SECONDARY AIR INJECTION CONTROL SYSTEM

With the secondary air injection control system, air can be injected into either the exhaust port or the catalytic converter to help in reducing harmful emissions. Injecting air into the exhaust port helps in burning hydrocarbons (raw unburnt fuel) that have not been burnt in the combustion chambers. It also brings extra air into the catalytic converters for better oxidation.

This above shows the air control valve (ACV) which controls air injection. The relief valve, switching valve, and anti-afterburn valve (AAV) are vacuum operated via a diaphragm. When vacuum is applied to the relief valve, the diaphragm moves to the left and the path from air pump to the switching valve and AAV is open. When vacuum is applied to the switching valve, the diaphragm moves up and the path to port air is open. When vacuum is applied to the AAV, the diaphragm moves down and the air path to the intake manifold is open. The vacuum applied to the relief valve is controlled by the relief solenoid valve. The vacuum applied to the switching valve is controlled by the switching solenoid valve. The vacuum applied to the AAV comes directly from the intake manifold. Check valves are used in the port air tract and in the split air tract to make any air flow unidirectional, that is, from the ACV to the exhaust port or the main converter. The port air solenoid valve and split air solenoid valve are standalone valves, that is, they are controlled by the ECU directly via voltage. These two valves will not be disccused any further here since their importance seems limited. The AAV will not be discussed any further here since it is part of the deceleration system (see the deceleration control system).

When the relief valve has no vacuum, the air coming from the air pump is not used as it goes through the air chamber and air silencer. When the relief valve has vacuum, the air coming from the air pump is directed to:

either the port air tract (if switching valve has vacuum) or the split air tract (if switching valve has no vacuum)
the intake tract if the anti afterburn valve has vacuum

This above shows the location of the relief solenoid valve and the switching solenoid valve. The relief solenoid valve belongs to the "left" pair of solenoid valves. They do not conduct vacuum at rest, they conduct vacuum only when energized. The switching solenoid valve belongs to the "right" pair of solenoid valves. They conduct vacuum at rest, they do not conduct vacuum when energized.

This above shows the electrical connections for the relief solenoid valve and the switching solenoid valve. For the relief solenoid valve, the B/W (black with white stripe) wire connects to the main relay and the L/Y (blue with yellow stripe) connects to pin 2P of the engine control unit (ECU). For the switching solenoid valve, the B/W (black with white stripe) wire connects to the main relay and the L/R (blue with red stripe) connects to pin 2O of the ECU. The voltage at the main relay is 12V when the engine is running. The status of either valve (energized or not) depends upon the voltage reading at the ECU pin.

PRIMITIVE TESTING PROCEDURES FOR RELIEF AND SWITCHING SOLENOID VALVES

When sitting in car with transmission in neutral, the voltage at pin 2P of the ECU (relief solenoid valve input) is:

under 2 volts from idle to about 3,500 rpm (this means the valve is energized and therefore conducts vacuum)
12 volts after 3,500 rpm (this means the valve is not energized and therefore does not conduct vacuum)

When sitting in car with transmission in neutral, the voltage at pin 2O of the ECU (switching solenoid valve input) is:

12 volts from idle to about 3,500 rpm (this means the valve is not energized and therefore conducts vacuum)
under 2 volts after 3,500 rpm (this means the valve is energized and therefore does not conduct vacuum)

This means that, under no load, air is pumped:

into the port air tract from idle to 3,500 rpm
into the air chamber and silencer (i.e., not used) past 3,500 rpm

It also means that, under no load, no air is pumped into the split air tract, that is, no pumped air goes to the main converter directly.

Before going any further, it is of interest to look at the procedure described in the factory service manual (FSM) to adjust the throttle position sensor (TPS), since it relates to the relief and switching solenoid valves.

The "checker lamp" is actually a pair of 12V led lamps whose positive leads have been tied together to form a single common positive lead. The check connector (EM-18) can be seen schematically on the electrical diagram above. This connector picks up the voltage from main relay (to connect to common lead of the checker lamps), the voltage from the relief solenoid valve (to connect to other lead of one checker lamp), and the voltage from the switching solenoid valve (to connect to other lead of other checker lamp). The goal of the checking procedure is to make sure that the relief solenoid valve is energized while the switching solenoid valve is not.

FSM TESTING PROCEDURES FOR RELIEF AND SWITCHING SOLENOID VALVES

This above shows the FSM test to check for proper operation of the relied solenoid valve. Step 8 has already been covered above. Steps 5 and 6 are not very clear in terms of what they are supposed to check. The key aspect of those two steps is that they require the operator to unplug the TPS but it is not indicated what the ECU thinks is happening when the TPS is disconnected. According to 86-88 ECU error codes, disconnecting the TPS on a 86-88 model makes the ECU defaults to a 100% throttle position, that is, wide open throttle (WOT). So, when the FSM says to disconnect the TPS, it is, for the purpose of a better understanding, similar to saying that WOT is applied, in other words, the ECU thinks the engine is under heavy load. If the engine is turning at over 1,500 rpm and WOT happens, the ECU puts vacuum on the relief valve for 120s and then stops. Then, if engine speed is brought down to idle, the ECU puts vacuum on the relief valve for 8s. It is still difficult to interpret these tests. It would seem that, under heavy load, pumped air is used for a limited amount of time only (before being relieved).

This above shows the FSM test to check for proper operation of the switching solenoid valve. When WOT happens, the supply of vacuum to the switching valve is cut off past about 1,100 rpm, meaning, any pumped air (if any) goes into the split air tract (instead of port air tract).

At this point, it is of interest to have a look at the FSM for the mazda rx-7 89-91 in search of better explainations for under which conditions the pumped air is used or relieved and, when it is used, whether it goes to port air or split air.

This above shows the on/off status for the relief solenoid valve and the switching solenoid valve on the mazda rx-7 89-91. It should be well noted that the behavior for the mazda rx-7 86-88 may be different but it is likely to be similar. The pumped air is injected at idle, under medium load (when warm), and upon deceleration. The pumped air is relieved under medium load (when cold) and under acceleration or heavy load. This makes quite a bit of sense. When accelerating or under heavy load (high speed cruising), there is enough intake air for proper operation of the converter(s), so pumped air can be relieved. Upon deceleration or at idle, air injection is needed since intake air supply is limited (air is also needed to feed the AAV). Unrelieved pumped air is injected into the split air tract only under medium load (when warm), that is, moderate speed cruising.