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Heater control valve + vacuum + flapper logic and operation unclear 700 1987

posted by  Gregg Shadduck on  Thu Jul 15 04:13 CST 2010
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last visit: Fri Mar 7 12:10 CST 2014
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Could someone please clarify the heater control (HVAC) logic and control?

(I am trying to turn the HEAT off, off, off.
In our 240 I went to far as to insert a ball valve shutoff the the heater line, since it was easily accessible, and I was weary of games.)

Despite reading the FAQs and Haynes, monitoring Brickboard, and observation, I still don't get it. :-(

I see: the heater control valve is operated ONLY by vacuum.
Infer: it does NOT modulate, but is supposed to be full on or full off?

(I still have lots of trouble trying to follow those little hoses in tight places, to apply MityVac suck.)

I see: in the manual HVAC system, the lever from low heat to high heat moves a flap, about a foot above the passenger's left toes.
Infer: THIS is the (only) modulator?
(I know its motion or limits are adjustable, as John Sargent commented in the FAQs, and have done so.)

I see: diagrams of the vacuum hoses at the control show a special position, I think. (I have not yet seen any diagrams with explanations.)
Infer: When the lever hits the left limit of travel it is supposed to activate a shutoff circuit in the vacuum? THAT closes the heater control valve, and is the ONLY way the valve is closed?

Thank you for your help!






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