QuestionThe AC on my 1998 Ford F-150 4.6ltr 4WD pickup is behaving strangely. If the truck is at idle, the AC is normal. But when I accelerate, the AC blower stops coming out of the dash vents and starts coming out the defrost vents. Then when I reach cruising speed and no longer trying to accelerate, it returns to the dash vents. There is also an audible "popping" sound from dashboard when the air returns to the dash vents. It is very irritating, especially on hot summer days in Texas. By the way, this problem only happens when the truck is in gear...ie when I am sitting in the driveway in park and I rev the engine, the AC behaves normally. What is the matter with this thing ? Appreciate any help you can give.
Thanks,
--Greg
AnswerWhat you have is the typical ever occurring loss of vacuum from the engine to the inside controls of the A/C-Heater.
Look under the hood for a small black or purple plastic tube or rubber hose that may be routed within the wiring harnesses that go thru the firewall.
This small plastic vacuum tube has a tendency to brae and when this happens all your air goes to the defrost under acceleration. The POP noise is the sound of the air diaphrams receiving the vacuum that makes them rapidly expand and causes the mode doors to slam shut.
Look very carefull under the hood for any signs of a broken vacuum line and fix it.
If you have had ANY work done on the vehicle look in the areas where work was performed for a broken hose.
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