Engine Controls - Theory & Operation - Gasoline Dodge Caravan II

INTRODUCTION

This article covers the basic description and operation of engine performance related systems and components. Read this article before working on unfamiliar systems.

SINGLE BOARD ENGINE CONTROLLER (SBEC)

SBEC is a digital computer that controls ignition timing, air/fuel ratio, emission control devices, cooling fan, charging system, idle speed, cruise control (if equipped), fuel pump and tachometer. The control unit is located in the engine compartment. SBEC uses data from various input sources to control output devices in order to achieve optimum engine performance for all operating conditions.

SBEC has voltage converters that convert battery voltage to regulated 5-volt and 8-volt outputs. The 8-volt output powers the distributor pick-up and crankshaft (angle/position) sensor. The 5-volt output powers coolant temperature sensor, charge temperature sensor, manifold absolute pressure sensor and throttle position sensor.

Note. Basically, components are grouped into 2 categories. The first category, INPUT DEVICES, includes components that control or produce voltage signals monitored by the SBEC. The second category, OUTPUT SIGNALS, includes components controlled by the SBEC (this is accomplished by the SBEC grounding individual circuits).

INPUT DEVICES

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage on a specific model, see appropriate WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section. Available input signals include

A/C Switch

Switch signals SBEC that A/C has been selected. SBEC then activates A/C compressor clutch relay and maintains idle speed at a scheduled RPM. This is done through control of Idle Speed Control (ISC) actuator or Automatic Idle Speed (AIS) motor.

Battery Voltage

SBEC monitors battery voltage to determine fuel injector pulse width and alternator field control.

Brake Switch

SBEC uses this input to maintain idle speed at a scheduled RPM when brakes are applied. If SBEC receives an input signal from brake switch when speed control system is on, SBEC will turn the speed control system off.

Camshaft Angle Sensor (3.3L)

Camshaft angle sensor is mounted on top of timing chain cover. This sensor reads slots in cam timing sprocket. The SBEC uses this information along with information from crankshaft sensor to determine if fuel injectors and ignition coils are properly sequenced for correct cylinders.

Charge Temperature Sensor

Sensor measures temperature of incoming intake air. This information is used by SBEC to adjust air/fuel mixture and turbocharger boost.

Clutch Switch

This input prevents engine from starting until clutch pedal is depressed.

Coolant Temperature Sensor (CTS)

The CTS monitors engine coolant temperature. Sensor is mounted in intake manifold, next to thermostat housing. SBEC uses coolant temperature information to adjust air/fuel mixture and idle speed and to control cooling fans as necessary.

Crankshaft (Angle/Position) Sensor (3.3L, 3.9L & 5.2L)

Crankshaft sensor is mounted on transaxle bellhousing. Sensor reads slots on flywheel/torque converter drive plate. SBEC uses this information to determine fuel injection sequence, ignition signal and spark timing.

Detonation Sensor (3.3L)

Sensor is mounted on engine block. Positioning of detonation sensor enables it to detect detonation in any cylinder. Sensor generates an input signal to SBEC when detonation occurs. The SBEC uses this input to adjust spark advance and to eliminate detonation.

Hall Effect Switch

Hall Effect switch, sometimes called a Hall Effect pick-up, is located inside distributor. This switch supplies SBEC with engine RPM data and ignition timing information. SBEC uses this information to advance or retard ignition timing as necessary.

Idle Contact Switch

Idle contact switch provides an input signal that enables SBEC to increase or decrease throttle stop angle in response to engine operating conditions.

Ignition Sense Circuit

Ignition sense circuit informs SBEC that ignition circuit has been activated. SBEC then prepares for engine operation.

Manifold Absolute Pressure (MAP) Sensor

The MAP sensor monitors manifold vacuum. Sensor transmits information on manifold vacuum and barometric pressure to SBEC. MAP sensor information is used with information from other sensors to adjust air/fuel mixture.

