Engine Controls - Theory & Operation: Other — Engine Controls - Theory & Operation — Theory & Operation

COMPUTERIZED ENGINE CONTROLS

Computerized engine control system monitors and controls engine operation. Input sensors supply information to the Powertrain Control Module (PCM). PCM processes information from input devices and sends output voltage signals to various control devices. See INPUT DEVICES and OUTPUT SIGNALS . Information received from input devices is processed by PCM to calculate proper fuel delivery, ignition timing, fuel evaporative purge, exhaust gas recirculation and other emission control systems.

POWERTRAIN CONTROL MODULE

Note. Powertrain Control Module (PCM) may also be referred to as Electronic Control Module (ECM) or Vehicle Control Module (VCM). PCM is located either in front of shifter below center console, or on fenderwell in engine compartment.

PCM analyzes all electrical data signals from input devices to control fuel injection, ignition and emissions. PCM includes a backup fail-safe control system. If a malfunction should occur in any PCM controlled systems, in most cases PCM will substitute default settings to maintain necessary engine functions and permit operation of vehicle.

Electronically Erasable Programmable Read Only Memory (EEPROM) is factory-programmed engine calibration data, which modifies PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. It can be reprogrammed using Tech II scan tool. EEPROM is permanently soldered into PCM and is cannot replaced separately. If battery voltage is removed, EEPROM information is retained.

INPUT DEVICES

Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the ECM. The second category covers OUTPUT SIGNALS, which are components controlled by the PCM.

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage on specific model, see WIRING DIAGRAMS article. The available input signals include the following

58X Reference PCM Input

The Powertrain Control Module (PCM) uses 58X signal from Crankshaft Position (CKP) sensor to calculate engine RPM and crankshaft position at all engine speeds. PCM also uses pulses on this circuit to initiate injector pulses. If PCM does not receive pulses on this circuit, DTC P0337 will set. Engine will not start and run without 58X reference signal.

A/C Request - A/C On

A/C compressor provides PCM with a simple ON (A/C Request) signal. PCM makes calculations for extra load, fuel enrichment and idle, before enabling A/C compressor. A/C will not operate if A/C ON signal is not sent to PCM.

Bank 1 & Bank 2 Heated Oxygen Sensors No. 1

Bank 1 and Bank 2 Heated Oxygen Sensors No.1 (H02S-1) are mounted in the exhaust stream before catalytic convertors where they can monitor oxygen content of exhaust gas. Oxygen present in exhaust gas reacts with the HO2S to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV to 900 mV. HO2S voltage can be monitored with a scan tool. By monitoring voltage output of the oxygen sensor, PCM calculates pulse width command for fuel injectors to produce proper combustion chamber mixture. Low H02S voltage is a lean mixture which will result in a rich command to compensate. High H02S voltage is a rich mixture which will result in a lean command to compensate. An open Bank 1 H02S 1 signal circuit will set a DTC P0134 and scan tool will display a constant voltage between 400-500 mV. A constant voltage below 300 mV in sensor circuit (circuit grounded) will set DTC P0131. A constant voltage above 800 mV in the circuit will set DTC P0132. Faults in the Bank 2 H02S 1 signal circuit will cause DTC P0154 (open circuit), DTC P0151 (grounded circuit), or DTC P0152 (signal voltage high) to set. A fault in the Bank 1 H02S 1 heater circuit will cause DTC P0135 to set. A fault in the Bank 2 H02S 1 heater circuit will cause DTC P0155 to set. The PCM can also detect H02S response problems. If the response time of an H02S is determined to be too slow, the PCM will store a DTC that indicates degraded H02S performance.

Bank 1 & Bank 2 Heated Oxygen Sensors No. 2

Three-way catalytic converters are used to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx). Catalyst within the converters promotes a chemical reaction. This reaction oxidizes HC and CO present in the exhaust gas and converts them into harmless water vapor and carbon dioxide. Catalyst also reduces NOx by converting it to nitrogen. PCM monitors this process using Bank 1 and Bank 2 Heated Oxygen Sensors No. 2 (HO2S-2). HO2S-2 sensors are located in exhaust system after catalytic convertors. Bank 1 and Bank 2 H02S No. 1 sensors produce an output signal which indicates amount of oxygen present in exhaust gas entering three-way catalytic converter. Bank 1 and the 2 H02S No. 2 produce an output signal which indicates oxygen storage capacity of catalyst. This signal indicates catalyst's ability to efficiently convert exhaust gases. If catalyst is operating efficiently, Bank 1 and Bank 2 H02S No. 1 signals will be more active than signals produced by Bank 1 and Bank 2 H02S No. 2 sensors. The catalyst monitor sensors operate same as Bank 1 and Bank 2 H02S No. 1. Bank 1 and Bank 2 H02S 2 sensors main function is catalyst monitoring, but they also have a limited role in fuel control. If a sensor output indicates a voltage either above or below 450 mV bias voltage for an extended period of time, PCM will make a slight adjustment to fuel trim to ensure that fuel delivery is correct for catalyst monitoring. A problem with the Bank 1 H02S No. 2 signal circuit will set DTC P0137, P0138, or P0140, depending on specific condition. A problem with Bank 2 H02S No. 2 signal circuit will set DTC P0157, P0158, or P0160, depending on specific condition. A fault in the heated oxygen sensor heater element or its ignition feed or ground will result in lower sensor response. This may cause incorrect catalyst monitor diagnostic results.

