Mazda Trucks 1987-1993 Repair Guide

Deceleration Control System

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OPERATION



The deceleration control system on B2200 carburetor equipped Pickups 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 deceleration control system used on carburetor equipped B2600 models and on the MPV reduces the fuel flow to decrease HC emissions and to improve fuel economy during deceleration and cut the fuel flow when the ignition switch is OFF to prevent "run-on''.

SERVICE



2.2L Carbureted Engine
SLOW FUEL CUT SYSTEM TESTING
  1. Warm the engine to normal operating temperature.
  2.  
  3. Disconnect the neutral switch or inhibitor switch wire connectors.
  4.  
  5. Remove the air cleaner assembly.
  6.  
  7. Connect a voltmeter to the F (LG) terminal of the carburetor connector.
  8.  
  9. Increase the engine speed to about 300 rpm.
  10.  
  11. Lift the idle switch arm.
  12.  
  13. Verify the voltmeter reading is about 12 volts when the engine speed is above 2500 rpm.
  14.  
  15. Verify that the voltmeter reading is less than 1.5 volts when the engine speed is less than 2500 rpm.
  16.  
  17. If not as specified, check the 2D terminal of the emission control unit and the slow cut solenoid valve.
  18.  
  19. Start the engine and run at idle.
  20.  
  21. Disconnect the carburetor wire connector.
  22.  
  23. Verify that the engine stops running.
  24.  
  25. If not as specified, inspect the carburetor ports or replace the solenoid valve.
  26.  

RICHER SYSTEM TESTING
  1. Warm the engine to normal operating temperature.
  2.  
  3. Disconnect the neutral switch or inhibitor switch wire connectors.
  4.  
  5. Remove the air cleaner assembly.
  6.  
  7. Connect a voltmeter to the H (BR/B) terminal of the carburetor connector.
  8.  
  9. Increase the engine speed to about 3000 rpm.
  10.  
  11. Lift the idle switch arm.
  12.  
  13. Verify the voltmeter reading is about 12 volts when the engine speed is above 2500 rpm.
  14.  
  15. Verify the voltmeter reading is less than 1.5 volts, one second after engine speed is 1400-2500 rpm.
  16.  
  17. Verify that the voltmeter reading is about 12 volts when the engine speed is below 1400 rpm.
  18.  
  19. If not as specified, inspect terminal 2H of the emission control unit and the coasting richer solenoid valve.
  20.  
  21. Start the engine and run at idle.
  22.  
  23. Ground the H (BR/B) terminal of the carburetor connector with a jumper wire.
  24.  
  25. Verify that the engine speed increases.
  26.  
  27. If not as specified, inspect the carburetor ports or replace the solenoid valve.
  28.  

ADVANCE SYSTEM TESTING
  1. Warm the engine to normal operating temperature.
  2.  
  3. Disconnect the neutral switch or inhibitor switch wire connectors.
  4.  
  5. Remove the air cleaner assembly.
  6.  
  7. Connect a voltmeter to the W/G terminal of the coasting advance solenoid wire connector.
  8.  
  9. Increase the engine speed to about 3000 rpm.
  10.  
  11. Lift the idle switch arm.
  12.  
  13. Verify that the voltmeter reading is about 12 volts when the engine speed is above 2500 rpm.
  14.  
  15. Verify that the voltmeter reading is less than 1.5 volts when the engine speed is 1700-2500 rpm.
  16.  
  17. Verify the voltmeter reading is about 12 volts when the engine speed is below 1700 rpm.
  18.  
  19. If not as specified, inspect terminal 1S of the emission control unit and the vacuum solenoid valve.
  20.  
  21. Remove the vacuum solenoid valve.
  22.  
  23. Connect vacuum hoses to the valve.
  24.  
  25. Blow air through the port closest to the solenoid wire connector.
  26.  
  27. Verify that air comes out of the valve air filter.
  28.  
  29. Apply battery voltage to the terminals of the solenoid valve.
  30.  
  31. Blow air through the solenoid port closest to the wire connector and verify that air comes out the opposite vacuum port.
  32.  
  33. If not as specified, replace the vacuum solenoid valve.
  34.  