Optical Distributor (3.0L)

Optical distributor provides engine speed and crankshaft position signals. SBEC uses this information to control fuel injection, ignition timing and idle speed.

Oxygen (O2) Sensor

The O2 sensor produces a small electrical voltage (.1-.9 volt) when exposed to oxygen in exhaust gas flow. O2 sensor is electrically heated for faster switching. Heating element is powered through Auto Shutdown (ASD) relay.

The O2 sensor acts like a rich/lean (air/fuel ratio) switch by monitoring the oxygen content in exhaust gas. This information is used by SBEC to adjust air/fuel ratio.

The O2 sensor produces a low voltage when oxygen content in exhaust gas is high. When oxygen content in exhaust gas is low, O2 sensor produces a higher voltage.

Park/Neutral (P/N) Switch

Switch is available on automatic transmission vehicles only. The P/N switch is located on transmission housing. Switch prevents engine starter from engaging if vehicle is in any gear except Park or Neutral. Also see TRANSMISSION GEAR SELECTION.

Serial Communication Interface (SCI) Receive

SCI Receive is serial communication link used when diagnosing vehicle using DRB-II tester. SBEC receives data and device activation commands from DRB-II on this circuit.

Speed Control Switch

Speed control switch provides SBEC with 3 separate inputs. ON/OFF informs SBEC that speed control system has been activated. SET/COAST informs SBEC that set vehicle speed has been selected, or if depressed will decelerate until switch is released. RESUME/ACCEL informs SBEC that a previously set speed has been selected or, if depressed, will increase speed until released. SBEC uses these inputs to control speed control servo.

Sync Pick-Up (3.9L & 5.2L)

Sync pick-up is locate inside distributor and is used along with crankshaft position sensor to determine fuel injection sequence. Sync pick-up consists of a single shutter blade that passes through Hall Effect switch causing a signal to be sent to SBEC.

Leading edge of shutter indicates cylinder No. 5 on 3.9L or cylinder No. 8 on 5.2L. Trailing edge of shutter indicates cylinder No. 2 on 3.9L or cylinder No. 5 on 5.2L.

SBEC compares signals received from crankshaft position sensor and sync pick-up to determine which piston is coming up to top dead center. SBEC then fires injectors in proper order for engine to operate.

Throttle Body Temperature Sensor (5.9L)

Sensor is mounted in throttle body. Sensor monitors throttle body temperature so SBEC can adjust air/fuel mixture for a hot restart condition.

Throttle Position Sensor (TPS)

TPS is mounted on throttle body and monitors opening angle of throttle valve. Sensor varies its output voltage according to angle of throttle blade opening. SBEC uses this information and other sensor inputs to adjust air/fuel ratio. If SBEC senses wide open throttle (WOT) position during cranking, it assumes engine is flooded and turns off fuel injectors until throttle is released.

Transmission Gear Selection

The P/N safety switch on the transmission housing provides an input to SBEC to indicate if transmission is in Park, Neutral or Drive. SBEC uses this input to determine necessary changes to idle speed, fuel injector pulse width and ignition timing advance. Also see PARK/NEUTRAL (P/N) SWITCH.

Transmission Overdrive Override Switch (A/T Models)

On models with Overdrive, SBEC regulates overdrive upshift and downshift through the overdrive solenoid. The transmission overdrive override switch is mounted in the instrument panel.

The switch is normally closed. If switch is depressed and switch opens, transmission will not enter Overdrive. Transmission will downshift if it is in Overdrive and switch is depressed.

Vehicle Speed Sensor (VSS)

The VSS generates 8 pulses per axle shaft revolution. SBEC interprets speed sensor input along with TPS closed throttle input.

This input enables SBEC to determine if a closed throttle deceleration or normal throttle idle (vehicle stopped) condition exists. During deceleration, SBEC controls Automatic Idle Speed (AIS) motor or Idle Speed Control (ISC) actuator to maintain a desired manifold pressure. During idle (vehicle stopped), SBEC controls AIS motor or ISC actuator to maintain a desired idle speed. SBEC also uses VSS signal to maintain set speed during cruise control operation.

