2003 Ford F-150 A/C and Heat blows only through defrost vent

  • 2003 FORD F-150
  • V8
  • 4WD
  • 160,000 MILES
A/C started blowing through defrost only upon acceleration. Found info on worn vacuum lines and did find one near the battery. Spliced the line with extra line and electrical tape and it worked fine for a couple of days, then same problem returned. Now it will blow only through defrost regardless of acceleration. This is identical problem with either A/C or heater. Thought at this point we'd take it to a mechanic to check all the vacuum lines. They said it was the electronic controller. We replaced the electronic controller a little over a year ago for similar symptoms and everything did work fine for awhile. But it doesn't seem to make sense this part would fail like that. How can we verify this is the problem again and are there any alternatives we could rule out first? Seems strange we would find this broken line and then the fix worked for a short time and then immediately after that the controller failed?
Do you
have the same problem?
Wednesday, November 18th, 2009 AT 9:08 PM

1 Reply

I say you're looking for a vacuum leak. Follow the supply line that goes to the control panel. Make sure there is good vacuum there. If there isn't, trace it under the dash, through the firewall, and to the engine (source). My guess is you will find a broken hose near the firewall.

Let me know what you find. Also, if the engine is running, you may get lucky and hear a hissing sound from the leak.

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Wednesday, November 18th, 2009 AT 9:29 PM

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