Mazda Trucks 1972-1986 Repair Guide

Deceleration Control System

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The deceleration control system is designed to maintain a balance air/fuel mixture during periods of engine deceleration. Although the components used vary in some years, the basic theory remains the same: To more thoroughly burn or dilute the initial rich mixture formed when throttle is suddenly closed, and to smooth out the transition to a lean mixture by enriching the mixture slightly after the throttle has closed. Although the process may seem contradictory, they act in sequence to provide an overall ideal mixture.

The 1972 49 States and Canada Mazdas use a vacuum control valve, an accelerator switch, a servo diaphragm, and a set of spark retard points in the distributor. The vacuum control valve controls spark retard, through the ignition points, and throttle opening through the servo diaphragm, and a set of spark retard points in the distributor. The vacuum control valve controls spark retard, through the ignition points, and throttle opening through the servo diaphragm. During deceleration, the vacuum control valve applies vacuum to the servo diaphragm, slightly opening the primary throttle plate to admit additional fuel to the lean mixture. At the same time, it activates the retard points through a vacuum switch. At idle, the accelerator switch activates the retard points; above idle speed, the accelerator switch is off and the standard point set is used.

1972 Mazdas sold in California, and all 1973-74 Mazdas use an anti-afterburn valve, and a coasting richer valve on the carburetor which is controlled by three switches: speedometer, accelerator, and clutch. 1975-76 Mazdas use the same system, but the clutch switch is not used. The anti-afterburn valve has a diaphragm controlled engine vacuum. During deceleration, the diaphragm lifts allow the air pump to inject air into the intake manifold. This dilutes the incoming rich mixture, preventing detonation in the exhaust system, which would occur if the injected air from the air pump were to burn this rich mixture in the exhaust manifold. The coasting richer valve acts to add additional fuel to the lean intake mixture as soon as the anti-afterburn valve has shut off. The speed, accelerator and clutch switches must be closed to allow the coasting richer valve to operate. The accelerator switch closes when the accelerator pedal is released. The speedometer switch closes when the truck speed is above 17-23 mph. The clutch switch, used from 1972-74, is closed when the clutch pedal is released.

1977-78 1,796cc 49 States and Canada Mazdas use an air by-pass valve, a carburetor dashpot, and a throttle opener system comprised of a servo diaphragm connected to the throttle and a vacuum control valve. This is essentially the same as the 1972 49 States model. The air by-pass valve prevents afterburn in the exhaust by shutting off air to the exhaust manifold from the air pump during deceleration. The dashpot holds the throttle open slightly for an instant during sudden deceleration. The throttle opener system is the same as that used in the 1972 49 States models.

1977-78 California Mazdas use an anti-afterburn valve and a throttle opener system. The anti-afterburn valve is the same as that used from 1973-76, and the throttle opener is the same as that used in 1977-78 49 States models.

1979 and later 1,970cc 49 States models use an anti-afterburn valve and a throttle positioner. California models have an air by-pass valve and a throttle positioner. Canadian models have an air by-pass valve, a dashpot, and a throttle positioner.

On 1974 models with automatic transmission, a kick-down control system is used. Regardless of the gear selected, the transmission will not go above Second gear when the choke knob is pulled out.

TESTING



Combination Anti-Afterburn and Coasting Valve
1974 VEHICLES
  1. Disconnect the hose which runs from the air cleaner to the combination valve at the air cleaner end.
  2.  
  3. Start the engine and run it at curb idle.
  4.  
  5. There should be no vacuum present at the end of the hose which you disconnect in Step 1.
  6.  
  7. Turn the engine off.
  8.  
  9. Disconnect the hose which runs from the coasting valve portion of the combination valve to the intake manifold from the coasting valve end and plug up the port.
  10.  
  11. Operate the engine at idle.
  12.  
  13. Disconnect the anti-afterburn valve solenoid connector.
  14.  
  15. Check for vacuum at the end of the hose which you disconnect in Step 1; there should be vacuum present. If not, the anti-afterburn valve is defective.
  16.  
  17. Turn the engine off. Reconnect the anti-afterburn valve electrical leads and the hose to the coasting valve.
  18.  
  19. Disconnect the intake manifold-to-anti-afterburn valve vacuum line at the valve end, and plug the vacuum fitting on the valve.
  20.  
  21. Start the engine and allow it to idle.
  22.  
  23. Disconnect the coasting valve solenoid at the multiconnector.
  24.  
  25. Hold your hand over the end of the vacuum line which you disconnected in Step 10. Vacuum should be felt; if not, replace the defective coasting valve.
  26.  
  27. Turn the engine off and reconnect the leads and hoses which were disconnected above.
  28.  