OUTPUT SIGNALS

Note. Each vehicle may be equipped with different combinations of computer-controlled components. The following components may NOT be used on all models. For theory and operation on each output component, refer to the indicated system.

A/C Clutch Relay

See MISCELLANEOUS CONTROLS.

Air Switching Solenoid

See AIR INJECTION SYSTEM under EMISSION SYSTEMS.

Alternator

See MISCELLANEOUS CONTROLS.

Automatic Idle Speed (AIS) Motor

See IDLE SPEED under FUEL SYSTEM.

Auto Shutdown (ASD) Relay

See MISCELLANEOUS CONTROLS.

Canister Purge Solenoid

See EVAPORATIVE EMISSIONS SYSTEM under EMISSION SYSTEMS.

CHECK ENGINE Light

See SELF-DIAGNOSTIC SYSTEM.

Direct Ignition System

See IGNITION SYSTEM.

Electric EGR Transducer (EET)

See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.

Emission Maintenance Reminder (EMR) Light

See EMISSION MAINTENANCE REMINDER LIGHT under EMISSION SYSTEMS.

Exhaust Gas Recirculation (EGR) Solenoid

See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.

Fuel Injectors

See FUEL CONTROL under FUEL SYSTEM.

Idle Speed Control (ISC) Actuator

See IDLE SPEED under FUEL SYSTEM.

In-Tank Fuel Pump

See FUEL DELIVERY under FUEL SYSTEM.

Limp-In Mode

See MISCELLANEOUS CONTROLS.

Lock-Up Torque Converter Solenoid

See MISCELLANEOUS CONTROLS.

Magnetic Ignition System (Hall Effect)

See IGNITION SYSTEM.

Optical Ignition System

See IGNITION SYSTEM.

Overdrive Override Switch Indicator Light

See MISCELLANEOUS CONTROLS.

Overdrive Solenoid

See MISCELLANEOUS CONTROLS.

Radiator Fan Relay

See MISCELLANEOUS CONTROLS.

Serial Communication Interface (SCI) Transmit

See SELF-DIAGNOSTIC SYSTEM.

Shift Indicator Light

See MISCELLANEOUS CONTROLS.

Speed Control Servo

See MISCELLANEOUS CONTROLS.

Tachometer (Dakota)

See MISCELLANEOUS CONTROLS.

See AUTO SHUTDOWN (ASD) RELAY under MISCELLANEOUS CONTROLS.

Fuel Pressure Damper (3.3L)

Damper is located downstream of fuel pressure regulator. (Scheme 1) Damper dampens fuel pressure pulsations caused by injectors opening and closing to keep fuel pressure constant across injectors. Pressure pulses are absorbed by an internal rubber diaphragm with air pocket on one side.

Scheme 1

Fuel Pressure Regulator

Fuel pressure regulator is a mechanical device. Pressure regulator is located on top of throttle body on TBI engines. Pressure regulator is located on fuel injector rail on PFI engines. Regulator maintains constant fuel pressure across injectors. See FUEL PRESSURE table.

Inside pressure regulator is a spring-loaded diaphragm. When fuel pump is energized, fuel flows past fuel injectors into fuel pressure regulator. Pressure regulator restricts fuel from flowing any farther until proper pressure has been reached. (Scheme 2)

When proper fuel pressure is reached, fuel pressure pushes on spring behind diaphragm. As fuel pressure moves spring and diaphragm, a return line to fuel tank is uncovered. This allows excess fuel to return to fuel tank, keeping fuel pressure constant across injectors.

FUEL PRESSURE

Scheme 2

In-Tank Fuel Pump (PFI)

Fuel pump is a positive displacement, immersible gerotor pump with a permanent magnet motor. The pump incorporates a filter sock attached to pump pick-up. This fuel pump contains 2 check valves. One check valve is used to relieve internal pump pressure and regulate maximum fuel pump output. The other check valve, located near pump outlet, is used to restrict fuel movement in either direction when pump is not operational. Voltage to operate pump is supplied through ASD relay.

