Mazda Trucks 1972-1986 Repair Guide

Throttle Positioner SystemB1600


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.


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.
  3. Disconnect the vacuum sensing tube between the servo diaphragm and the vacuum control valve at the servo diaphragm.
  5. Remove the intake manifold suction hole plug.
  7. Connect the intake manifold and the servo diaphragm with a tube so that the intake manifold vacuum goes directly to the servo diaphragm.
  9. Connect a tachometer and remove the vacuum sensing tube between the carburetor and distributor.
  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.

SERVICING Servo Diaphragm Replacement

  1. Remove the air cleaner.