I have a 1975 Mustang ll with a 2.3 L automatic, factory air. It doesn't matter if I'm driving with the heat or air on but when I have the fan blowing and accelerate the dash air switches from either the floor or dash vents to the defroster vents. After letting off the accelerator it switches back to where it should be on either the a/c or heat setting. I had this problem in a Mustang although not to this degree. Any little bit of acceleration sets it off. Any ideas? Faulty vacuum motor or bad vacuum source?
Vacuum Question on Acceleration
- Thread starter Hawkk72
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Power Surge
Founding Member
Sounds like your vacuum storage tank isn't hooked up or has a leak and doesn't hold.
The heater/ac box is spring loaded to move to the defroster position in case of loss of vacuum. It is a safety thing. You roll on the throttle, manifold pressure increases and your box turns to default.
These cars are 30 years old, hard to say what equipment is still installed or plumbed in.
Assuming all factory apparatus is intact you could have a cracked hose or fitting, bad check valve, or failing / failed vac can. Could even be ruptured diaphrams in the heater box motors.
OTOH you wrote Any little bit of acceleration sets it off which leads me to believe the vacuum source is plumbed directly into the H/AC controller. No check valve or anything.
These cars are 30 years old, hard to say what equipment is still installed or plumbed in.
Assuming all factory apparatus is intact you could have a cracked hose or fitting, bad check valve, or failing / failed vac can. Could even be ruptured diaphrams in the heater box motors.
OTOH you wrote Any little bit of acceleration sets it off which leads me to believe the vacuum source is plumbed directly into the H/AC controller. No check valve or anything.
78CobraII
Moderator
I had to replace the vacuum canister on my '76 hatch back in the late '80's. Even here in the south it had rusted through in several places.
The Ford replacement canister had a 1979 part number!
The Ford replacement canister had a 1979 part number!
So far I've only had a chance to check under the hood. I found a set of 3 vacuum lines together on a T fitting which was disconnected from something on the driver side fender apron. After I connected it I drove around and it did nothing to correct the problem. Still would like to know if the vacuum can is located behind the front driver side fender. No one has answered that question. If it is, I assume I'll have to remove the fender to gain access?
78CobraII
Moderator
Correct, the vacuum canister is behind the passenger-side fender. If you open the passenger door and look into the fender, you can see it.
However you don't have to remove the fender to access it. Jack up the car on the passenger side, remove the wheel, then remove the fender liner.
The liner is held in with sheetmetal tabs stuck through slots in the liner and bent over, and maybe some screws into the inner fender, depending on the year.
However you don't have to remove the fender to access it. Jack up the car on the passenger side, remove the wheel, then remove the fender liner.
The liner is held in with sheetmetal tabs stuck through slots in the liner and bent over, and maybe some screws into the inner fender, depending on the year.
beenstanged
New Member
i'm going to say its the charcoal can for emissions.I'm not sure but i think it returns back to the fuel tank from the black coffe can looking can
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