Mazda Trucks 1972-1986 Repair Guide

Air Injection System

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See Figure 1

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Fig. Fig. 1: Typical Thermactor system

Because of the many variables under which the engine operates, some hydrocarbon and carbon monoxide gases escape unburnt from the combustion chamber. To burn these gases more thoroughly, a belt-driven pump is used to supply fresh air to an air injection manifold located on the exhaust manifold. The injection of fresh air supports combustion of the hot unburned HC and CO gases within the exhaust manifold.

The Thermactor system, used on California trucks only in 1972 and 1973, and all trucks thereafter, consists of an air pump with relief valve, a check valve, four air injection nozzles (one for each cylinder) connected to an air injection manifold, an air by-pass valve, and hoses from 1972-74. The air pump pushes air through the one way check valve into the injection manifold and through the nozzles. The check valve prevents any backfire of gases into the system. An air by-pass valve is used to prevent air injection during cold choke operation. Pulling the choke cable closes both the choke and the air by-pass valve, venting all pumped air into the air cleaner. This prevents overheating of the rich mixture exhaust. The air pump relief valve vents excess pressure to the atmosphere.

1975-76 trucks use a slightly modified version of the system. The principle and main components remain the same, but an air control valve, incorporating two relief valves, replaces the air by-pass valve. The no. 1 relief valve is activated by a control unit (an engine rpm switch) located on the kick panel beneath the parking brake lever; the No. 2 relief valve is activated by engine vacuum. Below 4,000 rpm (4,300 with automatic transmission), no. 1 directs air to the injection manifold. Above that engine speed, no. 1 opens and vents air to the air cleaner. Number 2 opens when intake manifold vacuum exceeds 6.3 in.Hg, reducing air flow to the injection manifold during low engine loads.

1977-78 1,796cc engines are the same, except that only trucks sold in California have the air control valve, with one relief valve activated by intake manifold vacuum. In addition, trucks sold in California have a vacuum delay valve.

The systems used in 1979 and later are slightly different. 1,970cc engines sold for 49 States use do not have an air pump. Instead, air injection is controlled by exhaust system back pressure. The air injection pipe runs from the air cleaner case to the exhaust manifold. A one-way check valve is installed in the pipe. During periods of negative exhaust pulsation, air is drawn from the air cleaner, through the check valve, and into the exhaust manifold. During periods of positive exhaust back pressure (positive pulsation), the check valve closes and no air is drawn into the exhaust system.

Models sold in California have an air pump, a check valve, and an air control valve. The air control valve has a no. 1 relief valve, activated by intake manifold vacuum, and a No. 2 relief valve, which modulates the amount of secondary air. The No. 2 relief valve is not used with automatic transmissions.

TESTING Air Pump Drive Belt



Be sure that the air pump drive belt is adjusted to the proper tension. See General Information And Maintenance .

Air Pump See Figure 2



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Fig. Fig. 2: An example of connecting an air pressure gauge to the air pump outlet in order to check outlet pressureif the reading is under 1 psi, replace the pump

  1. Disconnect the air pump outlet hose from the air by-pass valve.
  2.  
  3. Connect a T-fitting and pressure gauge into the outlet line. The other end of the fitting should have a plug with an 11 / 32 " hole drilled through it.
  4.  
  5. Start the engine and run it briefly at 1,500 rpm. Be sure the choke is fully open (pushed in).
  6.  
  7. If the pressure reading is below 1 psi, replace the pump.
  8.  

Air Pump Relief Valve See Figure 3



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Fig. Fig. 3: Example of how to check the air pump relief valve

  1. Operate the engine at idle.
  2.  
  3. Check the relief valve for airflow. No flow should be evident. If airflow out is noted, replace the relief valve and air pump.
  4.  
  5. Increase the engine speed to 3,000 rpm (4,500 rpm, 1978 and later). If air flows out of the relief valve, the valve is in good condition. If air does not flow from the relief valve, or if the valve is excessively noisy, the relief valve and air pump assembly should be replaced.
  6.  

Air Manifold Check Valve

Remove the check valve from the air injection manifold. Blow through the valve from the intake side of the outlet side. Air should pass through the valve from the intake side only. If air passes through the valve from the outlet side, replace the valve.

