My A/C is blowing cold but only thru the defrost vents. First it was blowing normally from the dash but during acceleration or under load, the a/c would randomly blow from any vent, from the floor, dash, defrost. Now that stopped and it only blows from the defrost vents above the dash-board. Any suggestions?

1997 F-150 XLT, 4.2 Liter, V6, manual transmission.

 

my 92 f250 started that after i replaced the heater core. i think it is a vacuum related problem. i turn it to vent then back to a/c to get it to come out the vents. just had compressor clutch go out, so while im working on the truck anyway, might as well tear the dash apart. if i figure it out, i will post it.

 

You have lost vacuum to the hvac controls. Def is the default setting. I would look for broken vacuum lines under the hood first. There should be 1 or 2 plastic lines coming out of the firewall somewhere. Probably near the blower motor.
Jimmy

 

Same problem with my truck. Found the vacuum controller under the dash that regulates flow between dash and defrost. After shutting off the truck, the contoller will move about 2-1/2 times before it uses the vacuum. If it is a vacuum leak, why does it hold enough vacuum to move this often? Also, what is the best way to check for leaks? The visual inspection is fine for the lines that are visible, but there are a lot of hidden hoses and connections. Hope "pcdocta" can get his problem solved. Let us know how is works out.

 

I found the problem!! Just behind the Battery, coming out of the firewall is a plastic black tube that connects to about 4 other vacuum tubes. That black tube was cracked. I cut out the bad piece and connected the tube using a rubber vacuum hose. My A/C works!!!!!! Thanks lxman1 for the tip!!!!

 

I just got a 2003 Limited, V6, 4x4. My climate control behaves strangely. If it's hot out (85 - 90) and the temp is set reasonable (72) and I set it to auto, while I'm sitting after starting, the air comes out the front vents with the blower at high speed and blows cool air. As soon as I get moving and accelerate, the air moves to the defrost or the defrost and floor. If I drive for a little at hiway speeds and then back off for traffic, the air moves back to the dash vents (chest). As soon as I step on it again the air goes back to the defrost and maybe the floor. If I drive steadily at hiway speeds, it sometimes drops back down to the dash and then goes back to the defrost all by itself. Once the car cools down and the blower slows down it seems to work what I would consider normal (air out the dash and a slower fan). If during the period while the air is coming out the defrost, I select the dash vents manually, it stays up in the defrost vents. Even then it moves back and forth with changes in acceleration and deceleration.

Is this normal behavior?? It seems wrong to me and once I saw this thread, I thought I'd ask if I should try to get it fixed. It sure doesn't seem correct to me and I wanted to get some opinions.

Thanks, in advance, for any help.

 

Did you ever get a satisfactory answer to your question about the AC only working through the defrost vents? I wondered because I am having the same problem with my 1990 F-150.

 

I haven't received any reply at all other than your question.

What I'd really like an answer to is whether or not this is normal behavior. My dealer says it is normal because it's vaccuum operated and there's always a vaccuum drop during acceleration.

I think mine is worse than normal as it'll move from the dash to the defrost, stay there for awhile then move back and forth at random intervals while I'm holding a steady speed (50 - 70 mph). It seems like if it was just normal acceleration vaccuum decrease, it would move right back as soon as a steady speed was reached.

Please, someone comment on how this should work!

 

No, it's not normal and any competent service tech or advisor knows it. There's a check valve between the engine and the HVAC vacuum reservoir (glued to the evaporator cover on older trucks) that prevents the low vacuum in the engine during acceleration from draining the vacuum stored in the reservoir. You must have a leak in the HVAC vacuum system somewhere, probably under the dash, but maybe in the engine compartment.

 

Thanks Steve83. That's what I wanted to make sure of.

 

I am having the same probablem as the first guy my ac/vent is only coming from the defrost vents and not coming out the panel. I checked under the hood and I didnt find any visable cracks or breaks in the vacum lines.

I unplugged the line from where it enters the firewall and heard the pressure/vacum release so I am thinking it is under my dash somewhere.

The red vacum line to the accuator that lifts the lever for the vents and it doesn't get any vacum what so ever at that point.

There is a small cylinder shaped object that has 2 other vacum lines going into it and when I move it, it seems to leak, but I don't want to just go out buying all the parts through the system till I find the right one anyone have any ideas on more detailed trouble shooting?

 

My 1990 hose arrangement appeared to be a little different, however, it was the only black vaccuum hose going through the firewall. The hose was cracked where the large rubber mounting portion meets the small black hose. Anyway, it is all fixed and I am happy and I thank everyone for all the help.

 

Same problem of AC blowing through defrost vents. I looked and listened, but cannot find any leaks in the vacuum lines. I did correct the problem without curing the vacuum leak. I simply disconnected the lever arm that connects the vacuum controller to the vent/defrost control. Then I secured one end of a piece of stiff wire to the vent/defrost control and the other end to the dash closer to the steering wheel. Now I can manually regulate air flow by pulling or pushing on the wire to flip to either vent or defrost. I have to work both the dash control and the wire depending on heat, AC, defrost, or vent. It isn't the most convenient and does not solve the vacuum leak, but I do have AC out the vents all the time.

Also, I had to move the vacuum controller since it could interfere with the manual controls I stuck in place. The vacuum controller is mounted under the dash with two screws. I moved it over so it is only holding by one screw. This way it is free to move without hitting anything under the dash. I was concerned that if I took it off and plugged the line, I might mess up the vacuum somewhere else in the system.

I did test the system and everything works fine on all settings. I still want to find the vacuum leak. Any more suggestions on that end?

 

HibbardSmith, check the vacuum line plugged into the water control valve on your Explorer Limited, the often pull off or the water valve itself can have a vacuum leak as well.

F150 owners with 4X4 there is also a vacuum reservoir under the right front fender, and the main reservoir for all 97 and up F150 is mounted behind the battery. These can easily be the cause of vacuum problems because many vehicles get into fender benders and the tanks get cracked but body shop fails to replace them. Also check the various vaccum connectors under dash at the various vacuum door actuators.

 

On Ford F-150s...at least on 1998 models...
Check the black vacuum line at the connection near the line filter (blue + white disc) for a break or crack. The black line should be coming through the firewall on the passenger side...close to the battery.
I found that this line was actually touching the back side of my battery. The battery had a little moisture on it that leaked down and came in contact with the vacuum line...and ate right through it !
Take the time to disconnect the battery and remove it. This makes access to the whole series of vacuum lines a lot easier. If it is NOT the black line that came disconnected...at least you can easily check all of the other line connections.
Cut off the bad portion of the line and take out the piece of line still in the rubber boot. Insert the repaired line into the boot and reconnect it.
HINT: While you still have the battery removed...re-route the line so that it doesn't come in contact with any part of the battery. I even put an aluminum foil barrier between my battery and the vacuum lines to prevent future corrosion.