PCM/ECM Operating Modes

PCM AND ECM OPERATING MODES
As input signals to the Engine Control Module (ECM) change, the ECM adjusts its response to output devices. There are several different modes of operation that determine how the ECM responds to the various input signals.

Ignition Switch ON (Engine OFF) Mode
When the ignition switch is in the "ON" position, the following actions occur:
1. When the PCM and ECM receive an ignition sense input, they power up. Battery voltage is supplied through the battery voltage input.
2. The wait-to start, water-in-fuel, check engine, check gauges, and transmission temperature (auto trans only) lamps are turned ON by the PCM, ECM or BCM for at least 2 seconds as a bulb test. This is done by direct wire or through the Bus to the smart cluster.
3. The ECM monitors the intake air temperature signal. If the intake air temperature is below a predetermined temperature, the ECM will start the air intake heater preheat cycle. The ECM will energize the intake heaters by grounding the air intake heater relays. The ECM will turn the wait-to-start lamp ON until the preheat cycle is over. If the ECM receives a crank signal before the preheat cycle is over, the preheat cycle will be aborted.
4. The ECM will request the water-in-fuel lamp ON if a signal is received from the water-in-fuel sensor.

NOTE: IF THE WATER-IN-FUEL LAMP REMAINS ILLUMINATED AFTER THE 2-SECOND BULB CHECK, THE VEHICLE SHOULD NOT BE STARTED UNTIL THE FUEL FILTER HAS BEEN DRAINED AND PRIMED.

Engine Start-up Mode
When the ignition key is held in the "start" position, the following occurs:
1. The ECM receives the crank signal input from the crank position sensor.
2. The air intake heaters are not energized during cranking.
3. The ECM monitors engine speed and when the engine speed is above 650 rpm, the ECM determines that the engine is running.
4. If the ECM receives a crank signal before the preheat cycle is over, the heaters will be deenergized. However, the post-heat cycle will begin if the engine is in the run state or speed is above 1200 rpm.

Engine Warm-up Mode
1. The ECM may start the air intake heater post-heat cycle depending on intake air temperature. If manifold air temperature was at 66 °F or less when the ignition was turned ON, the cycle is started.
2. If the transmission temperature sensor shows it is below 59 °F, the automatic transmission will not be allowed to enter overdrive.
3. If the transmission temperature sensor shows it is below 69 °F, the transmission's torque converter clutch will not be allowed to engage.
4. The ECM will request the water-in-fuel lamp ON if a signal is received from the water-in-fuel sensor.

Cruise or Idle Mode
During the cruising speed, the following inputs are received by the ECM:
1. The ECM monitors intake manifold air temperature through the intake air temperature sensor.
2. The intake air heater post-heat cycle will be complete, if it is not already over.
3. The vehicle speed, and throttle position sensors are used by the PCM to control the transmission overdrive and torque converter clutch operation.
4. If the transmission temperature sensor shows it is below 59 °F or above 275 °F, the automatic transmission will not be allowed to enter overdrive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmission temperature drops to 240 °F.
5. The water-in-fuel sensor signal is used by the ECM to request the water-in-fuel lamp ON, if water is sensed in the fuel.

Acceleration Mode
When there is an abrupt demand for increased engine output and vehicle acceleration, the following occurs:
1. The vehicle speed, and throttle position signals are used to control transmission overdrive and torque converter clutch operation.
2. If the transmission temperature sensor shows it is below 59 °F, the torque converter clutch will not be allowed to engage.
3. If the transmission temperature sensor shows it is below 59 °F or above 275 °F, the automatic transmission will not be allowed to enter overdrive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmission temperature drops to 240 °F.
4. If the speed control system resume/accelerate function is being used, the PCM will only allow the vehicle to accelerate at a predetermined rate. If a speed control has been set and the resume/ accelerate button is momentarily pushed in, the PCM will increase vehicle speed by two miles per hour.
5. The PCM will disable the speed control when:
- the brakes are applied
- excessive engine speed
- the clutch is disengaged (manual transmission)
- the gear selector is shifted to neutral (automatic transmission)
- the speed control OFF switch is activated

Deceleration Mode
During deceleration, the following inputs are received by the PCM:
1. The vehicle speed, crank position, and throttle position signals are used to control transmission overdrive and torque converter clutch operation.
2. The torque converter clutch will disengage, if the brakes are applied.
3. If the transmission temperature sensor shows it is below 59 °F or above 275 °F, the automatic transmission will not be allowed to enter overdrive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmission temperature drops to 240 °F.
4. The PCM will disable the speed control when:
- the brakes are applied
- excessive engine speed
- the clutch is disengaged (manual transmission)
- the gear selector is shifted to neutral (automatic transmission)
- the speed control OFF switch is activated

Ignition Switch OFF Mode
When the ignition switch is turned to the "OFF" position, the PCM and ECM still stores in memory trouble codes and the throttle position sensor value from the previous key-ON.