Defrost Only
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Joined: May 2006
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Defrost Only
Ok, I have a stumper. I have an 88' F150 4.9 with heater only, no A/C unit. I currently have defrost only. Checked all vacuum lines, all ok. Disconnected main vacuum line (black) from back of control panel harness, tapped it into each door vacuum source and they work. Plug main source back into harness and get defrost only. Seems like it's not getting enough vacuum through the damper door switch (which I replaced with a new unit). What is the electrical plug in that goes into the damper door switch for? I thought is was totally vacuum operated? I also ran a direct vacuum line from the intake manifold tee directly to the vacuum source input in the back of the control panel harness, and still little vacuum to the door actuators when switched on the panel. I hope I explained this well enough. Any ideas?
Last edited by brew92; Aug 9, 2006 at 10:10 PM. Reason: Clarification
Elder User
Joined: Jun 2006
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Are you talking about an electrical connection with umm, say 4 or 5 wires? More than likely you're looking at the connections for the speed control resistors for the fan. They're mounted in the air box plenum so that the fan blows across them. Otherwise, the wattage required would make the resistors about the size of a beer can for continuous duty. All domestic manufacturers have been cheating with series resistors and keeping them cooled with the very air that's blowing into the passenger compartment. If one ever lets go while you're driving, you'll know it. (Does not smell very nice at all) Tell tale sign if I understand what you're asking, is the electrical connection of which you speak is very near the fan itself.
Post Fiend
Joined: Jan 2004
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From: Elkhart IN
im not sure about what all your saying, but usually when you only have defrost it's the black with white stripe or just white vacuum line on the passenger side that goes to the actuator on the firewall. its just a problematic line, and dryrots
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I replaced the blower resistor in the engine compartment near the fan. That too was not working. Have all speeds now. The switch I am describing bolts to the control unit and has the vacuum lines plugged into it. The black vacuum line plugs into the middle and seems to be the main vacuum source to this switch. If I direct this line to each vacuum line to each door, they operate. But they will not operate through the switch at the control panel even though it is a new unit. This damper door switch has a wire plugin that goes to it, and then into the harness that the fan speed switch plugs into.