Battery Voltage

Battery voltage is monitored by PCM. PCM must have 12 volts to operate.

Camshaft Position Sensor

Camshaft Position (CMP) sensor is located in timing cover or engine block. CMP sensor continuously sends PCM a signal that is used as a "sync pulse" to trigger injectors in proper sequence. CMP sensor signal indicates to PCM position during power stroke of cylinder No. 1. PCM is then able to calculate true sequential fuel injection operation. If signal is lost, PCM reverts back to calculated sequential mode based on the last fuel injection pulse received.

Crankshaft Position Sensor

Crankshaft Position (CKP) sensor is located in engine block. CKP sensor provides a signal used by PCM to calculate ignition sequence. CKP sensor initiates 58X reference pulses which PCM uses to calculate RPM and crankshaft position.

EGR Pintle Position Sensor

This EGR Pintle Position (EGR-PS) sensor is mounted inside linear EGR valve, and informs PCM of EGR valve position. PCM uses this information to control EGR flow. PCM uses information from ECT sensor, TP sensor and MAF sensor to control EGR pintle position.

Engine Coolant Temperature Sensor

Engine Coolant Temperature (ECT) sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant temperature produces a high resistance of 100,000 ohms at -40°C (-40°F). High temperature causes a low resistance of 70 ohms at 130°C (266°F). PCM supplies a 5-volt reference signal to ECT sensor through resistors in PCM and measures voltage. ECT signal voltage will be high when the engine is cold and low when engine is hot. By measuring ECT voltage, PCM calculates engine coolant temperature. Engine coolant temperature affects most systems that PCM controls. Scan tool displays engine coolant temperature in degrees. After engine start-up, temperature should rise steadily to about 85°C (185°F). It should then stabilize when thermostat opens. If engine has not been run for several hours (overnight), engine coolant temperature and intake air temperature displays should be close to each other. A hard fault in ECT sensor circuit will set DTC P0117 or DTC P0118. An intermittent fault will set DTC P1114 or P1115.

Fuel Vapor Pressure Sensor

FVP sensor is a three-wire strain gauge sensor similar to a common MAP sensor. However, the FVP sensor has very different electrical characteristics due to its pressure differential design. FVP sensor measures the difference between air pressure (or vacuum) in the fuel tank and outside air pressure. FVP sensor mounts at top of the fuel pump assembly. A three-wire electrical harness connects it to the PCM. PCM supplies a 5-volt reference voltage and ground to the sensor. FVP sensor will return a voltage between 0.1 and 4.9 volts. When air pressure in fuel tank is equal to outside air pressure, such as when fuel cap is removed, output voltage of the FVP sensor will be 1.3 to 1.7 volts. When air pressure in the fuel tank is 4.5 in. H 2 O (1.25 kPa), FVP sensor output voltage will be 0.3- 0.7 volts. When there is neither vacuum nor pressure in the fuel tank, FVP sensor voltage should be 1.5 volts. When air pressure in fuel tank is -14 in. H 2 O (-3.75 kPa), FVP sensor voltage should be 4.3-4.7 volts.

Fuel Level Sensor

Fuel level sensor is an important input to the PCM for enhanced EVAP system diagnostic. PCM needs fuel level information to know volume of fuel in fuel tank. Fuel level affects rate of change of air pressure in EVAP system. Several enhanced EVAP system diagnostic sub-tests are dependent upon correct fuel level information. Fuel level diagnostic will not run when the tank is less than 15 percent or more than 85 percent full. Diagnose any Fuel Level Sensor DTCs first, as they can cause other DTCs to set.

