AC/DEFROST
Thread Starter
|
Junior User
Joined: Mar 2013
Posts: 67
Likes: 0
From: Olympia, WA
AC/DEFROST
Hello everyone, I have a situation here that is strange to me. I have a 1983 Ford F150 with a 351W (that replaced original 302). I have AC that works great when on AC or Max AC and blows out of the regular vents, but when I try using the defrost, the AC kicks on and no air blows out of the defrost vents on the windshield, only out of the normal vents. The heat will work out of the normal vents and floor and AC will not kick on then (which is right). So, what could possibly be wrong?
I have no ideal on where to start. Thanks for any help you all can give me.
I have no ideal on where to start. Thanks for any help you all can give me.
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
The A/C is supposed to come on when you put it to Defrost. There may be a temp sensor that determines when it comes on, but it does above a certain temp. Look in the owner's manual.
The problem is that it doesn't go into Defrost mode on the ducting. I think there is a separate vacuum motor for that, but can't check that out until tomorrow. If so, that's where the problem is. I'll check and get back w/you tomorrow.
The problem is that it doesn't go into Defrost mode on the ducting. I think there is a separate vacuum motor for that, but can't check that out until tomorrow. If so, that's where the problem is. I'll check and get back w/you tomorrow.
More Turbo
Joined: May 2008
Posts: 585
Likes: 11
AC running in the defrost position is normal, which is intended to help dehumidify the windshield. I'd start by checking the vacuum lines. It sounds like you have vacuum to the control if it switches between the floor and panel vents, but the line for the defrost actuator may be disconnected or leaking. The vacuum switch on the heater control could also be bad: check for vacuum at the defrost actuator. Another possibility is the diaphragm inside the defrost actuator may be leaking or the linkage between the actuator and door may be broken or disconnected. If all that checks out, something may have slid off the dash, down through the defrost vent, and became stuck against the door. Over the years I've seen lots of pens, pencils and other objects stuck in the heater box after dropping down through the defrost vents.
Thread Starter
|
Junior User
Joined: Mar 2013
Posts: 67
Likes: 0
From: Olympia, WA
Gary and Rowdy, thanks for the response. I had no idea that the AC came on with the defrost. So, in the winter when I go start my truck before the drive to work the AC will still kick on when I want to defrost the windshield? Maybe I am just used to the other cars I have that I can control the heat or AC. I understand the part about the vacuum leak opening or closing the flap, I just don't know where to look...
Is there a vacuum line running form the control panel where you switch from floor/vent to defrost? and if so, where is that line supposed to go? The truck was parked for at least three years in the southern NM desert, so would I be correct in thinking that some sand bunched up around the "flap" and is not letting it open and close correctly? Where do I find this flap? Where do I even start looking for a leak? I am going to remove the control panel for the heater/ac tomorrow and see what is on the back side, the vacuum line would come out of the firewall into the engine compartment and plug into?? before I really get into taking off the whole dash I will wait for your response. Its probably something simple, I just haven't learned yet. Thanks guys I hope I don't ask too many dumb a$$ questions!
Is there a vacuum line running form the control panel where you switch from floor/vent to defrost? and if so, where is that line supposed to go? The truck was parked for at least three years in the southern NM desert, so would I be correct in thinking that some sand bunched up around the "flap" and is not letting it open and close correctly? Where do I find this flap? Where do I even start looking for a leak? I am going to remove the control panel for the heater/ac tomorrow and see what is on the back side, the vacuum line would come out of the firewall into the engine compartment and plug into?? before I really get into taking off the whole dash I will wait for your response. Its probably something simple, I just haven't learned yet. Thanks guys I hope I don't ask too many dumb a$$ questions!
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
IIRC there is a temp sensor that cuts the A/C off below something like 32 degrees. So, in winter the compressor shouldn't come on.
The vacuum runs from the engine, usually to a little plastic manifold on the firewall, then over toward the passenger's fender to a tee/check valve. One side of the tee goes to a reservoir, either a plastic ball or a juice can, and the other side goes into the firewall almost where it meets the fender.
Inside if you either take the glove box out or get down low you can see it come in. There are two lines - one in and one going back out to the fresh air/recirculate door. The "in" line goes on to the HVAC controls, and it is obvious you get vacuum that far since you said the air will come out of the vents and go down to the floor. So, Rowdy is right, it is either the vacuum motor for Defrost or something has jammed it.
With the glove box out you should be able to see the Defrost vacuum motor - almost in the center and up fairly high. It won't hurt to try to move it with the engine off, and if you can't move it then you know it is jammed or stuck. In fact, the default is Defrost, so after the engine has been off for a while it should go to Defrost as the vacuum leaks out of the system.
The vacuum runs from the engine, usually to a little plastic manifold on the firewall, then over toward the passenger's fender to a tee/check valve. One side of the tee goes to a reservoir, either a plastic ball or a juice can, and the other side goes into the firewall almost where it meets the fender.
Inside if you either take the glove box out or get down low you can see it come in. There are two lines - one in and one going back out to the fresh air/recirculate door. The "in" line goes on to the HVAC controls, and it is obvious you get vacuum that far since you said the air will come out of the vents and go down to the floor. So, Rowdy is right, it is either the vacuum motor for Defrost or something has jammed it.
With the glove box out you should be able to see the Defrost vacuum motor - almost in the center and up fairly high. It won't hurt to try to move it with the engine off, and if you can't move it then you know it is jammed or stuck. In fact, the default is Defrost, so after the engine has been off for a while it should go to Defrost as the vacuum leaks out of the system.
Thread Starter
|
Junior User
Joined: Mar 2013
Posts: 67
Likes: 0
From: Olympia, WA
Okay Gary, I have a much better picture of what I am looking for. I will try looking at that tomorrow if I have time and see what happens, I will let you all know what I find, fix or need help on... Thanks for the help!! 
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
If it is stuck because of something in the ductwork then the best way to clear it is probably to pull the dash cover. It isn't as difficult as you might think and provides much improved access to the ductwork.
Thread
Thread Starter
Forum
Replies
Last Post
oldcoupe42
1987 - 1996 F150 & Larger F-Series Trucks
2
Aug 14, 2016 07:39 AM
crazie0025
Cooling, Heating, Ventilation & A/C
2
Oct 19, 2006 11:12 PM
eltakko
1987 - 1996 F150 & Larger F-Series Trucks
9
Jul 31, 2006 10:25 PM