In-Tank Fuel Pump (TBI)

Fuel pump is an immersible electric pump with permanent magnet motor. The pump incorporates a sock attached to pump pick-up. Fuel pump also contains a check valve which restricts fuel movement in either direction to maintain fuel line pressure when pump is not operating. Voltage to operate pump is supplied through ASD relay.

Fuel injectors are powered by electric solenoids and controlled by SBEC. SBEC determines when and length of time injectors should operate. Current is supplied to injectors through ASD relay, which is controlled by SBEC. When ground is supplied to injector by SBEC, armature and pintle inside injector move a short distance against spring and open a small orifice. Since fuel is under high pressure, a fine spray is developed.

Port Fuel Injection (PFI)

Individual, electrically pulsed injectors (one per cylinder) are located in intake manifold fuel rails. These injectors are next to intake valves in cylinder head.

PFI system features sequential injection. SBEC controls injection timing based on input from optical distributor on 3.0L, crankshaft and camshaft angle sensors on 3.3L and crankshaft position sensor or sync pick-up on 3.9L and 5.2L. Fuel pressure regulator maintains constant fuel pressure to injectors. Air/fuel mixture is regulated by the length of time injector stays open (pulse width). ECM controls pulse width using information provided by various sensors.

Throttle Body Injection (TBI)

2.5L uses single-injector throttle body. 5.9L uses dual-injector throttle body. Battery voltage is supplied to injector when ignition is on. SBEC energizes injector solenoid by providing a ground path through its internal circuitry. By regulating injector ground circuit, SBEC controls injector on-time (pulse width) to provide proper amount of fuel to engine.

Pressure regulator maintains pressure to injector at 39 psi(2.7 kg/cm 2 ) on 2.5L and 14.5 psi (1.0 kg/cm 2 ) on 5.9L. Excess fuel passes through pressure regulator and returns to fuel tank.

In the "run" mode, SBEC uses tach (RPM) signal to determine when to pulse injector. On models equipped with dual injectors in the throttle body, injectors are pulsed alternately.

Internal SBEC calibration controls fuel delivery during starting, clear flood mode, deceleration and heavy acceleration.

Modes Of Operation

As input signals to SBEC change, SBEC adjusts its response to the output devices. Modes of operation come in 2 types, open loop and closed loop. In open loop mode, SBEC is not using input from oxygen sensor and is responding to preset programming to determine injector pulse width and ignition timing. In closed loop mode, SBEC is adjusting ignition timing and using the input from oxygen sensor to fine tune injector pulse width.

The following inputs are used to determine SBEC mode.

1. Coolant Temperature Sensor (CTS)
2. Idle Contact Switch
3. MAP Sensor
4. Engine Speed
5. Throttle Position Sensor (TPS)
6. Gear Position
7. A/C Switch
8. Battery Voltage
9. Oxygen (O2) Sensor
10. A/C Control Positions

From these inputs, SBEC determines which mode vehicle is in and the appropriate response. Not all inputs are used in all modes. The 8 modes of operation are

