Mazda Trucks 1972-1986 Repair Guide

Throttle Positioner SystemB1600

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The throttle positioner system consists of a servo diaphragm connected to the throttle lever and a vacuum control valve which controls intake manifold vacuum through the servo diaphragm.

TESTING THE SYSTEM Servo Diaphragm



See Figures 1 and 2

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Fig. Fig. 1: View of the servo diaphragm (arrow), located on the intake manifold next to the carburetor



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Fig. Fig. 2: View of the vacuum control valve, located next to the servo diaphragm and bolted to the intake manifold

  1. Start the engine and set the idle speed to 800 rpm. Stop the engine.
  2.  
  3. Disconnect the vacuum sensing tube between the servo diaphragm and the vacuum control valve at the servo diaphragm.
  4.  
  5. Remove the intake manifold suction hole plug.
  6.  
  7. Connect the intake manifold and the servo diaphragm with a tube so that the intake manifold vacuum goes directly to the servo diaphragm.
  8.  
  9. Connect a tachometer and remove the vacuum sensing tube between the carburetor and distributor.
  10.  
  11. Start the engine and read the speed. If the engine is running between 1,300-1,500 rpm, the servo diaphragm is operating normally. If the engine speed is 800-1,500 rpm, adjust the speed with the throttle opening screw. If the engine speed remains normal, about 800 rpm, the servo diaphragm is defective and should be replaced.
  12.  

SERVICING Servo Diaphragm Replacement



  1. Remove the air cleaner.
  2.  
  3. Disconnect the vacuum sensing tube from the servo diaphragm.
  4.  
  5. Remove the cotter pin and link.
  6.  
  7. Loosen the locknut and remove the servo diaphragm.
  8.  
  9. Installation is the reverse of removal. Adjust the servo diaphragm.
  10.  

Vacuum Control Valve Replacement

  1. Remove the air cleaner.
  2.  
  3. Disconnect the vacuum sensing tubes from the vacuum control valve.
  4.  
  5. Unbolt and remove the vacuum control valve.
  6.  
  7. Installation is the reverse of removal.
  8.  

ADJUSTMENTS Throttle Opener



  1. Install a tachometer on the engine.
  2.  
  3. Start the engine and set the idle speed.
  4.  
  5. Stop the engine.
  6.  
  7. Disconnect the vacuum sensing tube between the servo diaphragm and the vacuum control valve from the servo diaphragm.
  8.  
  9. Remove the plug from the intake manifold suction hole.
  10.  
  11. Attach a test tube between the intake manifold and the servo diaphragm to route intake manifold vacuum directly to the servo diaphragm.
  12.  
  13. Start the engine and note the speed.
  14.  
  15. Set the engine speed to 1,400 rpm using the throttle opener screw. Turning the adjusting screw clockwise increases engine speed.
  16.  
  17. Stop the engine and disconnect the tachometer. Reconnect all lines.
  18.  

 
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