Air Bypass Valve

1972-74 VEHICLES

See Figure 4

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Fig. Fig. 4: Diagram of air bypass valve operation for 1972-74 trucks

  1. Disconnect the air line at the check valve.
  2.  
  3. Push the choke knob all the way in.
  4.  
  5. Run the engine at 1,500 rpm.
  6.  
  7. Hold your hand over the end of the air pump air line. Air should flow from the hose.
  8.  
  9. Pull the choke knob all the way out. No air should flow from the air line.
  10.  
  11. If the valve is not operating properly, replace the valve.
  12.  

Air Control Valve

1975-76 VEHICLES
  1. Disconnect the outlet hose from the bottom of the air control valve at the air cleaner. Start the engine and let it idle. Air should not be discharged from the outlet.
  2.  
  3. Unplug the air control valve solenoid electrical connector. With the engine still idling, check for air discharge from the valve outlet.
  4.  
  5. Reconnect the electrical connectors. Increase the engine speed above 4,000 rpm (4,300 with automatic transmission), and check that air is discharged from the valve outlet. When the engine speed falls, the air discharge should stop.
  6.  
  7. If the valve is not operating properly, check the hoses for leaks, breaks, kinks, or improper connections. Replace the valve as necessary.
  8.  

Control Unit

1975-76 VEHICLES

See Figures 5 and 6

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Fig. Fig. 5: The control unit is located on the kick panel, beneath the parking brake lever



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Fig. Fig. 6: Connecting a test light to the 1975-76 air injection control unit

  1. Attach a test light to the unit connector as shown.
  2.  
  3. Start the engine. The light should be on with the engine idling.
  4.  
  5. Increase the speed above 4,000 rpm (4,300 with automatic transmission). The light should go off.
  6.  
  7. If the light does not go off when specified, or does not remain on with the engine idling, replace the unit.
  8.  

Air Control Valve

1977-78 VEHICLES

This procedure is for all 1,796cc engines. Only California trucks have this valve.

  1. Disconnect the outlet hose at the bottom of the valve. Start the engine and let it idle.
  2.  
  3. Check that air is not discharged from the outlet.
  4.  
  5. Disconnect the small diameter hose from the top of the valve. Air should now be felt at the valve outlet.
  6.  
  7. If the valve is not operating properly, check the hoses for leaks, kinks, breaks, or improper connections, and inspect the check valve for proper operation. Replace the air control valve if necessary. Reconnect the hoses after the test.
  8.  

Air Control Valve

1980 AND LATER VEHICLES WITH MANUAL TRANSMISSION

This test is for all 1980 models sold in California with a manual transmission.

  1. Warm the engine to normal operating temperature, then shut it off.
  2.  
  3. Disconnect the air hose at the bottom of the air control valve. Start the engine and allow it to idle.
  4.  
  5. Disconnect the thin vacuum hose from the top of the air control valve. This is the hose which runs to the intake manifold. Air should be discharged from the air control valve outlet. Connect the hose again; the airflow from the outlet should stop.
  6.  
  7. Disconnect the vacuum sensing hose (which connects to the No. 2 relief valve on the side of the air control valve) at the T-fitting.
  8.  
  9. Obtain a T fitting and a length of vacuum hose. Disconnect the vacuum hose from the top of the air control valve (same hose as disconnected in Step 3). Connect this hose to the T-fitting and the hose fitting on the top of the air control valve from which the other length of hose (intake manifold vacuum hose) was just removed. You should now have a T-fitting with two hoses connected to it: one running from the top of the air control valve to the T-fitting, and the other running from the intake manifold to the T-fitting.
  10.  
  11. Connect the No. 2 relief valve vacuum hose (disconnect from its own T-fitting in Step 4) to the T-fitting. This will allow the No. 2 relief valve to be directly connected to intake manifold vacuum.
  12.  
  13. With the engine idling, air should be discharged from the outlet of the air control valve.
  14.  
  15. Disconnect and plug the vacuum hose from the No. 2 relief valve fitting (on the side of the air control valve). Airflow from the air control valve should stop.
  16.  
  17. If the air control valve does not operate correctly, replace it.
  18.  

Check Valve

1979 AND LATER VEHICLES49 STATES

This is a test for the check valve installed in the hose which runs from the air cleaner case to the exhaust manifold, on 1979 and later 49 State models without an air pump.