1. Ignition Switch In ON Position This is an open loop mode. SBEC determines atmospheric pressure from MAP sensor and determines basic fuel strategy. SBEC modifies fuel strategy according to coolant temperature input.
2. Engine Start-Up This is an open loop mode. When starter is engaged, SBEC receives distributor signal and energizes Auto Shutdown (ASD) relay. See AUTO SHUTDOWN (ASD) RELAY under MISCELLANEOUS CONTROLS. Once ASD relay is energized, SBEC determines proper injector pulse width and ignition timing from input signals. During engine start-up, SBEC pulses each fuel injector 4 times per engine revolution instead of the normal 2 pulses per revolution.
3. Engine Warm-Up This is an open loop mode. SBEC determines injector pulse width using information from various inputs and fires each injector 2 times per engine revolution. SBEC controls engine idle speed, throttle stop angle and ignition timing.
4. Cruise When engine is at operating temperature, this is a closed loop mode. Using information from various inputs, SBEC determines injector pulse width and fires each injector 3 times per engine revolution. SBEC controls engine idle speed, throttle stop angle and ignition timing. SBEC determines proper air/fuel ratio using input from oxygen sensor.
5. Acceleration This is a closed loop mode on Dakota 2.5L and all FWD models. All other models are in open loop mode. When SBEC recognizes an abrupt increase in throttle position or manifold pressure as a demand for increased engine output, it increases injector pulse width in response to increased fuel demand.
6. Deceleration This is an open loop mode on Dakota 3.9L and 5.2L models. All other models are in closed loop mode. When SBEC receives inputs signaling a closed throttle and an abrupt decrease in manifold pressure, it may reduce injector firing to one pulse per engine revolution to lean air/fuel mixture. SBEC also prevents EGR and canister purge functions during deceleration by grounding EGR and evaporative purge solenoids. SBEC may cycle air switching solenoid for short periods of time in response to a high vacuum signal from MAP sensor.
7. Wide Open Throttle This is an open loop mode. When SBEC senses wide open throttle, it grounds EGR and evaporative purge solenoids to prevent EGR and canister purge functions. Oxygen sensor input is not utilized and SBEC adjusts injector pulse width to supply a predetermined amount of additional fuel.
8. Ignition Switch In OFF Position This is an open loop mode. All fuel injection is stopped. SBEC de-energizes auto shutdown (ASD) relay and extends Idle Speed Control (ISC) actuator (if equipped) in anticipation of next start-up.

Automatic Idle Speed Motor (2.5L, 3.0L, 3.3L, 3.9L & 5.2L)

Automatic Idle Speed (AIS) motor adjusts idle speed to compensate for engine load and ambient temperature by adjusting amount of air flowing through by-pass in throttle body. SBEC uses coolant temperature, distance (speed) sensor, throttle position and various switch input operations to adjust AIS to obtain optimum idle conditions. Deceleration stall is prevented by increasing airflow when throttle is closed suddenly.

Idle Speed Control (ISC) Actuator (5.9L)

The ISC actuator is a motor mounted to throttle body and controlled by SBEC. Using inputs from various engine control system sensors, SBEC extends or retracts actuator to control engine idle speed and to set throttle stop angle during deceleration.

Note. DO NOT attempt to correct a high idle speed condition by turning ISC adjustment screw. This will not change idle speed of warm engine but may cause cold start problems due to restricted airflow.

DIRECT IGNITION SYSTEM (3.3L)

A Direct Ignition System (DIS) eliminates mechanical ignition components that can wear out. SBEC has complete ignition control and uses crankshaft and camshaft sensors to control ignition timing. Crankshaft position sensor senses slots (4 per cylinder, 20 degrees apart) around an extension of the drive plate. Basic timing is preset by crankshaft sensor position and is not adjustable. By using a crankshaft sensor, spark scatter has been eliminated.

A cam sensor is located on timing chain cover. Cam sensor senses slots on cam timing gear. Fuel injection synchronization and cylinder identification are provided through cam sensor. The unique combination of slots on cam gear are used to identify individual cylinders and initiate fuel and spark for start and run conditions.

This system uses 3 molded coils mounted on the intake manifold. Coil fires 2 spark plugs every power stroke. One cylinder is on compression stroke and the other cylinder is on exhaust stroke. A low primary resistance allows SBEC to fully charge ignition coil for each firing.

OPTICAL IGNITION SYSTEM (3.0L)

The timing member is a thin disc mounted on distributor shaft and driven at 1/2 crankshaft speed. Disc has 2 sets of slots on its surface. The outer, high data rate set of slots is positioned at intervals of 2 degrees of crankshaft rotation. Disc is used for ignition timing at engine speeds up to 1200 RPM to increase timing accuracy.