Idle Switch See Figure 1



Click image to see an enlarged view

Fig. Fig. 1: View of the idle switch (arrow), located on the carburetor

  1. Unfasten the idle switch leads.
  2.  
  3. Connect the test meter to the switch terminals.
  4.  
  5. With the engine at idle, the meter should indicate a complete circuit.
  6.  
  7. Depress the plunger on the idle switch; the circuit should be broken (on meter reading). If the idle switch is not functioning properly, replace it with a new one.
  8.  

Coolant Temperature Switch

1974-75 VEHICLES

Start this test with the coolant temperature below 68ºF (20ºC).

  1. Disconnect the electrical lead from the temperature switch.
  2.  
  3. Connect a test light between one terminal of the switch and a 12v battery. Ground the other terminal.
  4.  
  5. The test light should light.
  6.  
  7. Start the engine and allow it to warm up. Once the engine reaches normal operating temperature, the test light should go out.
  8.  
  9. Replace the switch if it doesn't work as outlined.
  10.  

Choke Switch (Semi-Automatic Choke)

  1. Working underneath the instrument panel, disconnect the lead at the back of the choke switch.
  2.  
  3. Connect an ohmmeter to the terminals on the choke switch side of the connector.
  4.  
  5. With the choke knob on (off), the meter should show continuity (resistance reading).
  6.  
  7. Pull the choke knob out, about 1 / 2 " for manual transmission trucks or 1" for automatics. The meter should show no continuity (read zero).
  8.  
  9. Replace the switch if defective.
  10.  

Anti-Afterburn Valve See Figure 2



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Fig. Fig. 2: View of the anti-afterburn valve, located on the intake manifold

  1. Remove the outlet hose from the anti-afterburn valve.
  2.  
  3. Hold a hand over the outlet fitting and raise the engine rpm. Quickly release the accelerator. Air should flow for approximately three seconds. If the valve passes air for more than three seconds, or does not pass air at all, it should be replaced.
  4.  

Coasting Richer Valve (Deceleration Valve) See Figure 3



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Fig. Fig. 3: View of the coasting richer valve, located on the carburetor

  1. Remove the coasting richer valve from the carburetor.
  2.  
  3. Connect the coasting richer valve to the battery.
  4.  
  5. As power is applied to the valve, the solenoid plunger should be pulled into the valve body.
  6.  
  7. Reinstall the coasting richer valve. Connect a test light.
  8.  
  9. Raise the rear wheels and support the truck on stands. Block the front wheels.
  10.  
  11. Start the engine and raise the engine speed above 30 mph. Release the accelerator pedal. The test light should come ON and remain ON until the speed falls below 17-23 mph.
  12.  
  13. If the system is operating properly, no further tests are required. If not, proceed with the other tests.
  14.  
  15. Remove the stands and lower the truck. Disconnect the test light.
  16.  

Clutch Switch

The clutch switch is activated by the clutch pedal. When checking the circuit, the test light should be ON when the clutch pedal is fully released, and should be OFF when the clutch pedal is fully depressed.

Accelerator Switch See Figure 4



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Fig. Fig. 4: Checking the accelerator switch with a test lighton 1972-78 trucks, the switch is located on the carburetor; on 1979 and later trucks, the switch is located on the arm of the accelerator pedal

The accelerator switch is actuated by a throttle lever link on the carburetor through 1978; the switch is located on the accelerator pedal on 1979 and later models. When checking the switch with a circuit tester, the test light should be ON when the accelerator pedal is fully released and should be OFF when the pedal is depressed.

