ac coming out of defroster

[BROWNIEBOY525]

i have ac only coming out of the defroster im guess it has a vacum leak but im not sure. what do i look for first?

 

[Michael Fowler]

Do your brakes still work correctly? A/C being directed to the wrong vents is often a sign of low vacuum, although I think more often the A/C just goes to the floor; not Defroster.

 

[Manwiththevan]

9 times out of 10 it is the main vacuum line to the HVAC, it just falls off.

 

[BROWNIEBOY525]

brakes work fine. Where is the main vac line?

 

[typ4]

Mine was broken in the heater harness, follow the ac/blower wiring thru the firewall.
Almost all vac controlled cars and trucks default to defrost when there is no vacuum for safety reasons.

 

[OLDBULL8]

This is for the 89 trucks.

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[MIDNIGHT RIDER]

On every vehicle I have had to do that, it has always ended up being the vacuum-switch made onto the A/C-Heater control itself

When these go bad, they default to defrost.


Usually what kills the vacuum-switch is excessive heat over long periods of time from a too-small fan switch; the fan/blower switch gets hot and melts the plastic vacuum-switch; after a while, the constantly heated/melted plastic becomes brittle and develops a crack, thus losing vacuum.

 

[reklund]

Mine's been acting funny lately too. If I shut the truck down and leave it in "Max AC" position it'll restart and discharge in defrost mode until I cycle it to "Normal AC" and then back to "Max AC". It works fine otherwise, but I don't remember it doing that until this summer. Any ideas?

Ryan

 

[MIDNIGHT RIDER]

Mine's been acting funny lately too. If I shut the truck down and leave it in "Max AC" position it'll restart and discharge in defrost mode until I cycle it to "Normal AC" and then back to "Max AC". It works fine otherwise, but I don't remember it doing that until this summer. Any ideas?

Ryan

I would check this first.

On every vehicle I have had to do that, it has always ended up being the vacuum-switch made onto the A/C-Heater control itself

When these go bad, they default to defrost.


Usually what kills the vacuum-switch is excessive heat over long periods of time from a too-small fan switch; the fan/blower switch gets hot and melts the plastic vacuum-switch; after a while, the constantly heated/melted plastic becomes brittle and develops a crack, thus losing vacuum.

 

[EMD_DRIVER]

I know that the A/C compressor is supposed to run, while the defroster is on. That helps get the moisture off the window faster. If it's not blowing cold from the vents, it may be the vacuum switch/selector switch.

 

[pfarmer]

"brakes work fine. Where is the main vac line?"

I've had a line come off before, it was a vac line located on the blower side of the engine compartment, just above the blower at the top of the fire wall. In fact, I crazy glued mine back on which solved that immediate problem. But the problem I have is not that it ALWAYS blows through the defroster. I am a crop insurance adjuster and am in and out of wheat and cotton fields all day long and when I first go into a field (creeping along at just above idle speed) and if the pick up gets jarred around really good, the air will transfer from the dash vents to the defroster. It always goes back to normal once I have had the chance to get out of the field and generate more power. I have been told it's not a line and it's not a "mechanical" gate or valve..but that it is electrical, probably the vacuum selector switch like "EMD Driver" told you. I haven't done anything about the problem because as soon as I build power, it quits and I forget all about it. Let me know what you do, I think I need to do it too!

Good Luck,
Priscilla

 

[BROWNIEBOY525]

found my problem there was a small crack in the black line that goes to the heater box. cut out the crack and slipped a piece of hose over the line and its working fine
thanks guys