Mazda Trucks 1972-1986 Repair Guide

Spark Timing Control System

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A spark delay system is used on 1977-78 1,796cc engines, and 1979 and later 1,970cc engines. Its purpose is to reduce the formation of CO and NO 2 emissions by delaying the vacuum advance to the distributor during normal acceleration. The system consists of a spark delay valve installed in the vacuum hose between the carburetor and the distributor vacuum advance diaphragm, on all 1977-78 models, as well as 1979 and later 49 States models. The spark delay valve is installed in the vacuum line between the water thermo valve and the distributor diaphragm on 1979 and later models sold in California. The water thermo valve is installed in the intake manifold. The spark delay valve has an internal restrictor to slow the air flow in one direction and a check valve which allows air to flow freely in the opposite direction.

SYSTEM TESTING



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Spark delay valve test

  1. Remove the air cleaner. Disconnect the vacuum hose from the distributor.
  2.  
  3. On 1977-78 models, disconnect the vacuum control valve hose at the intake manifold fitting, and install the distributor hose on that fitting. On 1979 and later 49 States models, disconnect the anti-afterburn valve line at the intake manifold fitting and install the distributor vacuum line to the intake manifold fitting. On 1979 and later California models, disconnect the air bypass valve hose from the intake manifold fitting and install the distributor vacuum line to the intake manifold fitting.
  4.  
  5. Remove the vacuum hose from the carburetor side of the spark delay valve. Plug this hose, and attach a vacuum gauge to the delay valve.
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  7. Start the engine and allow it to idle.
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  9. Disconnect the vacuum hose from the intake manifold fitting (the hose from the spark delay valve to the distributor which has been connected to the intake manifold in Step 2) and note the time for the vacuum gauge reading to drop to 11.8 in. Hg. It should drop to this figure within 2-7 seconds (1-10 seconds on California models), 1977-78, or 4-6 seconds, 1979 and later. If the reading is not correct, replace the spark delay valve.
  10.  
  11. After all tests have been completed, return the hoses to their original positions.
  12.  

DISTRIBUTOR SOLENOID VALVE TEST



  1. Start the engine and allow it to idle.
  2.  
  3. Separate the connector at the distributor solenoid valve and apply battery power to the terminals to energize the valve.
  4.  
  5. Disconnect the vacuum hose from the vacuum advance unit. There should be no vacuum at the hose.
  6.  
  7. Remove the battery power to the solenoid valve. The valve is operating properly if air is drawn (by vacuum) into the vacuum hose as soon as power is removed.
  8.  

WATER THERMO SWITCH TEST



  1. Remove the switch.
  2.  
  3. Connect an ohmmeter across the switch terminals and place the sensor in water. Heat the water gradually. Using a thermometer check the water temperature as it is heated.
  4.  
  5. Note the temperature at which continuity exists between both poles. The specified temperature is 131ºF (55ºC). If the temperature is not within specifications, replace the switch.
  6.  

TIMER TEST



See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: Submerge the end of the temperature sensor in cold or hot water and check resistance

  1. Remove the timer.
  2.  
  3. Connect suitable test wires to terminals A, B, and C, respectively.
  4.  
  5. Connect a test wire with a 30K omega resistor to terminal D.
  6.  
  7. Connect a test wire with a 3.4w lamp to terminal E.
  8.  
  9. Connect the positive test lead of a voltmeter to terminal D and the negative lead to the battery negative terminal.
  10.  
  11. Connect test wires A, D, and E to the positive terminal. Do not connect Terminal B in this step.
  12.  
  13. Connect the wire from terminal C to the battery negative terminal.
  14.  
  15. Check the voltmeter reading. It should read about 6v or above.
  16.  
  17. Connect the test wire from terminal B to the positive battery terminal. At the moment the connection is complete, the voltmeter should read less than 1 volt, and the lamp will light. Maintain this connection for 2 minutes. The voltmeter should read about 8v.
  18.  

 
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