Bosch Lh-Jetronic - Engine Control System Volvo 740 I

DESCRIPTION

The Volvo Computerized Engine Control system is used on both Volvo 4-cylinder and 6-cylinder engines. It consists of a computer controlled electronic ignition with microprocessor and LH-Jetronic 2.2 fuel system integrated with a Constant Idle Speed system.

The computer controlled ignition system consists of a distributor, Electronic Control Unit (ECU), ignition coil, ignition switch, throttle switch, knock sensor, coolant temperature sensor and air mass meter.

LH-Jetronic fuel system consists of Electronic Control Unit (ECU), air mass meter, injectors, injector ballast resistors, coolant temperature sensor, air control valve, lambda sond, fuel pump and in-tank pump.

ELECTRONIC IGNITION SYSTEM

Electronic controlled ignition ECU incorporates a fault monitoring circuit. If a component develops a fault that can cause knocks, circuit automatically lowers timing 11-15 degrees to avoid damage to engine. Throttle switch signals ECU to set ignition timing point at idle. At higher speeds, ECU sets firing point to obtain best possible emissions.

Knock sensor produces current within a frequency range which occurs when cylinder knocks. When ECU receives signal, it retards timing in 3 degree intervals until knocking stops. ECU progressively advances ignition timing in.37 degree steps until programmed firing point is reached, or if cylinder starts to knock again. (Scheme 13)

Coolant temperature sensor provides information to both ignition ECU and fuel injection ECU. It cuts out information signals from knock sensors when engine is cold and adjusts for engine temperature when determining firing points.

Air mass meter measures air drawn into engine by means of a wire filament located in intake air stream. A temperature of 212°F (100°C) is maintained at filament. Current needed to maintain temperature at filament is measured, providing precise information on air mass.

Scheme 13

ELECTRONIC FUEL INJECTION SYSTEM

Note. Only those components directly used in the Constant Idle Speed system will be covered in this section. For information on fuel injection system, refer to appropriate FUEL INJECTION SYSTEM - BOSCH LH JETRONIC II article.

The LH-Jetronic fuel injection system consists of 3 functional sections: ECU, fuel system and sensors. ECU receives and evaluates readings from various sensors around engine. Microprocessor evaluates variations in engine running conditions, calculates correct amount of fuel to be injected, and produces corresponding signals to injectors. It also operates air control valve for constant idle speed by regulating airflow opening. (Scheme 14)

Scheme 14

CONSTANT IDLE SPEED (CIS) SYSTEM

CIS system is incorporated into LH-Jetronic fuel system. Its main function is to control engine at idle or near idle by regulating air control valve. Combined information from throttle switch, engine speed sensor and coolant temperature sensor, is processed through fuel system ECU to operate air control valve.

Air control valve increases or decreases amount of air which is taken from in front of throttle valve, by-passed around throttle valve, and injected into intake manifold (behind throttle valve). It consists of a small electric motor, mounted between connecting hoses at air flow throttle valve. Electric motor rotates clockwise or counterclockwise, depending on electrical signals received from ECU. (Scheme 15)

Scheme 15

ENGINE WILL NOT START

Check fuses No. 1 and 11 in fuse block at rear of engine compartment. Check all ground connections on intake manifold. Check ignition system, starting system, charging system, and all electrical connectors for proper connections, routing and function.

Check air intake system and fuel system. Check computer system and fuel pump relays. Ensure that Lambda sond's test socket is not grounded (located at right front fender).

ENGINE HARD TO START (COLD & WARM)

Check starting and charging systems. Check electrical and fuel connectors for proper connections and routing. Check air intake system and fuel system. Check air temperature sensor.

POOR OR NO IDLE

Check air intake system and fuel system. Check air mass meter.

EXCESSIVE FUEL CONSUMPTION

Check fuel system for excessive pressure and leaks. Check throttle valve switch and mixture adjustment.

POOR PERFORMANCE (LOW TOP SPEED)

Check air intake system, fuel pressure, and fuel pump. Check throttle valve switch and mixture adjustment.

ERRATIC IDLE

Check air intake system and fuel injectors. Check air control valve hoses for proper connection and blockage. Check throttle valve adjustment and attaching screws.

IDLE SPEED TOO HIGH

Check throttle valve and throttle valve switch. Check idle speed. Check air temperature sensor.

IDLE SPEED TOO LOW (ENGINE COLD)

Check air temperature sensor and air control valve. Check idle speed control system.

JERKING, POOR ACCELERATION

Check acceleration enrichment.

