air vent issues

[jfp3205]

Ok everyone, help me out

My 1992 f350 idi develeoped a problem today. When i turn on the a/c the air only blows out the deforster, not the dash vents. No matter what fan speed i use or what setting i use, it only blows out of the defrost vents
any ideas where i need to start and what it might be/
Thanks

 

[LCAM-01XA]

You're losing vacuum to your air doors, so they default to defrost - it's a safety measure built in to ensure truck is still drivable in winter even if this malfunction happens. Do you get cold air at least out the defrost vents? Or is it hot? Does it only happen when A/C is on? Or does the control knob do nothing at all regardless on which setting you put it on?

 

[jfp3205]

the air is cold but no matter what setting i put the switch on it blows out the defrost. the fan speeds work just out the defrost. Where is the first place to check for a vacuum leak? It just started this today

 

[jfp3205]

i can turn it off when i put the switch to off so i know that part works

 

[jfp3205]

I am thinking that my replaced air pump just went bad. How can O tell if it is pumping what it is suppose too. I pulled a line from the manifold going into the cruise control servo and it does not feel like a lot of vacuum

 

[junk]

Is your brake light on? How are the brakes? The vacuum run brake booster uses the same vacuum pump as the dash vents. So if your not getting vacuum and still have vacuum fed brakes your brakes should be stiff and the brake light should be on. If the brakes are OK then I'm leaning towards the dash switch unless you have a bad vacuum line in the dash. I would probably pull the heater controls and check for vacuum to the controls. Then check to see if they have vacuum elsewhere when changing the switch. I think most of the vacuum stuff is accessible from the bottom side of the dash.

One last item is you could have a vacuum actuator or door inside the vents that is broken.

Good luck!

 

[icanfixall]

If your brakes are hard to press then you have lost the vacuum in the system. We don't have much for vacuum hoses so just check each one closely. If you have cruise control I have found that tomato looking can under the master cylinder gets rusted out so vacuum leaks there. You need to remove it from the crappy metal holder and check for rust out holes. Mine rusted out and I cleaned it and rtv on the holes. Never any more issues. Also on the ac side of the firewall there is a bundle of wires going around the heater fan motor. In the bundle is a very small vacuum hose too. Mine continues to break off and I end up with no vents working inside the cab. So I cut back more into the wire harness and fix the broken vacuum line.

 

[jfp3205]

I found a small vac hose droke near the blower motor. The line was coming off a three way block and it goes into a wireing harness and I do not know from there where it goes. How do I repair this hose

 

[LCAM-01XA]

Use a short piece of 5/32" rubber vacuum hose (sold by the foot at any decent parts store) to connect the two ends of the broken plastic line. Not a big deal, it's pretty normal actually for the plastic line to get petrified with age and heat. Black or white line? One is your system vacuum supply and the other is for the fresh air door. Regardless, it's a simple and cheap fix there.

 

[jfp3205]

Thanks everyone, I got it fixed and it is working again. Now until the next piece breaks lol

 

[LCAM-01XA]

Shhh! Don't jinx yourself now! Still, keep an eye on the white plastic line over on the engine side of the firewall, it comes out on top of the heater box and loops over towards the fender to the fresh air door vacuum servo - that line simply loves to disintegrate into dust, then you have a vacuum leak and your HVAC won't run right again.

 

[rlb245]

Now thats what I'm talking about. I've had my truck for 3 years now and have never had the dash vents work. With the info from this post I now have a working heating system and freash air that I've never had sence I bought my girl in cody Wyoming. Thank you gentleman for the easy fix.

 

[icanfixall]

Well guys your all very welcome for the cheap simple easy fix to a big problem in the ac system. I too had these issues and still do have them. As posted. Thats small plastic hose likes to break and then you only have defrost to the windshield as a saftey thing and thats a good thing. I just continue to cut off the tape on that wire bundle to reach new fresh vacuum hose. I try everything I can to protect it from breaking again but its hopeless. Maybe its time to abandon that hose in the wire loom and run a seperate vacuum hose to the ac vents in the cab.. Sounds like an all day sucker of a job too. This site is famous for finding out the whys things happen and the easy fix. then its shared with everyone. Thats what makes us better than the rest. We share without telling someone they are stupid for asking this question. They should know this. We don't name call and thats the second time in over 16,800 posts that I have used the word stupid. Now its three times...

 

[Zaggnutt]

I am having the same issues with air out of the defrost only. I found the white and black hoses under glove box where the junction is were disconnected, but connecting them changed nothing... When I switch temp knob to a/c the blend door moves closed? open?, but changing the selector knob to floor, defrost, etc., does nothing. I checked vacuum pressure at the black and white hoses and have none. Hose from actuator at pass side corner has none. Not sure where to look next.

 

[icanfixall]

Check the plastic hose in the engine bay that is near the fan motor. It comes into the wire harness and usually breaks off causing this problem. Just cut the harness wrapping and splice in to the line where its available. Then wrap some electrical tape around the splice so the vacuum line is supported better than the factory did.