Mazda Trucks 1972-1986 Repair Guide

Airflow Control System

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The airflow control system consists of a thermosensor, thermodetector, vacuum switch and a control box.

The system determines when the trailing distributor should be used, depending upon engine temperature and speed (rpm). The airflow control system also operates the solenoid on the airflow control valve.

When the engine is cold, the thermosensor sends a signal to the control box which, in turn, activates the distributor. The thermodetector is used to keep the thermosensor from being influenced by ambient temperatures. This ensures easier cold starting and better drivability. The thermosensor and control box are located, in order, beneath the dash, next to the fuse box, and behind the grille.

TESTING No. 1 Thermosensor



See Figure 1

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Fig. Fig. 1: The No. 1 thermosensor is located next to the thermostat housing

Begin this test procedure with the engine cold.

  1. Remove the air cleaner.
  2.  
  3. Examine the no. 1 thermosensor, which is located next to the thermostat housing, for leakage around the boot and for signs of wax leakage.
  4.  
  5. Disconnect the multiconnector from the thermosensor and place the prods of an ohmmeter on the thermosensor terminals. The ohmmeter should read over 7k omega with the engine cold and less than 2.3k omega after the engine has been warmed up.
  6.  
  7. Replace the thermosensor with a new one, if the reading on the ohmmeter is not within specifications.
  8.  
  9. If the no. 1 thermosensor is functioning properly, proceed with the appropriate test for the thermodetector below.
  10.  

Thermodetector

Use the following chart to determine the correct ohmmeter reading for the ambient temperature at the time of the test:

No. 1 Control Box See Figures 2, 3 and 4



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Fig. Fig. 2: Testing the No. 1 control box



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Fig. Fig. 3: 1972 idle switchthe 1973 switch uses a multi-connector



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Fig. Fig. 4: Using a jumper wire to short together the pins of the No.1 thermosensor

If all of the other components of the airflow control system are functioning properly and the system wiring and vacuum lines are in good condition, then the fault probably lies in the no. 1 control box. Perform the following tests to verify this.

  1. Disconnect the no. 1 thermosensor. Disconnect the idle switch multiconnector.
  2.  
  3. Start the engine and run it to the speeds specified below. The timing light should come on in these speed ranges:

    Manual Transmission: 3,600-4,400 rpm
     
    Automatic Transmission: 4,300-5,300 rpm
     

  4.  

These speeds should be held for an instant only.

  1. Connect an ammeter to the air control valve solenoid leads and to ground.
    1. Current should flow when the engine speed is between 900-4,000 +- 200 rpm, manual or 7,500-5,200, automatic.
    2.  
    3. Current flow should cease above 3,600-4,400 rpm, manual transmission or 4,300-5,300 rpm, automatic transmission.
    4.  

  2.  
  3. Short together the pins of the no. 1 thermosensor multiconnector with a jumper wire. Connect the timing light to the trailing side of the distributor, if it is not already in place.
    1. The timing light should go on when the engine is below 3,600-4,400 rpm, automatic, or below 4,300-5,200 rpm, manual.
    2.  
    3. On automatic transmission equipped models, connect an ammeter to the air control valve solenoid. Current should flow to the solenoid when the engine speed is below 3,400 +- 200 rpm and should cease flowing above this speed.
    4.  

  4.  
  5. Remove the jumper wire from the multiconnector and reconnect the No. 1 thermosensor. Reconnect the vacuum switch if it was disconnected.
  6.  
  7. Connect the prods of the ammeter to the coasting valve solenoid terminals.
  8.  

For a further description of coasting valve operation, see Deceleration Control Systems.

  1. No current should flow to the solenoid with the engine at idle. Increase the engine speed; current should begin flowing between 1,250 and 1,500 rpm. Decrease engine speed; current should cease flowing between 1,300 and 1,100 rpm for manual transmissions; 1,400 rpm for automatic transmissions. If the no. 1 control box proves to be defective, replace it. Remember to disconnect all of the test equipment and reconnect the system components when the tests are completed.
  2.  

REMOVAL & INSTALLATION No. 1 Thermosensor



  1. Remove the air cleaner assembly.
  2.  
  3. If necessary, remove the starter motor as detailed under Engine Electrical.
  4.  
  5. Unplug the thermosensor multiconnector.
  6.  
  7. Withdraw the boot from the thermosensor.
  8.  
  9. Unfasten its securing nuts and remove the switch.
  10.  
  11. Installation is the reverse of removal.
  12.  

Control Box

1974-75 VEHICLES

WARNING
Be sure that the ignition switch is turned off to prevent damage to the control box!

  1. Working from underneath the instrument panel, locate and disconnect the control box multiconnector.
  2.  
  3. Remove the screws which secure the control box.
  4.  
  5. Remove the control box.
  6.  
  7. Installation is performed in the reverse order of removal.
  8.  

Other Components

Any of the other components used in the airflow control system are removed by unfastening their multiconnectors and removing the screws which secure them.

 
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