During cranking and idle, engine speed changes with firing pulse of each cylinder. High data rate signal is used to trigger ignition at correct crankshaft position regardless of these speed changes.

The inner, low data rate set contains 6 slots, which are correlated to piston TDC for each cylinder. This set is used to trigger fuel injection system operation at speeds greater than 1200 RPM, where speed changes because individual firing pulses are small. This set of slots is also used for ignition timing. Light Emitting Diodes (LEDs) and photo diodes are mounted in facing positions on opposite sides of disc, in-line with slots. (Scheme 3)

Masks over LEDs and photo diodes focus light beams onto photo diodes. As each slot passes between diodes, light beam is turned on and off. This creates an alternating voltage in each photo diode, which is converted into on-off pulses by an integrated circuit within distributor. SBEC uses these pulses to control timing.

Scheme 3

2.5L, 3.9L, 5.2L & 5.9L

Ignition timing is controlled by Single Board Engine Controller (SBEC). SBEC uses engine RPM data from the Hall Effect switch to control ignition timing. Hall Effect switch is located inside distributor on 2.5L and 5.9L and is incorporated into crankshaft position sensor on 3.9L and 5.2L.

Hall Effect distributor has a shutter attached to distributor shaft. On 2.5L and 5.9L, shutter contains one blade per engine cylinder. (Scheme 4) On 3.9L and 5.2L, a single-blade shutter is used. On all models, switch plate is mounted to distributor housing above shutter. Switch plate contains distributor pick-up (a Hall Effect device and magnet) through which shutter blade(s) rotate.

SBEC applies and monitors voltage to Hall Effect switch. As shutter blade(s) pass through pick-up, magnetic field is interrupted and monitored voltage is toggled between high and low. On 3.9L and 5.2L, same effect is generated when windows in flywheel pass magnet in crankshaft position sensor.

SBEC monitors voltage signal switching and uses these pulses to calculate engine speed (2.5L and 5.9L) or fuel injection sync signal (3.9L and 5.2L).

Scheme 4

IGNITION TIMING CONTROL SYSTEM

The Single Board Engine Controller (SBEC) completely controls ignition system. During a crank/start mode, SBEC will set a fixed amount of spark advance for an efficient engine start.

Ignition Timing Advance Control

The amount of spark advance or retard is determined by inputs that SBEC receives from coolant temperature, engine vacuum and engine RPM. During engine operation, SBEC can supply an infinite number of advance curves to ensure proper engine operation.

AIR INJECTION SYSTEM

This system adds a controlled amount of air to exhaust gases to assist oxidation of hydrocarbons and carbon monoxide in exhaust stream. System does not interfere with ability of EGR system to control oxides of nitrogen emissions. Air is injected at either the exhaust manifold or the catalytic converter through air switching/relief valve. Air switching/relief valve is controlled by SBEC through air switching solenoid.

5.9L Heavy-Duty Emissions engines are equipped with a dual air pump system, which does not include air switching/relief valve.

SBEC uses this solenoid to control discharge of air from air pump into exhaust system. This solenoid controls flow of vacuum to air switching/relief valve of air pump system. When solenoid is not energized, airflow is in downstream mode which means no vacuum is supplied to air switching/relief valve and air pump output is directed to catalytic converter. When solenoid is energized, airflow is in upstream mode, which means that vacuum is supplied to air switching/relief valve and air pump output is directed to exhaust manifolds.

EMISSION MAINTENANCE REMINDER LIGHT

SBEC activates the Emission Maintenance Reminder (EMR) light at scheduled mileage intervals to indicate need for servicing of certain emission system components. SBEC will also illuminate this light whenever a fault occurs in the emission systems.

To reset mileage interval, connect Diagnostic Readout Box II (DRB-II) to vehicle diagnostic connector. Turn ignition switch to RUN position. Access EMISSIONS EMR TESTS on DRB-II. Select EMR MEMORY CHECK. Select RESET EMR LIGHT. Reset EMR light.

