Engine Control Module - ECM

Note: The following positions may not be applicable to all engines:

ENGINE SPEED - Engine speed is computed by the ECM from the fuel control reference input. It should remain close to desired idle under various engine loads with engine idling.

DESIRED IDLE - The idle speed commanded by the ECM. The ECM will compensate for various engine loads to keep the engine at the desired idle speed.

ENGINE COOLANT TEMP - The Engine Coolant Temperature (ECT) sensor is mounted in the intake manifold and sends engine temperature information to the ECM. The ECM supplies 5 volts to the coolant temperature sensor circuit. The sensor is a thermistor which changes internal resistance as temperature changes. When the sensor is cold (internal resistance high), the ECM monitors a high signal voltage which it interprets as a cold engine. As the sensor warms (internal resistance decreases), the voltage signal will decrease and the ECM will interpret the lower voltage as a warm engine.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR - The MAP sensor produces a low signal voltage when manifold pressure is low (high vacuum) and a high voltage when the pressure is high (low vacuum). With the ignition "ON," and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude and is referred to as BARO. Comparison of this BARO reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Readings should be the same +/- .4 volt.

THROTTLE POSITION - Used by the ECM to determine the amount of throttle demanded by the driver. Should read .45-.85 volt at idle to above 4 volts at Wide Open Throttle (WOT).

THROTTLE ANGLE - Computed by the ECM from TP signal voltage (Throttle position) should read 0% at idle, 100% at Wide Open Throttle (WOT).

O2S - Represents the exhaust oxygen sensor output voltage. Should fluctuate constantly within a range between 100 mV (lean exhaust) and 1000 mV (rich exhaust) when operating in "Closed Loop."

RICH/LEAN STATUS - Indicates whether exhaust oxygen sensor voltage is above (rich) or below (lean) the 450 mV oxygen sensor threshold voltage. Should change constantly indicating that the ECM is controlling the air/fuel mixture properly.

O2S CROSS COUNTS - The number of times the oxygen sensor voltage crosses over the rich/lean threshold during a one second interval.

LOOP STATUS - This position will indicate whether the engine control system is operating in "Open" or "Closed Loop." Most systems go "Closed Loop" after a certain amount of run time, when coolant temperature is high enough, and the oxygen sensor becomes active.

KNOCK SIGNAL - Tech 1 Scan Tool Displays "YES" OR "NO" - Indicates whether or not a knock signal is being detected by the ECM.

SHORT TERM FUEL TRIM (Formerly Integrator) - Represents a short-term correction to fuel delivery by the ECM in response to the amount of time the oxygen sensor voltage spends above or below the 450 mV threshold. If the oxygen sensor voltage has mainly been below 450 mV, indicating a lean air/fuel mixture, short-term fuel trim will increase to tell the ECM to add fuel. If the oxygen sensor voltage stays mainly above the threshold, the ECM will reduce fuel delivery to compensate for the indicated rich condition.

LONG TERM FUEL TRIM - Is derived from the short-term fuel trim value and is used for long term correction of fuel delivery. A value of 128 counts indicates that fuel delivery requires no compensation to maintain a 14.7:1 air/fuel ratio. A value below 128 counts means that the fuel system is too rich and fuel delivery is being reduced (decreased injector pulse width). A value above 128 counts indicates that a lean condition exists and the ECM is compensating by adding fuel (increased injector pulse width).

IDLE AIR CONTROL (IAC) - The system is used to control engine idle speed to the desired RPM for different operating conditions. In this mode, the numbers will indicate the position the ECM thinks the valve is in. The ECM moves the IAC in counts and these counts are displayed on a Tech I scan tool.

A/C CLUTCH - Displays 'ON" when the ECM has commanded the A/C clutch "ON."

MPH KM/H - Vehicle speed is an ECM internal parameter. It is computed by timing pulses coming from the Vehicle Speed Sensor (VSS). Vehicle speed is used in checking TCC lock-up speed or speedometer accuracy. Speed is displayed in both miles per hour (mph) or kilometer per hour (km/h).

SHIFT LIGHT - The shift light is used in manual transmissions to indicate the optimum point for upshifting. The ECM uses engine speed vehicle speed, and engine load to determine the state of the shift light.

SYSTEM VOLTS - Battery/ignition voltage is an analog input signal read by the ECM. It is the ignition switched battery voltage and is mainly used for diagnostics. Certain ECM functions will be modified if the battery voltage falls below or rises above programmed thresholds.

FUEL PUMP VOLTS - This parameter is a reading of the voltage found at the fuel pump. It is used by the ECM as the system voltage. The ECM uses fuel pump volts as a reference to determine electrical loading.

CALIBRATION ID - The PROM (CAL-PAK) identification parameter describes the particular PROM (CAL-PAK) used in the ECM being tested. The PROM (CAL-PAK) contains the ECM program. PROM ID is used when it is necessary to replace the PROM (CAL-PAK). PROM ID must be specified when ordering new PROM(s). PROM ID should not be confused with "Part Number."

AIR CONTROL SOL - When the Air Injector Reaction Divert Solenoid (Air Control) is "ON" (LOW, NORMAL, PORT), air from the AIR pump is sent to the AIR switch When it is "OFF" (HIGH, DIVER, ATMOSPHERE), the air is vented to the atmosphere or air cleaner.