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Answer to Heater control valve + vacuum + flapper logic 700 1987

posted by  Gregg Shadduck on  Wed Jul 21 14:19 CST 2010
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last visit: Fri Mar 7 12:10 CST 2014
[RELATED]

Success. :-)

While I remain inexpert, I would like to pass on what I learned, which is very basic, but new to me, anyway.

1. The heater control valve is operated ONLY by vacuum.
It does NOT modulate, but is full on or full off.

2. The cable-operated flap (above passenger's left foot) is the (cool <--> hot) modulator.

3. When the lever hits the left limit of travel it activates a shutoff circuit in the vacuum. yes. THAT closes the heater control valve, and is the ONLY way the valve is closed.

I determined that the hose (gray) between the lever control and the shutoff valve (near the A/T dipstick) was bad. Would not suck. (MityVac to the rescue.)

I ran a replacement hose (generic NAPA fuel/vacuum) between the two ends, by the most convenient route (that term is relative), and orphaned the old hose.

We are now able to enjoy ambient air flowing through our vents in July.






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