EVAPORATIVE EMISSIONS SYSTEM

This system stores fuel vapors from fuel tank, preventing vapors from reaching the atmosphere. As fuel evaporates inside fuel tank, vapors are routed inside vent hoses to charcoal canister, located in wheelwell area, where they are stored until engine is started.

Charcoal canister purging is controlled by a canister purge solenoid. Canister purge solenoid is controlled by SBEC. During engine warm-up and for a short period after hot restarts, SBEC grounds canister purge solenoid winding causing solenoid to energize.

When canister purge solenoid is energized, engine vacuum signal to charcoal canister is interrupted. After engine reaches a predetermined operating temperature and SBEC internal timer has expired, SBEC will de-energize canister purge solenoid. This will allow engine vacuum to purge charcoal canister. Canister purge solenoid will also be de-energized during certain idle conditions so SBEC can update fuel delivery calibration.

EXHAUST GAS RECIRCULATION (EGR)

EGR system allows a predetermined amount of exhaust gas to enter cylinder with the air/fuel mixture. This dilution of cylinder air/fuel volume reduces oxides of nitrogen (NOx) and helps prevent spark knock by reducing peak temperatures inside combustion chamber.

EGR system is a backpressure type. Backpressure transducer measures amount of exhaust gas backpressure on exhaust side of EGR valve and varies amount of vacuum applied to EGR valve. (Scheme 5)or (Scheme 6).

This system allows backpressure transducer to provide proper vacuum signal to EGR valve for all engine operating conditions. EGR system is controlled by an EGR vacuum solenoid using a manifold vacuum signal from throttle body.

Scheme 5

Exhaust Gas Recirculation (EGR) Solenoid (5.9L)

EGR solenoid does not allow EGR at idle. EGR system does not function when ambient temperature is less than 40°F (4°C). EGR system is activated when coolant temperature reaches 170°F (77°C).

On 2.5L, 3.0L, 3.3L, 3.9L and 5.2L, EGR system uses an Electric EGR Transducer (EET). This system incorporates backpressure transducer and EGR solenoid into one unit. (Scheme 6)

Scheme 6

HEATED AIR INLET SYSTEM

This device is an air preheater that controls air temperature entering throttle body when ambient temperatures are low. By maintaining temperature, throttle body can be calibrated much leaner to reduce hydrocarbon and carbon monoxide emissions, improve engine warm-up characteristics and minimize icing.

The heated air inlet system is a 2-circuit airflow system. When ambient temperature is greater than control temperature, airflow is through outside air inlet alone. When air temperature is less than control temperature, airflow is through outside air inlet and through heated air inlet. The cooler the ambient temperature, the greater the flow through heated air inlet. Airflow is controlled by a vacuum-operated, heat-controlled door, located in snorkel.

POSITIVE CRANKCASE VENTILATION (PCV)

Crankcase blow-by gases are removed from crankcase with manifold vacuum. These gases are introduced into incoming air/fuel mixture and become part of the calibrated mixture. No fresh air enters crankcase with this PCV system.

SELF-DIAGNOSTIC SYSTEM

The Single Board Engine Controller (SBEC) monitors several different circuits of engine control system. If a problem is sensed with a monitored circuit, SBEC will store a trouble code to aid technician in diagnosis of system. The CHECK ENGINE light or Diagnostic Readout Box II (DRB-II) can be used to read trouble codes.

CHECK ENGINE light comes on and remains on for 3 seconds as a bulb test each time ignition switch is turned to ON position. If SBEC receives an incorrect signal or receives no signal from battery voltage input, charging system, coolant temperature sensor, manifold absolute pressure sensor or throttle position sensor, CHECK ENGINE light will glow. On California vehicles only, light will also glow if an emission-related fault exists. This warns driver that SBEC is in limp-in mode and immediate repairs are necessary. CHECK ENGINE light can also be used to display fault codes. For additional information, see appropriate G - TESTS W/ CODES article in the ENGINE PERFORMANCE Section.