LH-Jetronic Fuel System

1. Check all ground connections on inlet manifold. Ensure that lambda sond's test socket (Green/White) at front right fender is not grounded. Check Fuses No. 1 and 11 (pump relay and tank relay).
2. Ensure all connectors for air mass meter, air control valve, knock sensor, coolant temperature sensor and injectors make good contact.
3. Check for air leaks occurring between air mass meter and engine. Leaks will cause air/fuel mixture to be too lean. Check all connections, joint couplings, seals and hoses on intake manifold and throttle housing.
4. Clean throttle housing where necessary. Disconnect connector from throttle switch. Remove throttle housing and clean with suitable solvent. Do not allow solvent to come in contact with throttle switch.
5. To adjust throttle, loosen lock nut. Unscrew adjustment screw until throttle switch is completely closed. Screw in adjustment screw until it just contacts link arm and then 1/4 turn further. Tighten nut. Use a new gasket when installing throttle housing. Connect air hoses and throttle switch connector.
6. Check throttle switch adjustment. Open throttle slightly and listen. A click should be heard as soon as throttle is opened (indicating that idle switch has opened). Adjust if click is not heard.
7. To adjust. Loosen retaining screws using 3 mm Allen wrench. Turn switch clockwise slightly, then turn switch counterclockwise until click is heard. Tighten retaining screws and check setting.

Note. Test procedures described require standard test equipment using volt/ohmmeter. Although Volvo LED Tester (9995280-6 ) can be used with proper instructions.

B230F/FT

1. Check that ground connections are properly connected to intake manifold. Ignition ground may have one or 2 leads depending on model.
2. Disconnect lead from one of the spark plugs and connect a separate spark plug to it. Ground spark plug and crank starter motor. A strong, Blue/White spark indicates engine or fuel system malfunction. If there is no spark, or if a weak spark occurs, turn off ignition.
3. Connect a lead, plus spark plug, to ignition coil High Tension (HT) terminal. Ground spark plug and crank starter motor. No spark or weak spark indicates ignition system malfunction. If spark plug fires, check distributor rotor and cap, and HT leads. Replace if necessary.

B280F

1. Ensure that ground connections (BN) from control unit (ignition ECU), and power stage unit (next to left side front fender), are properly connected to intake manifold ground.
2. Disconnect lead from one of the spark plugs and connect a separate spark plug to it. Ground spark plug and crank starter motor. A strong, Blue/White spark indicates engine or fuel system malfunction. If there is no spark, or if a weak spark occurs, turn off ignition.
3. Connect a lead, plus spark plug, to ignition coil (HT) terminal. Ground spark plug and crank starter motor. No spark or weak spark indicates ignition system malfunction. If spark plug fires, check distributor rotor and cap, and HT leads. Replace if necessary.

B280F Only

Operate engine at light load until normal operating temperature is reached. Engine temperature below 122°F (50°C) could effect timing check. Check timing at idle speed (750 RPM). Timing specification is 15 to 17 degrees BTDC.

Note. There is no timing adjustment. If timing is incorrect, proceed to CIRCUIT CHECKS .

CIRCUIT CHECKS

Note. Unless otherwise mentioned, the connector referenced in following checks is the control unit (ignition ECU) connector. (Scheme 16)

Scheme 16

1. Turn off ignition. Remove panel from lower left of dash board. Disconnect wiring harness from ignition control unit (located above accelerator pedal). Remove cover from connector. Check that all pin connection sleeves are the same height.
2. Using an ohmmeter, measure resistance between pin No. 2 of connector F (on left suspension tower), and ground. If reading is incorrect, repeat measurement directly at the sensor. See «COOLANT TEMPERATURE SENSOR RESISTANCE»(/volvo/740/i-1983-1992/remont/testing-diagnostics/#bosch-lh-jetronic-engine-control-system) table. Replace as applicable.

COOLANT TEMPERATURE SENSOR RESISTANCE

Cylinder No. 1 Position Sensor (B280F)

1. Connect ohmmeter leads between connector pins No. 18 (Black/Yellow) and No. 19 (Yellow/Red). Resistance should read zero ohms. Ensure that shield wire is connected to connector pin No. 21. If problem still exists, continue to next step.
2. Check that wires from engine RPM/crankshaft sensor are correctly connected to connector "F" on left suspension tower. Replace any broken wire(s). If problem still exists, go on to next step.
3. Turn ignition off. Check that wires from cylinder No. 1 position sensor are correctly connected to connector" F" on left suspension tower. Check continuity of wires and replace any broken wire(s).

Throttle Switch B280F

1. Connect ohmmeter leads between ground and control unit connector pin No. 7. Resistance should be 0 ohms. Slowly depress accelerator to open throttle switch and observe meter reading. Resistance should increase.
2. If reading is incorrect, measure resistance at throttle switch to determine if wire is broken (open circuit), or if throttle switch is defective. Adjust or replace as necessary.

Knock Sensor B280F

1. Check wires at knock sensor. Disconnect connector at fuel distribution rail. Connect a jumper wire between pins No. 1 and 2 of connector.
2. Measure resistance between control unit connector pins No. 12 (SB) and 13 (GR), and pins No. 24 (BN) and 25 (GN). Resistance should be zero ohms in both measurements. If resistance is high, there is an open circuit. Remove jumper wire and check each wire for continuity. Replace any broken wires.

Knock Sensor B230F/FT

1. Disconnect wiring harness from knock sensor. Connect jumper wire between pins at rear of connector. Be careful not to damage pins.
2. Measure resistance between control unit connector pins No. 12 and 13. Resistance should be zero ohms. If correct, remove jumper wire. Meter should indicate infinity. If neither reading is correct, check for shorted or broken wires.

See also:
• CIRCUIT CHECKS
• COOLANT TEMPERATURE SENSOR RESISTANCE