Subaru Select Monitor Parameters for the EJ22

The SSM parameters are noted as being INFO, INPUT, or CONTROL type:
  • INPUT parameters are measured values detected by the various sensors connected to the ECU. The primary INPUT parameters are Engine Speed, Airflow Signal, and Load (a special case input parameter - calculated by the ECU). The other inputs can be used by the ECU to fine tune the control parameters, or in some cases cause the ECU to run in special modes.
  • CONTROL parameters are engine control values. These are found by the ECU via calculation or lookup tables in the ECU's ROM. The primary CONTROL parameters are fuel Injector pulse width and Ignition timing.
  • The typical value ranges given are for warm idle (except where noted) for a 1990-1994 EJ22, as specified by the 1992 Subaru Service Manual.
  • For parameter values under driving conditions, for EJ22 engines transplanted into Volkswagen Vanagon/Westfalias, see here.
To read more about interactions between ECU parameters, see here.

Continuous Parameters

ROM ID   - - the revision number of the ROM chip inside the ECU
  • the ID of the internal firmware version in the ECU's ROM varies between years, models, and market where the car was delivered
  • the locations in the ECU's memory of the monitored values are different for each different ROM; read why this matters here
  • System voltage   V 13-14 the voltage as measured at the ECU
  • this can be different than the voltage measured at the battery, depending upon the state of the grounding straps, connections, and wires between the battery and the ECU
  • the ECU makes corrections to its calculations for those that involve voltage-sensitive sensors
  • the ECU adjusts the fuel injector duration for an inherent time lag that is also voltage sensitive
  • Vehicle speed INPUT kph - speed of the vehicle as measured by the VSS
  • this is the speed measured by the vehicle speed sensor (VSS), not the speed shown on the speedometer which uses a mechanical device to measure speed
  • the ECU uses the vehicle speed for controlling the canister purge and radiator fan, and holding the idle higher than normal to prevent stalling at speed; some say it affects fuel trim also
  • the engine is able to run very well without the VSS
  • Engine speed INPUT rev/min - RPM as measured by the cam and crank angle sensors
  • this is one of the primary values used as input parameters in the internal lookup tables kept in the ECU's ROM
  • this is measured differently from the RPM shown on the tachometer
  • more on the cam and crank angle sensors here
  • Coolant temperature INPUT deg C 80-95 temperature of coolant in the heads
  • the temperature indicated by the gauge on the dashboard is measured from a completely different sensor
  • incomplete bleeding of the system can cause the two sensors to read wildly different - one from coolant and the other from trapped air
  • coolant temp is used by the ECU to decide when to override 'normal' operation in favor of a mode which works more effectively when the engine is cold, and to make adjustments to calculations for temperature sensitive sensors
  • Ignition timing CONTROL deg BTDC 12-28 (auto)
    18-22 (manual)
    9-21 (turbo)
    degrees BTDC that the spark is fired
  • this is one of the primary engine control parameters (outputs of the ECU)
  • in addition to basic timing, timing is altered when knocking is detected, and when angular velocity of the crankshaft fluctuates (engine is jerking)
    ([*1]: warm idle, gear in neutral)
  • Airflow signal INPUT V 0.8-1.2 amount of air entering the engine
  • the EJ22 uses a hot-wire MAF which measures the mass of the air entering the engine in a certain time
  • this varies with barometric pressure and ambient temperature (changes in pressure and temperature result in changes in air density, and thus a greater or lesser mass of air, to enter each piston stroke)
  • more on the MAF here
  • Load INPUT air mass/rev 15-20 (non-turbo)
    30-50 (turbo)
    how hard the engine is working - mass of air consumed per engine revolution
  • this is one of the primary values used as input parameters in the internal lookup tables kept in the ECU's ROM
  • the load is calculated by the ECU rather than read from a sensor
  • this is calculated from the MAF sensor (mass/time) and the RPM cam/crank sensor (rev/time):
    mass/time / rev/time = mass/rev
  • if load value is out of range, the MAF is usually at fault; can also potentially be the RPM sensors (cam/crank)
  • more on the load calculation here
  • Throttle position signal INPUT V 4.4-4.8 closed
    (0.7-1.6 open)
    how far the throttle is open
  • this is measured as the voltage drop across a potentiometer (variable resistor) which changes resistance as the throttle body opens
  • the ECU uses this partially to determine how hard the driver is attempting to push the engine (regardless of the current load)
  • there is another separate binary throttle signal from a switch on the throttle body which opens/closes just at the very closed (throttle off) end of gas pedal travel (see "Idle switch")
  • Zeroed throttle position ? ? ? ?
  • only on 90-91 manual and 91-94 turbo models
  • Injector pulse width CONTROL ms 3.0-3.7 (non-turbo)
    2.0-3.5 (turbo)
    time in msec that each fuel injector stays open
  • the amount of fuel injected into the cylinder
  • this is one of the primary engine control parameters (outputs of the ECU)
  • note that there is only a very rough reading on 90-91 models; later ECUs have much finer granularity
  • IAC valve duty cycle CONTROL % duty cycle 25-40 (non-turbo)
    30-45 (turbo)
    how much air is allowed to bypass the throttle for idling purposes
  • the IAC valve is a solenoid valve that is continuously opened and closed at a variable rate to effectively vary the amount of air that is allowed through the valve
  • the duty cycle is measured as a percent of the time that the solenoid is open; higher duty ratio means valve is open longer resulting in higher idle speed
  • this is non-zero even when not idling; however the amount of air the IAC allows into the engine becomes insignificant compared to the air entering the engine via the open throttle
  • compensates for idle speed decreases due to: a/c operation & electrical loads; prevents rich air/fuel during deceleration; mechanically compensates for engine temperature
  • note that on some EJ22s, there is a air bypass screw on the intake body which is used to adjust the idle air only enough so that the IAC duty cycle remains in range
    ([*2]: warm idle, A/C off, radiator fan off, battery>13V, sea level)
  • Oxygen sensor signal INPUT V 0-1 whether exhaust gasses indicate that the fuel/air mixture is currently burning lean or rich
  • lower values (towards 0) indicate a lean mixture, higher (towards 1) indicate rich
  • this oscillates rapidly during closed-loop operation; as the fuel/air mixture is changed to keep this centered, this varies rapidly between near 0 to near 1
  • Fuel trim CONTROL % -1.6 to 1.6 +/- incremental change added to the injector pulse width to compensate for the lean/rich condition shown by oxygen sensor
  • this is the incremental fuel added/subtracted to the main fuel/air mixture to keep the ratio correct
  • if OXS indicates lean, the fuel trim correction is positive (+)
  • the fuel trim is changed rapidly to keep the OXS signal centered
  • more on air/fuel trim here
  • Timing correction CONTROL deg -10 to 10 +/- the degrees of timing advance added to timing to correct for knock
  • this is changed to non-zero when pinging is detected by the knock sensor
  • this is used to temporarily advance (often only for several seconds) the timing to counteract pinging
  • Boost control duty cycle ? ? ? ?
  • turbo models only
  • Barometric pressure INPUT mm Hg - atmospheric pressure as measured from a sensor inside the ECU
    Boost/vacuum ? ? ? ?
  • turbo models only

