heater a/c control
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Senior User
Joined: May 2005
Posts: 315
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From: SW Kansas
heater a/c control
I read the thread in the tech tips, but didn't see my specific issue. When I first start the truck it will blow air where I have the selector set, but then as I'm driving it will blow out the defrost, but when I let off of the throttle is goes back to what's selected. All the vacuum lines look ok, and the cruise works without any issues.
Postmaster
Joined: Apr 2003
Posts: 2,703
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From: Harrisburg, NE
sounds like you have a vacuum pod that isn't connected., the only one I can think of is on the passenger side inner fender and looks like a coffee can, stores vacuum for stuff like the heater when you give it throttle and the engine vacuum goes to zero (the heater doors are vacuum operated)
be glad you don't have an older vehicle with a vacuum wiper motor!
be glad you don't have an older vehicle with a vacuum wiper motor!
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Joined: Nov 2007
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From: Denver Metro Area, CO
I would also suspect a vacuum reservoir isn't connected (properly).
I always thought the grapefruit-sized plastic ball was for HVAC and the fruit juice can was for the engine controls but I may be wrong there.
I always thought the grapefruit-sized plastic ball was for HVAC and the fruit juice can was for the engine controls but I may be wrong there.
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Senior User
Joined: May 2005
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From: SW Kansas
]Here's what I have, the grapefruit with one line and the do-hickey it connects to. The doo-hickey connects to a fitting on the firewall that in turn connects to engine vacuum. Hoses all look good.
Postmaster
Joined: Apr 2003
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From: Harrisburg, NE
the do-hickey (vacuum T with the round part) in your picture is a check valve so that when the engine vacuum drops it keeps vacuum in the grapefruit res. and heater controls.
I think that the "Help!" section of most part stores have vacuum check valves.
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Joined: Nov 2007
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From: Denver Metro Area, CO
If the guy has a hand-held vacuum pump, I wonder... can he somehow apply vacuum to the HVAC system and see if that check valve is doing its job.
Not that a new check valve would be re4al expensive, I just hate throwing parts at a problem without any real proof of the need to replace something.
He might also look on the other side of his firewall and make sure that vacuum line is correctly hooked up.
Not that a new check valve would be re4al expensive, I just hate throwing parts at a problem without any real proof of the need to replace something.
He might also look on the other side of his firewall and make sure that vacuum line is correctly hooked up.
Postmaster
Joined: Apr 2003
Posts: 2,703
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From: Harrisburg, NE
not a bad idea, i suppose that if you hooked it up in place of the controls or grapefruit, and stuck your finger over the engine side of the valve it should hold vacuum for at least a little while. i guess in a pinch you could use your mouth.
completely agree about tossing parts at stuff, but yeah, something like this wouldn't be very expensive.
completely agree about tossing parts at stuff, but yeah, something like this wouldn't be very expensive.
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Joined: May 2005
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From: SW Kansas
I'll check that out today. Lovely Kansas weather, 60's on Monday, snow yesterday. Supposed to be about 50 today. Problem seems to be vacuum supply rather than in the control, so might be the check valve.
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From: Sheffield, AL
Years ago, my son's 1984 F150 was doing the same as you describe, 9wire. He had just had a new stereo unit installed and during the installation, a vacuum line was pulled from the vacuum servo motor [under the dash] which determines the blend of air for defroster and floor [heat]. Anyway, that relatively small leak was enough to let the system leak down when the engine was under a load [going uphill or passing] and the system defaults to the defroster mode. Once he let off the gas, the system would quickly resume normal function.
Trace all vacuum lines looking for a loose fitting, split or crack of the old plastic vacuum lines.
Trace all vacuum lines looking for a loose fitting, split or crack of the old plastic vacuum lines.
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Senior User
Joined: May 2005
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From: SW Kansas
Check valve seems ok, and I have vacuum through the firewall going to the control. Looks like a bit of disassembly is required to get the control out? Last time I tried one was a awhile back, I seem to recall it being a pain. Any tips?
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