SERIAL COMMUNICATIONS INTERFACE (SCI)

SCI circuit is used by SBEC to send data to and receive data and sensor activation signals from DRB-II tester. DRB-II uses signals sent on SCI to display fault codes, sensor voltages and device states (ON/OFF). DRB-II uses SCI to send solenoid and switch activation commands to SBEC so that devices and circuits can be tested. SCI is also used to reset Emission Maintenance Reminder (EMR) light and to write EMR mileage to SBEC.

MISCELLANEOUS CONTROLS

Note. Although not strictly considered part of engine performance system, some controlled devices can adversely affect driveability if they malfunction.

A/C clutch relay is controlled by SBEC and A/C switch. A/C relay is powered by condenser fan relay. This relay is energized during engine operation when A/C switch is closed and blower is on.

When SBEC senses low idle speed or wide open throttle through throttle position sensor, SBEC will de-energize A/C clutch relay, preventing A/C operation.

Single Board Engine Controller (SBEC) controls voltage regulation.

The ASD relay is a cutoff relay for the following components.

1. Electric Fuel Pump
2. Fuel Injectors
3. Ignition Coil
4. O2 Sensor Heating Element

When ignition switch is turned to RUN position, SBEC energizes ASD relay which powers these components. If SBEC does not receive a distributor signal (cam or crankshaft signal on 3.3L) shortly thereafter, SBEC will de-energize ASD relay and power to these components is cut off.

Limp-in mode is the attempt by SBEC to compensate for failure of certain components by substituting information from other sources so that vehicle can still be operated. If SBEC senses incorrect data or no data at all from MAP sensor, throttle position sensor, coolant temperature sensor or battery voltage, system is placed into limp-in mode and CHECK ENGINE light on instrument panel is illuminated.

If faulty sensor comes back on line, SBEC will resume closed loop operation. On some vehicles, CHECK ENGINE will remain illuminated until ignition is shut off and vehicle is restarted. To prevent damage to catalytic converter, vehicle should NOT be driven for extended periods in limp-in mode.

SBEC controls torque converter lock-up through lock-up solenoid. SBEC controls lock-up according to various operating conditions.

SBEC controls indicator light on overdrive override switch on models equipped with overdrive automatic transmission.

On vehicles equipped with overdrive transmissions, SBEC controls the 3-4 overdrive upshift and downshift through the overdrive solenoid. SBEC determines optimum overdrive shift scheduling for all operating conditions.

Single Board Engine Controller (SBEC) controls radiator fan relay. Radiator fan relay is energized during the following conditions

1. Fan relay is energized when A/C clutch is engaged.
2. On non-A/C vehicles and A/C vehicles with A/C not engaged, fan relay energizes when vehicle speed is greater than 40 MPH and coolant temperature reaches 230°F (110°C). Fan relay turns off when coolant temperature decreases to 220°F (104°C). When vehicle speed is less than 40 MPH, fan relay switches on at 210°F (99°C) and switches off at 200°F (93°C).
3. Fan relay also prevents "steaming". "Steaming" occurs when moisture evaporates from outside of radiator and is not blown under the vehicle. Fan relay will energize when ambient temperature is less than 60°F (16°C), when coolant temperature is between 100-195°F (38-91°C), when engine is at idle and when vehicle is stopped. Fan relay will energize for 3 minutes only.

SBEC provides ground for shift indicator light on models equipped with manual transmission. SBEC, based on engine speed, throttle position, and vehicle speed, turns shift indicator light on to advise driver to shift to a higher gear for optimum driveability.

System is electrically actuated and vacuum operated. The controls are located on the steering wheel. Controls consist of 3 buttons: OFF/ON, RESUME/ACCEL and SET/DECEL. Speed control servo is controlled by SBEC. System will operate at 35-85 MPH.

SBEC provides signal to drive tachometer on Dakota.