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Why does my under dash blower work for A/C but not heat or defrost?

(1990 Ford F-150)
in Glen Burnie, MD on October 25, 2013
The under dash blower and under hood blower, run/blows air when AC is selected all speeds. When controller is slid to heat, the under dash blower stops, but under hood blower continues to run and blow - no air coming out of vents though. When controller is moved to defrost, under dash blower doesn't blow, under hood blower blows and a small amount of hot air comes out of the defrost vents. I know the main blend door is opening and closing, because I removed the under hood blower and checked.
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of G. T. Service Center on October 26, 2013
You only have 1 blower motor; You may hear it better on recirculate because the inside door to the blower is open. As far as how the controls work, the temp door is controlled by a cable from the control head. The rest are controlled by vacuum. Sounds like should you to check the cable adjustment, and/or vacuum from the engine to the control head, then to the vacuum actuators. Some older controls have a replacement vacuum control you can purchase for the slide on the back of the control head. Check that the vacuum is be directed to the proper actuator at the proper selection.
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on October 26, 2013
It has two blowers for sure. I can see the one under the dash and pulled the one under the hood, leaving it exposed to verify when it was running. They are both shaped like hamster wheels. The one under the hood appears to be attached to a housing that also holds the heater core. When I slide the control arm, I can check under the hood and feel where the door is opening and closing. Not understanding the exact functionality of the blower motor resistor, is it possible that it would function for the A/C and fail for the heater? There don't appear to be any vacuum lines in this area.
of G. T. Service Center on October 27, 2013
The resistor is used to change the resistance to the blower and cause it to slow or increase the speed of the blower. I double checked with Mitchell when you said it had 2 blowers, it only shows 1 and have only seen 1 blower on a single system, have seen 2 on a dual system. Anyway there should be a vacuum line from your engine to the firewall or blower case, usually at the blower case area. Then it runs to your heater/a/c control, then you have actuators at the heater case to work the mode, recirculation, and defrost doors. The blower works the same for a/c & heat, the doors will direct the air flow by how you set the controls. Can you pull the under hood blower, leave it out and then still see the blower under dash?
on October 27, 2013
Yes, I can see both blowers. I actually pulled the blower out of the housing under the hood and sat it where I could look through the windshield to watch it turn as I moved the control lever. I would then look under the dash to confirm when that blower was turning. Do you think someone could have modified the system, because I really can't find any vacuum hoses. A friend told me it looked like he remembered his father's old pickup, but couldn't remember what year it was.

I have been asking around, and found an older gentleman that used to have an older F150. He said that around that time AC wasn't always installed at the factory and the dealers had kits to convert them to AC and he thinks that's what I have. The blower for the AC was attached under the dash like mine is. Not great, but at least it makes sense why none of the documentation describes my configuration. It now makes sense that the inside blower only works when the selector is on AC. He also said that he thought the heat only blew out of the under dash vents and the AC used the upper dash vents. My AC is coming out of the upper dash vent all speeds. My heat doesn't appear to be coming out of the lower vents on the 1st two speeds, but does now slightly on high. It also slightly comes out of the defroster. I understand that when the resistor goes, you usually lose the lower speeds, but does the air flow also decrease on high? Does anyone know if there are two blower motor resistors with the dealer installed AC? The one I found is under the dash near the AC blower. Would there also be one under the hood near the standard heater blower motor? Sorry for the confusion, but I had no idea the back in the day it was normal for the dealer to install the AC and the customer never new the difference.
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