Engine Controls - Theory & Operation - Gasoline Dodge Dakota I

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, 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 pickup. 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).

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 diagram in appropriate WIRING DIAGRAMS article. Available input signals include the following

A/C SWITCH

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.

CAMSHAFT ANGLE SENSOR

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 SENSOR

Crankshaft sensor is mounted on transaxle bellhousing. Sensor reads slots (4 per cylinder) on torque converter drive plate. The SBEC uses this information to determine crankshaft position.

DETONATION SENSOR (3.3L)

The Detonation 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.

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, it produces a higher voltage.

PARK/NEUTRAL (P/N) SWITCH

Park/Neutral 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 .

THROTTLE BODY TEMPERATURE SENSOR (5.2L & 5.9L)

The Throttle Body Temperature 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)

The 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.

TRANSMISSION GEAR SELECTION

The P/N safety switch on the transmission housing provides an input to SBEC to indicate if transmission gear selection 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 .

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.

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 under appropriate heading.

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 EMISSION SYSTEM under EMISSION SYSTEMS.

CHECK ENGINE LIGHT

See SELF-DIAGNOSTIC SYSTEM .

ELECTRIC EGR TRANSDUCER (EET)

See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSION

SYSTEM.

EMISSION MAINTENANCE REMINDER (EMR) LIGHT

See EMISSION MAINTENANCE REMINDER LIGHT under EMISSION

SYSTEMS.

EXHAUST GAS RECIRCULATION (EGR) SOLENOID

See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSION

SYSTEMS.

FUEL INJECTORS

See FUEL CONTROL under FUEL SYSTEM.

IDLE SPEED CONTROL (ISC) ACTUATOR

See IDLE SPEED under FUEL SYSTEM.

DIRECT IGNITION SYSTEM

See IGNITION SYSTEM.

OPTICAL IGNITION SYSTEM

See IGNITION SYSTEM.

MAGNETIC IGNITION SYSTEM (HALL EFFECT)

See IGNITION 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.

OVERDRIVE SOLENOID

See MISCELLANEOUS CONTROLS.

RADIATOR FAN RELAY

See MISCELLANEOUS CONTROLS.

SPEED CONTROL SERVO

See MISCELLANEOUS CONTROLS .

See AUTO SHUTDOWN (ASD) RELAY under MISCELLANEOUS CONTROLS in this article.

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. (Scheme 2) 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.

When proper fuel pressure is reached, fuel pressure pushes on a 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 (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.

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.

Fuel injectors are powered by electric solenoids and controlled by SBEC. SBEC determines when and 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.

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 as follows

IGNITION SWITCH ON

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.

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.

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.

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.

ACCELERATION

This is a closed loop mode on Dakota 2.5L and all front-wheel drive vehicles. All other vehicles 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.

DECELERATION

This is an open loop mode on Dakota 3.9L and 5.2L vehicles. All other vehicles are in closed loop mode. When SBEC receives inputs signalling 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.

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.

IGNITION SWITCH OFF

This is an open loop mode. All fuel injection is stopped. SBEC de-energizes auto shutdown 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)

Automatic Idle Speed (AIS) motor adjusts idle speed to compensate for engine load and ambient temperature. AIS motor does this 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 (3.9L, 5.2L & 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) OPERATION

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 disk mounted on distributor shaft and driven at 1/2 crankshaft speed. Disk 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. Disk 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 (LED's) and photo diodes are mounted in facing positions on opposite sides of disk, in-line with slots. (Scheme 3)

Masks over LED's 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

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. Hall Effect distributor has a shutter attached to distributor shaft.

Shutter contains one blade per engine cylinder. (Scheme 4) A switch plate is mounted to distributor housing above shutter. Switch plate contains distributor pick-up (a Hall Effect device and magnet) through which shutter blades rotate. As shutter blades pass through pick-up, they interrupt the magnetic field. Hall Effect device in pick-up creates pulses by switching on and off when it senses change in magnetic field. These pulses generate the input signal to SBEC, which uses it to calculate engine speed.

Scheme 4

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.

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 a 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.

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.

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 above control temperature, airflow is through outside air inlet alone. When air temperature is below control temperature, airflow is through outside air inlet and through heated air inlet. The colder 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) SYSTEM

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.

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 vehicles' 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.

INTRODUSTION

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)

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.

EGR solenoid does not allow EGR at idle. EGR system on 2.5L engines allows EGR at all temperatures. On all other EGR-equipped engines, 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, EGR system uses an Electric EGR Transducer (EET). This system incorporates backpressure transducer and EGR solenoid into one unit. (Scheme 6)

Scheme 5

Scheme 6

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 illuminate. On California vehicles only, light will also illuminate if there is an emission-related fault. 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

* PLEASE READ THIS FIRST *

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.

The 3.0L and 3.3L engines use Nippondenso alternators. All other engines may be equipped with either a Bosch or Nippondenso unit.

The alternator consists of a rotor, stator, rectifiers, front and rear covers and drive pulley. On all vehicles, voltage regulation is controlled by Single Board Engine Controller (SBEC).

Alternator diodes convert AC current to DC current. Engine controller monitors critical input and output of charging system to ensure it is working properly.

The ASD relay is a cut-off 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. 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.

On vehicles equipped with A-999 or A-500 automatic transmission, SBEC controls torque converter lock-up through lock-up solenoid. SBEC controls lock-up according to various operating conditions.

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 drops 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.

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.

See also:
• TRANSMISSION GEAR SELECTION
• PARK/NEUTRAL (P/N) SWITCH
• MISCELLANEOUS CONTROLS
• EMISSION MAINTENANCE REMINDER LIGHT