Speed Switch

  1. Remove the instrument cluster and attach a test light to the speedometer switch.
  2.  
  3. Reconnect the speedometer cable and ground wire.
  4.  
  5. Raise both wheels off the ground and support the truck on stands. Block the front wheels.
  6.  
  7. Start the engine.
  8.  
  9. Depress the accelerator pedal to accelerate the engine and confirm that the speed switch is ON at speeds of 17-23 mph and OFF at speeds below 17-23 mph.
  10.  
  11. If not, replace the switch.
  12.  
  13. Lower the truck and remove the test light.Reinstall the instrument cluster.
  14.  

Speed Switch Relay

Check the speed switch relay with a test light to be sure that it is operating at 17-23 mph.

Three-Way Solenoid Valve See Figures 5 and 6

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Fig. Fig. 5: Three-way solenoid valve checkwith ignition switch OFF



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Fig. Fig. 6: Three-way solenoid valve checkwith ignition switch ON

1,970cc ENGINES
  1. Start the engine and allow it to reach normal operating temperature. Check the idle speed and adjust as necessary.
  2.  
  3. Disconnect the wire at the three way solenoid valve. This wire is coded either black with a white stripe or brown with a red stripe on some models.
  4.  
  5. When the wire is disconnected, the engine speed should increase to 1,000 rpm for 49 States trucks, or 1,100 rpm for California models.
  6.  
  7. If the engine speed does not increase, the three way solenoid valve or the servo diaphragm is not operating correctly. Check the servo diaphragm using the following procedure; if the servo diaphragm is operating correctly, the three way valve is faulty and should be replaced.
  8.  

Servo Diaphragm

  1. Start the engine and set the idle speed to specification.
  2.  
  3. Stop the engine and disconnect the vacuum line between the vacuum control valve and the diaphragm at the diaphragm.
  4.  
  5. Disconnect the vacuum line between the intake manifold and the vacuum control valve at the manifold on models through 1978. On 1979 and later models, disconnect the vacuum hose at the vacuum amplifier and the vacuum hose at the three way solenoid valve. Connect the vacuum hose from the servo diaphragm to the vacuum amplifier so that the intake manifold vacuum is applied directly to the servo diaphragm.
  6.  
  7. Disconnect and plug the vacuum line between the carburetor and distributor.
  8.  
  9. Connect a vacuum line from the intake manifold to the servo diaphragm on models through 1978.
  10.  
  11. Connect a tachometer and start the engine. The engine should idle at 1,300-1,500 rpm for models through 1978, or 900-1,100 rpm, 1979 (1,000-1,200 rpm for 1979 and later California models). If the engine speed is not correct, adjust by means of the servo diaphragm adjusting screw. If the correct speed is not obtainable, replace the diaphragm.
  12.  

Vacuum Control Valve

  1. Disconnect the vacuum hose between the vacuum control valve and the intake manifold at the manifold.
  2.  
  3. Attach a vacuum gauge in the line using a T-fitting.
  4.  
  5. Connect a tachometer to the engine. Start the engine and raise the speed to 3,000 rpm, then suddenly release the throttle. The vacuum reading should rise above 21.3 in.Hg, drop to that figure and hesitate there for one or two seconds, then drop to the normal idle vacuum of 16-18 in.Hg. Note that these readings are for sea level, and should be corrected accordingly.
  6.  
  7. If the vacuum reading is not within specification, adjust the vacuum control valve by turning the adjusting screw in the top of the valve. If the correct reading is unobtainable, replace the valve.
  8.  

Vacuum Switch

  1. Disconnect the vacuum hose between the vacuum switch and the vacuum control valve.
  2.  
  3. Using a T-fitting, connect a vacuum gauge between the vacuum switch and an external vacuum source.
  4.  
  5. Raise the vacuum reading above 8 in.Hg, then allow the vacuum to drop. The switch should click at approximately 6 in.Hg. If it does not, or if it clicks at a higher reading, replace the switch.
  6.  

Air Bypass Valve

  1. Disconnect the air hose from the side of the air by-pass valve.
  2.  
  3. Connect a tachometer to the engine. Start the engine and raise the speed above 2,000 rpm.
  4.  
  5. Release the throttle and check for air flow from the port on the side of the air by-pass valve. If there is no airflow, replace the valve.
  6.  