Intake Air Temperature Sensor

Intake Air Temperature (IAT) sensor is a thermistor which changes its resistance based on temperature of air entering engine. Low air temperatures produces a high resistance of 100,000 ohms at -40°F (-40°C). High air temperatures cause low resistance of 70 ohms at 266°F (130°C). PCM supplies a 5-volt reference signal to IAT sensor through a resistor in the PCM and monitors signal voltage. Voltage will be high when incoming air is cold. Voltage will be low when incoming air is hot. By measuring IAT voltage, PCM calculates incoming air temperature. IAT sensor signal is used to adjust spark timing according to incoming air density. Scan tool displays temperature of air entering engine. Temperature should read close to ambient air temperature when engine is cold and rise as underhood temperature increases. If engine has not been run for several hours (overnight), IAT sensor temperature and engine coolant temperature should read close to each other. A fault in IAT sensor circuit will set DTC P0112 or DTC P0113.

Knock Sensor

Piezo electric Knock Sensor (KS) is located in engine block. If detonation occurs, sensor generates a signal to PCM. PCM uses this signal to determine when to retard ignition timing.

Manifold Absolute Pressure Sensor

Manifold Absolute Pressure (MAP) sensor responds to changes in intake manifold pressure (vacuum). MAP sensor signal voltage to the PCM varies from below 2 volts at idle (high vacuum) to above 4 volts with ignition on, engine not running or at wide-open throttle (low vacuum). MAP sensor is used to determine following

Manifold pressure changes while linear EGR flow test diagnostic is being run.
Barometric pressure (BARO).

If PCM detects a voltage that is lower than the possible range of the MAP sensor, DTC P0107 will be set. A signal voltage higher than the possible range of the sensor will set DTC P0108. An intermittent low or high voltage will set DTC P1107. PCM can detect a shifted MAP sensor. PCM compares MAP sensor signal to a calculated MAP based on throttle position and various engine load factors. If PCM detects a MAP signal that varies excessively above or below calculated value, DTC P0106 will set.

Mass Airflow Sensor

The Mass Air Flow (MAF) sensor measures difference between volume and quantity of air that enters engine. Volume means size of space to be filled. Quantity means number of air molecules that will fit into the space. MAF information is important to the PCM because heavier, denser air will hold more fuel than lighter, thinner air. PCM adjusts air/fuel ratio as needed depending on MAF value. Scan tool reads MAF value and displays it in terms of grams per second (gm/s). At idle, scan tool should read between 4-7 gm/s on a fully warmed up engine. Values should change quickly on acceleration. Values should remain stable at any given RPM. A failure in the MAF sensor or circuit will set DTC P0101, DTC P0102, or DTC P0103.

Park/Neutral Switch - A/T (Mode Switch) & Inhibitor Switch - M/T

Note. The vehicle should not be driven with the transmission range switch disconnected, as idle quality will be affected. Park/Neutral switch can also be referred to as Transmission Range Switch (TRS) or Mode switch.

Transmission Range Switch (TRS) provides one of four inputs to the PCM. Position is selected by transmission selector lever. This information is used for starting, ignition timing, EVAP canister purge or EGR operation.

Power Steering Pressure Switch

When power steering pressure is high, Power Steering Pressure (PSP) switch sends a signal to PCM to increase idle speed. PCM will also turn off A/C clutch when high power steering pressure is detected.

Throttle Position Sensor

Throttle Position (TP) sensor is a potentiometer connected to the throttle shaft on the throttle body. PCM monitors voltage on TP sensor signal line and calculates throttle position. As throttle valve angle is changed (accelerator pedal moved), TP sensor signal also changes. At closed throttle position, output of the TP sensor is low. As the throttle valve opens, output increases so that at wide open throttle, output voltage should be above 92% (scan tool display). PCM calculates fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from an injector and unstable idle because PCM thinks throttle is moving. V6 engines are equipped with 2 TP sensors, while 4 cylinder models have one sensor.

Vehicle Speed Sensor

Vehicle Speed Sensor (VSS) is located at the rear of transfer case or transmission, and informs PCM of vehicle road speed. PCM determines speed of vehicle by converting a pulsing voltage signal from the VSS into miles per hour. PCM uses this signal to operate cruise control, speedometer, Torque Convertor Clutch (TCC) and transmission shift solenoids.

OUTPUT SIGNALS

Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation on each output component, refer to the system indicated after component.

CHECK ENGINE Light or Malfunction Indicator Light

See MALFUNCTION INDICATOR LIGHT under SELF-DIAGNOSTIC SYSTEM.

Cooling Fan Relays

See COOLING FAN RELAY testing procedure in appropriate SELF-DIAGNOSTICS article.

Distributorless Ignition System

See DISTRIBUTORLESS IGNITION SYSTEM under IGNITION SYSTEM.

Evaporative Canister Purge Control

See EVAPORATIVE EMISSION SYSTEM under EMISSION SYSTEMS.

Exhaust Gas Recirculation Valve

See EXHAUST GAS RECIRCULATION SYSTEM under EMISSION SYSTEMS.

Electronic Spark Advance

See IGNITION TIMING CONTROL SYSTEMS under IGNITION SYSTEM.