blend door or not
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Cross-Country
Joined: Oct 2004
Posts: 86
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blend door or not
If my truck heats up, and the fan speed control works fine, and the the hot cold blend control works fine, but the panel control does not work (it only blows from the defrost mode regardless of the setting) is this a problem with the blend door that so many have?
Post Fiend
Joined: Jan 2003
Posts: 9,748
Likes: 16
From: Connecticut
Climate Control Vacuum Leak Diagnosis
No, it doesn't sound like the blend door, since you can get hot and cold air. Instead it appears that you may have a vacuum leak in the climate control system somewhere. The system will default to defrost mode when there is a problem like that. You might want to take a peek at the vacuum lines and the vacuum control valves behind the dash.
Climate Control Vacuum Leak Diagnosis
The air handling system is designed to provide defrost when no vacuum is applied to any of the three air door vacuum motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. Instead, a leak in the vacuum control circuit will send all airflow to the defroster outlets. This condition may occur during acceleration (slow leak), may exist at all times (large leak) and may happen only when certain specific functions are selected, indicating a leak in that portion of the circuit.
The vacuum hoses used in the passenger compartment control circuit are constructed from PVC plastic material. The vacuum hoses used in the engine compartment are constructed of Hytrel. Because of the materials used, the vacuum hoses should never be pinched off during diagnosis to locate a leak. Use Rotunda Vacuum Tester or equivalent to locate vacuum leaks. A wood golf tee can be used as a plug when it is necessary to plug one end of a vacuum hose for leak test purposes.
If a momentary hiss can be heard as each function button is pressed, vacuum is available at the control assembly. Cycle the function buttons through each position with the blower on HI and check the location(s) of the discharged air. The airflow schematic and vacuum control chart show the vacuum control motors applied for each function selection along with an airflow diagram of the system. The airflow diagram shows the position of each door when vacuum is applied, along with the no-vacuum positions. With this chart, airflow for each position of the control assembly can be determined. If a vacuum control motor fails to operate, the vacuum control motor can readily be found because the airflow will be incorrect.
If a vacuum control motor is inoperative, check the operation of the vacuum control motor with a Vacuum Tester. If the vacuum control motor operates properly, the vacuum hose is probably pinched, kinked, disconnected or has a leak.
If the system functions normally at idle, but goes to defrost during acceleration, a small leak exists in the system. The leak can be located by shutting off the engine and using a gauge to check for vacuum loss while selectively blocking off vacuum hoses.
The air handling system is designed to provide defrost when no vacuum is applied to any of the three air door vacuum motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. Instead, a leak in the vacuum control circuit will send all airflow to the defroster outlets. This condition may occur during acceleration (slow leak), may exist at all times (large leak) and may happen only when certain specific functions are selected, indicating a leak in that portion of the circuit.
The vacuum hoses used in the passenger compartment control circuit are constructed from PVC plastic material. The vacuum hoses used in the engine compartment are constructed of Hytrel. Because of the materials used, the vacuum hoses should never be pinched off during diagnosis to locate a leak. Use Rotunda Vacuum Tester or equivalent to locate vacuum leaks. A wood golf tee can be used as a plug when it is necessary to plug one end of a vacuum hose for leak test purposes.
If a momentary hiss can be heard as each function button is pressed, vacuum is available at the control assembly. Cycle the function buttons through each position with the blower on HI and check the location(s) of the discharged air. The airflow schematic and vacuum control chart show the vacuum control motors applied for each function selection along with an airflow diagram of the system. The airflow diagram shows the position of each door when vacuum is applied, along with the no-vacuum positions. With this chart, airflow for each position of the control assembly can be determined. If a vacuum control motor fails to operate, the vacuum control motor can readily be found because the airflow will be incorrect.
If a vacuum control motor is inoperative, check the operation of the vacuum control motor with a Vacuum Tester. If the vacuum control motor operates properly, the vacuum hose is probably pinched, kinked, disconnected or has a leak.
If the system functions normally at idle, but goes to defrost during acceleration, a small leak exists in the system. The leak can be located by shutting off the engine and using a gauge to check for vacuum loss while selectively blocking off vacuum hoses.
Thread Starter
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Cross-Country
Joined: Oct 2004
Posts: 86
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Thanks for your help, I try to figure it out. Do you think that moisture that freezes could cause this vaccum problem? When it started it was like -35 c outside and it really hasn't let up. I was hoping that when the outside temp gets better it might correct itself.
New User
Joined: Feb 2006
Posts: 23
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I have the same problem at the moment, fan control and temp control work fine, but all air goes to the window defrost. It sounds like vacuum lines under the hood could affect this as well as vacuum lines in the cab. A vacuum line under the hood broke the other day(had no idea what it's for). I barely touched it and it snapped in half, must have been a little brittle. Hopefully repairing this line will solve my problem, although I don't know if these are the vacuum lines for the climate control or not.
Does anyone know the location of the climate control vacuum lines under the hood?
Does anyone know the location of the climate control vacuum lines under the hood?
Post Fiend
Joined: Jan 2003
Posts: 9,748
Likes: 16
From: Connecticut
If you pull off the passenger tire and splash guard, you will find the vacuum reservoir tank and from that you can follow all the vacuum lines to check for leaks. If you have the PCV 4x4 system, there will be an additional line between the reservoir and the PVH solenoid, and then from the solenoid to the wheel ends. The reservoir has a built-in check valve that I have heard can fail. I've also heard of the PVH solenoid failing and allowing vacuum to bleed off.
Here's another thread with information on the climate control air flow system:
https://www.ford-trucks.com/forums/s....php?p=2020808
Here's another thread with information on the climate control air flow system:
https://www.ford-trucks.com/forums/s....php?p=2020808
New User
Joined: Feb 2006
Posts: 23
Likes: 0
Thanks for the help.
After temporarily connecting the broken vacuum line, the mode select **** worked and allowed air to come out the panel or floor vents. While the truck is running it will basically hold together itself from the vacuum but comes apart while driving of course. I'm guessing I can just glue it together.
After temporarily connecting the broken vacuum line, the mode select **** worked and allowed air to come out the panel or floor vents. While the truck is running it will basically hold together itself from the vacuum but comes apart while driving of course. I'm guessing I can just glue it together.
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Post Fiend
Joined: Jan 2003
Posts: 9,748
Likes: 16
From: Connecticut
What kind of material is the broken line, plastic-like or rubber?
On the rack at a lot of parts stores there are small parts and pieces that can help you re-connect vacuum lines and things like that. Some silicone rubber stretch tape might also come in handy for the repair.
On the rack at a lot of parts stores there are small parts and pieces that can help you re-connect vacuum lines and things like that. Some silicone rubber stretch tape might also come in handy for the repair.
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