MIXTURE CONTROL VALVE TESTING
  1. Start the engine.
  2.  
  3. Block the intake port of the mixture control valve and verify that engine rpm does not increase.
  4.  
  5. Increase the engine speed and quickly decelerate.
  6.  
  7. Verify that air is pulled into the intake port of the mixture control valve for about one to two seconds after the accelerator is released.
  8.  
  9. If not as specified, replace the mixture control valve.
  10.  

DASHPOT TESTING
  1. Push the dashpot rod in, making sure the rod goes into the dashpot slowly.
  2.  
  3. Release the rod and make sure it comes out quickly.
  4.  
  5. Replace the dashpot if it does not perform as specified.
  6.  

DASHPOT ADJUSTMENT
  1. Run the engine until normal operating temperature is reached.
  2.  
  3. Connect a suitable tachometer to the engine.
  4.  
  5. Raise the engine speed to above 3500 rpm.
  6.  
  7. Slowly decrease the engine speed making sure the dashpot arm touches the lever at about 2700-2900 rpm.
  8.  
  9. Adjust the dashpot as required, for correct curb idle speed, by loosening the locknut and turning the dashpot in or out as necessary. Tighten the locknut after adjustment.
  10.  

B2600

SLOW FUEL CUT SYSTEM TESTING
  1. Warm the engine to normal operating temperature.
  2.  
  3. Connect a tachometer to the engine.
  4.  
  5. Connect a voltmeter to the solenoid valve connector (e) terminal (YL).
  6.  
  7. Check the voltage at the following conditions. At idle 0-0.6 volts. Accelerate to 4000 rpm and decelerate quickly, momentary voltage should be 13-15 volts.
  8.  
  9. If not to specifications, check the deceleration vacuum switch, feedback control unit and wiring of the slow fuel cut system.
  10.  

SLOW FUEL CUT SOLENOID VALVE TEST
  1. Run the engine at idle.
  2.  
  3. Disconnect the connector for the solenoid valves.
  4.  
  5. Check that the engine stops.
  6.  

DECELERATION VACUUM SWITCH TESTING
  1. Turn the ignition ON.
  2.  
  3. Connect a hand vacuum pump to the deceleration vacuum switch.
  4.  
  5. Apply 10.2-11.- in. Hg vacuum to the switch and check the voltage at (e) terminal (GB).
  6.  
  7. Voltages should be: with vacuum applied 0 volts. Vacuum not applied 12 volts.
  8.  
  9. If the voltages are not correct check the deceleration vacuum switch.
  10.  
  11. Turn the ignition OFF.
  12.  
  13. Connect the vacuum pump to the deceleration vacuum switch.
  14.  
  15. Disconnect the vacuum switch connector. Apply 10.2-11.- in. Hg of vacuum and check continuity between (e) and (f) terminals (GB) and (B). There should be continuity when vacuum is applied. If not correct, replace the deceleration vacuum switch.
  16.  

DASHPOT TESTING

Move the throttle lever to the full throttle position, then release the throttle lever and check that the dashpot rod extends slowly.

DASHPOT ADJUSTMENT
  1. Warm the engine to normal operating temperature.
  2.  
  3. Connect a tachometer to the engine.
  4.  
  5. Push the dashpot pushrod up into the pot.
  6.  
  7. Loosen the dashpot adjust screw.
  8.  
  9. Turn the adjust screw clockwise and set the curb idle speed to specification.
  10.  

MPV

DASHPOT ADJUSTMENT
  1. The dashpot is located at the throttle body. Run the engine until it reaches normal operating temperature and then let it idle.
  2.  
  3. Connect a suitable tachometer to the engine.
  4.  
  5. Raise the engine speed to above 4000 rpm.
  6.  
  7. Slowly decrease the engine speed making sure the dashpot rod touches the lever at about 3200-3800 rpm.
  8.  
  9. To adjust, loosen the locknut and turn the dashpot. Secure the locknut when adjustment is finished.
  10.  

 
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