ADJUSTMENTS Dashpot



  1. Check the engine idle speed and mixture, and adjust as necessary.
  2.  
  3. Remove the air cleaner.
  4.  
  5. With the tachometer still connected to the engine, loosen the dashpot locknut. Move the throttle lever and hold to maintain the engine speed at 2,400-2,600 rpm (2,100-2,300 rpm for California trucks).
  6.  
  7. Turn the dashpot until its rod contacts the throttle lever and tighten the locknut.
  8.  
  9. Move the throttle lever until it contacts the dashpot rod and recheck the engine speed. Repeat the adjustment if necessary.
  10.  

Accelerator Switch See Figure 7

Click image to see an enlarged view

Fig. Fig. 7: Adjusting the accelerator switch

1972-78 VEHICLES
  1. Be sure that the throttle valve is fully closed.
  2.  
  3. Loosen the switch adjusting screw and turn the switch off.
  4.  
  5. Gradually tighten the adjusting screw until the switch produces a clicking sound and is turned on.
  6.  
  7. Tighten the adjusting screw another 1 1 / 2 turns.
  8.  

1979 AND LATER VEHICLES

The accelerator switch is mounted on the arm of the accelerator pedal.

  1. Check the accelerator pedal to make sure that it moves freely.
  2.  
  3. Loosen the accelerator switch locknut.
  4.  
  5. Gradually turn the adjusting screw until the accelerator switch clicks.
  6.  
  7. Tighten the locknut.
  8.  

Idle Switch

  1. Warm up the engine until the water temperature is at least 156ºF (69ºC).
  2.  
  3. Make sure that the mixture and idle speed are properly adjusted.
  4.  
  5. Adjust the idle speed to 1,075-1,100 rpm on models with manual transmission; 1,200-1,300 rpm on models with automatic transmission, by rotating the throttle adjusting screw.
  6.  
  7. Rotate the idle switch adjusting screw until the switch changes from OFF to ON position.
  8.  
  9. Slowly turn the idle switch adjusting screw back to the point where the switch just changes from ON to OFF.
  10.  
  11. Turn the throttle screw back so that the engine returns to idle.
  12.  

Be sure that the idle switch goes on when the idle speed is still above 1,000 rpm.

REMOVAL & INSTALLATION Speed Switch



The speed switch is integral with the speedometer head located in the instrument panel. To replace the speed switch, refer to Instrument Cluster Removal and Installation in Chassis Electrical .

Anti-Afterburn Valve

  1. Remove the air cleaner assembly.
  2.  
  3. Disconnect the air hoses and vacuum lines from the valve.
  4.  
  5. Unfasten the solenoid wiring.
  6.  
  7. Remove the securing nuts and withdraw the valve.
  8.  
  9. Installation is performed in the reverse order of removal.
  10.  

Coasting Valve

The coasting valve is removed and installed in the same manner as the anti-afterburn valve.

Idle Switch

  1. Remove the coasting valve. See the section above.
  2.  
  3. Remove the carburetor as detailed in Fuel System .
  4.  
  5. Disconnect the wiring from the switch.
  6.  
  7. Unfasten the securing screws and remove the switch.
  8.  
  9. Installation is performed in the reverse order of removal. After installing the switch, adjust it as outlinedpreviously.
  10.  

Air Supply Valve

  1. Remove the air cleaner and the hot air duct.
  2.  
  3. Disconnect the air hose, the vacuum lines, and the solenoid wiring from the valve.
  4.  
  5. Unfasten the screws which secure the valve and remove it.
  6.  
  7. Air supply valve installation is performed in the reverse order of removal.
  8.  

Coolant Temperature Switch

  1. Drain the coolant from the radiator enough to bring the coolant level below the temperature switch.
  2.  


CAUTION
When draining engine coolant, keep in mind that cats and dogs are attracted to ethylene glycol antifreeze and could drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain coolant into a sealable container. Coolant should be reused unless it is contaminated or is several years old.

  1. Remove the alternator and drive belt if they are in the way.
  2.  
  3. Disconnect the switch multiconnector.
  4.  
  5. Use an open-end wrench to remove the switch.
  6.  
  7. Installation is the reverse of removal.
  8.  

 
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