A/C Switch
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Tuned
Joined: Sep 2001
Posts: 279
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From: Western PA
A/C Switch
O.K. when I am going up a hill the A/C quits blowing at the vents and blows out the defrost. You guys said it was a white plastic vacuum line. I haventd fixed it yet but now.....the compressor? hickie runs on VENT and MIX and DEFROST. I found this out becuase it is getting cooler here and I wanted some heat at night with the window down and noticed it still wasnt climbing the hill like when the A/C is on. SO I pulled over the popped the hood and the compressor runs in all the switch positions. While this might be nice to have A/C blow out other places than vents I dont what this to run all the time. Whats wrong?
Junior User
Joined: Jul 2004
Posts: 63
Likes: 0
From: Northwest Germany
Hi, don't know your type of truck, so some from the '94 F-series shop manual.
1. The white vacüüm line controls the outside-recirc door, which is under the hood behind the blower. In your case I would check the MIX door, means blue vacuum line, if here the vacuum fails it will shut off the vent path and flow goes to defrost.
2. The A/C compressor is powered (controlled by the pressure switch on the accumulator) in MAX, A/C NORM, MIX and DEFROST position. I would first check the press switch, if the contact is welded together (does not open). Second I would suspect the A/C-Heater function selector switch on the control panel.
LoL Dieter
1. The white vacüüm line controls the outside-recirc door, which is under the hood behind the blower. In your case I would check the MIX door, means blue vacuum line, if here the vacuum fails it will shut off the vent path and flow goes to defrost.
2. The A/C compressor is powered (controlled by the pressure switch on the accumulator) in MAX, A/C NORM, MIX and DEFROST position. I would first check the press switch, if the contact is welded together (does not open). Second I would suspect the A/C-Heater function selector switch on the control panel.
LoL Dieter
Senior User
Joined: Apr 2000
Posts: 396
Likes: 0
The part about going from vents to defrost. The doors are probably vacuum operated and as you give it more throttle, the vacuum drops off - normal at the engine. I have had to fix a few trucks with this problem. Get a vacuum check-valve and install it in the vacuum line going into the heater control supply. This should hold the doors where you put them until you top the hill. If not, get a small vacuum reservoir and 't' it into the supply line after the check valve.