    Binary Parameters

    (See this note on decoding binary parameters using Evoscan and other monitoring software.)

    Ignition switch   ON if ignition key switch is on
    A/T mode ? ON if ECU was shipped on an auto transmission model (and is intended to be paired with sensors shipped with those models
  • some engine sensors vary between MT and AT models
  • Test mode   ON when the EJ22s green 'test mode' connectors are connected
  • in this mode, the ECU flashes the 'check engine' light to indicate any fault codes that are currently being monitored
  • Read memory mode   ON when the EJ22s black fault code connectors are connected
  • in this mode, the ECU flashes the 'check engine' light to indicate any fault codes that are stored in memory
  • Neutral switch INPUT ON if transmission is in neutral
  • in most transplanted EJ22s, the original switch is missing and can be mocked up to avoid the neutral switch trouble code
  • ECU needs to detect an occasional transition from OFF to ON to avoid throwing the associated trouble code
  • Park switch INPUT ON if automatic transmission is in park
  • in most transplanted EJ22s, the original switch is missing and can be mocked up to avoid the park switch trouble code
  • ECU needs to detect an occasional transition from OFF to ON to avoid throwing the associated trouble code
  • California mode ? ON if ECU delivered in a california emissions equipped auto
    Idle switch INPUT ON if throttle is at completely closed end of travel
  • note this binary (ON/OFF) input is different than the continuously variable throttle position signal
  • indicates to the ECU that the gas pedal is fully released, indicating full deceleration or idle
  • some generic auto idle references indicate that the ECU input controls to the IAC may not become active unless this switch is triggered
  • Air conditioning switch   ON when air conditioning system is switched on (but not necessarily active)
    Air conditioning relay active INPUT ON when the air conditioning compressor clutch is currently engaged
  • the ECU adjusts for the increased load on the engine from the A/C
  • Radiator fan relay active ? ON when the radiator fan is active (?low or high speed?)
    Fuel pump relay active ? ON when the fuel pump is activated
    Canister purge valve active ? ON when the canister purge valve is activated
    Pinging detected INPUT ON when the knock sensor detects pinging in the engine
  • this detection is done by a piezoelectric sensor attached to the engine block, which detects the higher frequencies of vibrations sent out when the engine knocks
  • the ECU changes the Timing Correction in response to ping detection
  • may stay ON for a short duration after pinging stops
  • Pressure exchange ? Atmospheric pressure is being measured (solenoid is ON)
  • turbo models only
  • O2 Monitor ? Air/Fuel ratio is rich
  • this value is not read by most aftermarket monitoring programs, but appears in the SSM parameter list
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