  1. Warm the engine to normal operating temperature, then shut it off.
  2.  
  3. Disconnect the air hose from the check valve.
  4.  
  5. Start the engine and allow it to idle.
  6.  
  7. Place a finger over the check valve inlet; vacuum should be felt. If no air is being drawn into the valve, replace the valve.
  8.  
  9. Run the engine up to about 1,500 rpm. Check for exhaust gas leakage at the check valve air inlet fitting. If leakage is present, replace the valve.
  10.  

ADJUSTMENTS Air Pump Drive Belt



Adjustment of the air pump drive belt is covered in General Information And Maintenance .

Air Bypass Valve

1972-74 VEHICLES

See Figure 7

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Fig. Fig. 7: View of the adjustment points on the air bypass valve for 1972-74 trucks

  1. Push the choke handle all the way in. Be sure that the choke plate is fully open.
  2.  
  3. Loosen the cable retaining screw in the valve plunger and the screw in the cable retaining bracket.
  4.  
  5. Be sure that the plunger is fully bottomed.
  6.  
  7. Insert the cable in the plunger and tighten the retaining screw.
  8.  
  9. Push down on the cable as much as possible without bending the control wire, then tighten the bracket screw.
  10.  
  11. Pull the choke knob all the way out. The valve plunger should be pulled to the top of the bracket.
  12.  

REMOVAL & INSTALLATION Air Pump



  1. Remove the battery, alternator and alternator drive belt.
  2.  
  3. Disconnect the inlet and outlet hoses from the air pump.
  4.  
  5. Remove the adjusting bar bolt and disengage the air pump drive belt.
  6.  
  7. Remove the air pump mounting bolt and remove the air pump.
  8.  

To install:
  1. Position the pump on the mounting bracket and loosely install the attaching and adjusting bolts.
  2.  
  3. Install the air pump drive belt.
  4.  
  5. Adjust the air pump belt tension and tighten the mounting and adjusting bolts.
  6.  
  7. Connect the inlet and outlet lines to the air pump.
  8.  
  9. Install the battery, alternator and alternator drive belt. Adjust the drive belt tension.
  10.  

Air Pump Check Valve

  1. Disconnect the air hose from the check valve.
  2.  
  3. Unscrew the check valve from the air manifold.
  4.  

To install:
  1. Screw the check valve into the air manifold. Tighten the check valve to 20 ft. lbs.
  2.  
  3. Connect the air hose to the check valve.
  4.  

Air Manifold

1,796cc AND 1,970cc ENGINES
  1. Remove the check valve.
  2.  
  3. Loosen the nuts securing the manifold to the cylinder head. Remove the nozzles from the manifold by removing the attaching nuts from the nozzles.
  4.  
  5. Unbolt and remove the manifold.
  6.  

To install:
  1. Install the nozzles in the manifold. Torque the nuts to 20 ft. lbs.
  2.  
  3. Install the manifold and torque the nuts to 15 ft. lbs.
  4.  
  5. Install the check valve.
  6.  

Air Injection Nozzle

1,796cc AND 1,970cc ENGINES
  1. Disconnect the air line from the check valve.
  2.  
  3. Remove the nozzles from the manifold by unscrewing the nuts.
  4.  
  5. Remove the heat stove from the exhaust manifold.
  6.  
  7. Remove the nozzles.
  8.  

To install:
  1. Install the nozzles. Torque the nuts to 20 ft. lbs.
  2.  
  3. Install the heat stove on the exhaust manifold.
  4.  
  5. Connect the air line to the manifold check valve.
  6.  

Air Bypass Valve

1972-74 VEHICLES
  1. Loosen the cable attaching screws at the valve plunger and cable retaining bracket.
  2.  
  3. Pull the cable out of the valve.
  4.  
  5. Unscrew and remove the valve from the mounting bracket.
  6.  

To install:
  1. Install the valve on the mounting bracket.
  2.  
  3. Push the choke knob all the way in.
  4.  
  5. Insert the end of the cable in the valve plunger and tighten the retaining screw.
  6.  
  7. Pull the cable to remove all slack between the plunger and cable bracket. Tighten the cable retaining screw at the bracket.
  8.  

 
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