes, they will go to default (def and floor) as soon as vac is lost to the actuators, which is why you have this problem in the first place. When you test the check valve, put the gauge on the valve as though it would be the actuator, and connect the valve to the engine the way it would normally be. Start the engine, and then disconnect the valve from the engine, it should either hold the vac on the gauge, or else just delay the release of the vac by slowing it down. This would be like they used to like to put on mechanical distributors to slow the vacuum advance on them.
The only thing I can tell you about how to find the lines and actuators is that I know mine go through the firewall (bulkhead) near the fender, so look under the dash and see if you can trace them.
As far as the "redneck" way goes, that is a very good idea to try, but depending on how much of a load there is on it, it may be hard to move. Suck on the line to put a vacuum to the actuator, and try to hold it. If you can hold the vac to the actuator without it bleeding off, the actuator is good.
One quick question... When you say "a/c is on" to you mean that it is actually in the a/c part of the control, or does it react the same in the heat area? I'm thinking of the type of controller set up where the vent/floor/defrost (mode door selector) lever turns on the a/c on (as you move the lever from right to left it goes - MAX A/C NORM A/C VENT FLOOR MIX DEFROST. Am I assuming right? If so, then if the problem only occurs in the a/c portion of the selector, the control head may be bad. If it is in all positions of the lever, it is up to and may include the control head.
I know I'm giving info about an older model, but I figure any help will be better than none, and it may give you an idea an what to look for with your year truck.

 

If you had a vacum line hooked up to the actuator and sucked on it and was not able to get it to move then it is bad. I tested mine the exact same way and was able to actuate it. I ended up having a small crack in the vacum hose leading to the actuator. I had the same symptons as you.