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What 'controls' the Heater Control Valve vacuum ? (MCC) 940 NA 900 1993

uid 18083's pic
posted by  lucid on  Thu Jul 10 02:36 CST 2003
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last visit: Fri Oct 24 12:31 CST 2014
[RELATED]

I see the HC valve is vacuum-controlled, and am wondering how the MCC system influences this vacuum. (am replacing valve, hoses, and radiator today per FAQ advice).

I have diagrams for '93 940, but don't see anything obvious on page 156. My best guess is item 6/27, which is identified as "Servo Motor, throttle control", with inputs from 4/40 the MCC/CU. Could this be something like the IAC, but controlling the HC Valve vacuum?


--
Bruce Young, 940-NA, many 240s (one V8), '73 142, '71 144 (ex circle track car in CT), '67 144 (new), used 122 from '63 to '67








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    What 'controls' the Heater Control Valve vacuum ? (MCC) 940 NA 900 1993

    posted by  someone claiming to be Peter Penguin on  Thu Jul 10 03:18 CST 2003 [RELATED]

    I do believe the heater control vacuum is controlled by the dial on the dash... a totally mechanical system, no motors or electronics.

    In any event, I find it's incredibly sensitive... when I just flick it a fraction of a turn I can feel the difference in the heat almost immediately.







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      What 'controls' the Heater Control Valve vacuum ? (MCC) 940 NA 900 1993

      posted by  Kuba on  Thu Jul 10 06:48 CST 2003
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      IIRC, The only reason for heater control valve's existence is to prevent stealing of engine heat by interior heater when the engine is not fully warmed up. The vacuum that goes to it is ultimately controlled by the engine temperature, probably via ECU. It's a binary device -- either closed or open, no intermediate states.

      When my valve failed (cracked when I was replacing the heater hose) I simply removed it and replaced the 2-piece hose with a single piece going straight from the engine block to the heater core.

      As long as you remember to turn off your heating before engine gets warm, you're doing fine without that valve.

      Cheers, Kuba







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