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LH Fuel System

LH 2.4 fuel system

Operation

Cars with 16-valve engines and Lambda are equipped with the Bosch LH Jetronic 2.4 fuel injection system, which is an updated version of the LH Jetronic 2.2 system. LH is an abbreviation of Luftmassenmesser Hitzdraht (air mass meter with hot filament). The fuel injection system is controlled and supervised by control unit 200, which includes a microprocessor.

A number of new features have been introduced, such as:
^ Adaptive Lambda control system
^ AIC (Automatic Idling Control) valve with built-in "Limp home" function. When necessary, the valve provides a high, fixed idling speed of approximately 1200 r/min.
^ Adaptive idling control system. Normal changes are compensated for automatically. The idling speed is controlled by means of valve 272.
^ Built-in deceleration function. During overrun braking, it shuts off the fuel supply within a certain engine speed range.
^ A new, electrically-operated vent valve (321) for the charcoal canister. Controlled by signals from the control unit. The charcoal in the canister absorbs fuel fumes in the vent line from the tank.
^ Built-in fault diagnosis system. Gives fault codes via the CHECK ENGINE lamp or the SAT test instrument.
^ The pressure monitoring function is built into the control unit. If a fault should occur on the turbocharger pressure control system (T16 Lambda), the LH control unit will interrupt the signals to the injection valves.
^ The gear indication function is built into the control unit (US only).

The system receives a positive supply from fuse 22 when the ignition switch is in the start or drive position, and a constant supply from +30 to relays 229 and 102.

On the basis of the data stored in the control unit and the incoming information from various sensors, the control unit calculates and controls the opening times of the electrically operated fuel injection valves (206). Control unit 200 receives information on the engine speed by sensing the pulses from the ignition system.

Throttle angle transmitter 203 provides information to the control unit on the throttle angle. The transmitter has two contact positions, i.e. 0~ (idling speed) and 720 throttle angle.

Temperature transmitter 202 is of Negative Temperature Coefficient (NTC) type and provides continuous engine temperature information to the control unit. In the event of loss of this signal, the control unit will simulate, by default, an engine temperature of +45 °C (113 °F).

Air mass meter 205 is built into a plastic housing. In the event of a loss of signal from the air mass meter, e.g. if the filament should fail, an emergency system known as the "Limp home" function in the control unit will come into operation. The car can then be driven, although its driveablity will be limited. When the "Limp home" function is operative, warning lamp 47P, CHECK ENGINE, will light up. The lamp is located in the combined instrument, and is supplied from fuse 7. (The lamp can also be energised from the EZK ignition system.)

If a fault affecting the exhaust emissions should occur, the CHECK ENGINE lamp will light up.

The engine is supplied with fuel either by electrically driven fuel pump 103 and feed pump 101 or by fuel pump 323 with integrated feed pump. In both cases, the pumps draw fuel from the fuel tank and pressurise the fuel system.

The control unit 200 is connected to the air-conditioning system via pin 14 (AC compressor relay 156, pin 16). When the AC compressor is running, the control unit will compensate for the increased load applied by the compressor at engine idling speed. A signal (earth) is applied from pin 3 at full throttle, which disconnects the AC compressor.

Test connector 347 is used for fault tracing. When pin 2 in the connector is earthed, flashing codes will be obtained on the CHECK ENGINE lamp.


Exhaust gas emission control, Lambda

The car is equipped with an adaptive Lambda system, which compensates for variations in the fuel/air mixture caused by changes in the fuel system.

Lambda sensor 136 continually measures the oxygen content of the exhaust gases, thus enabling the control unit to adjust the mixture to as close to Lambda = 1 as possible. The sensor is heated by preheater 271 (protected by fuse 1).


Automatic transmission

Cars with automatic transmission are also equipped with switch 76 which will close when the selector lever is set to "Drive", and the control unit will then compensate for the increased load applied by the automatic transmission when the engine is running at idling speed.


Fault-tracing hints

When fault tracing in the wiring for the fuel system, always observe the following:
1. Read the fault codes that have been stored before disconnecting the control unit. (The fault codes are erased when the control unit is deenergised.) Use a test cable or the ISAT as described in Group 2:3 of the Service Manual.
2. Always disconnect the 35-pole connector on the control unit and the connector on the air mass meter. N.B. These two components can easily be seriously damaged if measurements are carried out on their terminal pins.
3. Always disconnect the connection to the suspected component.
4. Use an ohmmeter not a buzzer for checking the wiring.

Before checking the wiring, always start by checking fuse 22 and checking that the supply to it is live. Also check fuse 7 and fuse 1 for the Lambda sensor preheater.

Earthing point 201 on the engine lifting lug is important for correct operation of the fuel system. If the location of the earthing point is altered, such as when other work is being carried out on the engine, the performance of the system may be disturbed.