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Vacuum Control Function

  1. To test the Vacuum Control Function, start the engine and move mode lever slowly from one position to another, A momentary hiss should be heard as lever is moved to each position. A continuous hiss indicates a major leak within the system, but not necessarily within the control head.
  2. If a momentary hiss cannot be heard, check for pinched or disconnected vacuum supply hose. Ensure that the check valve between the vacuum reservoir and the intake manifold is working properly
  3. It a momentary hiss con be heard, vacuum is available at the control assembly Cycle the control fever through each position with blower on "HI" and check air discharge locations. The Air Flow and Vacuum Control Lever Position chart will give the vacuum motors applied for each mode lever and temperature control position. See Figure.
  4. The an flow will show the position of each door with vacuum applied as well as then no vacuum position, if a vacuum motor is not operating properly, the air flow will not be as the diagram indicates.
  5. If a vacuum motor appears inoperative, check motor operation with a vacuum pump and gauge. If the vacuum motor then operates, a vacuum line is leaking or pinched closed.
    NOTE: Vacuum and wiring diagrams are of end of article.
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