Bosch LH Technical Description

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Technical description 200-35LH fuel injection system,8202 injection engineGeneralThe Bosch LH-Jetronic L uftmassenmesser Hitzdraht) fuel injection system, comprising a mass airflow sensor incorporating a filament (hot-wire sen-sor), combines the best features of different fuel in-jection systems with measurement of the air mass, l.e. the density of the induction air is taken into ac-count - unlike earlier fuel injection systems.in the LH-Jetronic system, the air mass consumedby the engine is measured, which means that allow-ance is made for the composition of the air (temper-ature, pressure and humidity). Measurement of theair mass is performed by the mass air flow sensorwith the hot- wire sensor, which measure the air up-stream of the inlet manifold.This method provides a more exact measure of thequantity of oxygen supplied to the engine by the airmass, which determines the quantity of fuel requiredfor efficient combustion.In all previous fuel injection systems, only the airvolume (quantity of air) and/or the atmospheric pres-sure has been taken into account.The LH system has the following advantages overearlier types of fuel injection: Compensation for temperature variations. Adjusts the quantity of fuel to the quantity of ox-ygen at high altitudes. Fewer components. Lighter. Fewer moving parts. Wider scope for adapting the quantity of fuel toactual needs (warm-up period/accelerationphase).

Saab 900

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200-36 Technical descriptionFuel injection system, EU (1984-)

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21Fuel injection system, EU (1984-)1 Fuel tank2 Fuel pump3 Feed pump4 Fuel filter5 Fuel injection manifold6 Fuel pressure regulator7 Control module8 Distributor9 Ignition coil10 Coolant temperature sensor11 Injector12 Vacuum line13 Intake pipe14 Throttle position sensor15 Auxiliary air valve16 Mass air flow sensor

17 Pressure switch18 Main relay19 Fuel pump relay20 Battery21 Ignition switch

S aab 90 0

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Techn ical desc rip tion 200-37Fuel injection system, cars with catalytic converter

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Fuel injection system, cars with catalytic converter1 Fuel tank2 Fuel pump3 Feed pump4 Fuel fil ter5 Fuel injection manifold6 Fuel pressure regulator7 Control module8 Distributor9 Ignition coil10 Coolant temperature sensor11 Injector12 Vacuum line13 Intake pipe14 Throttle position sensor15 Auxiliary air valve16 Mass air flow sensor17 Oxygen sensor

18 Pressure switch (Turbo only)19 Main relay20 Fuel pump relay21 Battery22 Ignition switch

Saab 900

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200-38 Technical descriptionPrinciple of operationThe operating principle of the system is as follows.An electric fuel pump pumps fuel from the fueltank, building up pressure in the fuel system. Thelevel of the pressure is governed by the pressureregulator , which maintains a constant ratio be-tween the fuel pressure and the pressure in the in-take pipe. As a result, the quantity of injected fuel isunaffected by variations in the inlet manifold pres-sure and is influenced only by the amount of timethe injector is open.The fuel is injected by the injectors (electricallycontrolled solenoid valves) fitted in the intake pipeclose to the inlet valves and connected by a com-mon fuel injection manifold .The time the injectors remain open is determined bythe engine load.On M1989 and later cars the injectors are fitted witha plastic sleeve. A nozzle of new design improvesthe long-term characteristics.

The mass air flow sensor consists of an alumin-ium housing enclosing an air duct. On M1989 andlater Saab 900 Turbo models with catalytic con-verter (LH2.4), the mass air flow sensor housing ismade of plastic: Located in the centre of the duct isa tube which houses the platinum filament (hot wire).The temperature of the hot wire is maintained at ap-prox. 100C (212F) above the temperature of theinlet air, regardless of the corriposition of the air orthe air flow.The device for controlling the current required tomaintain the hot wire at a constant temperature con-sists of a bridge circuit and a detector resistor, thevoltage variation across which is directly propor-tional to the magnitude of the inlet air mass.The electrical components are located in a moduleon the mass air flow sensor housing. Since the hotwire is sited inside the inlet duct it can becomecoated with dirt which reduces its sensitivity and af-fects the results of measurement. To keep the hotwire free from dirt, it is heated to a temperature ofabout 1000C (1830F) for one second. This takesplace four seconds after the engine has beenswitched off.

Saab 900

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Technical description 200-39In the event of a break in the signal from the massair flow sensor (MAF sensor)caused by a broken hotwire, for instance, an emergency system built intothe control module and known as the "Limp- Home"mode will take over, enabling the car to be drivenwith impaired performance. When the Limp-Homemode is in operation, the CHECK ENGINE lamp onthe instrument panel will light up.

The electric signals from the temperature sensorand throttle position sensor (two positions), the ig-nition pulses from the ignition system and the signalfrom the mass air flow sensor are all fed into thecontrol module's microprocessor which then pro-cesses the information and determines the time thatthe injectors remain open. The hot wire burn-offfunction is also controlled by the control module'smicroprocessor.Other functions stored in the control module includefull- load enrichment at different engine speeds, ex-tra acceleration enrichment when the engine is cold,and idling control on cars equipped with lAC.The temperature sensor is of NTC (Negative Tem-perature Coefficient) type and it sends a continuousengine temperature signal direct to the control mod-ule. If there is a break in the signal from the tem-perature sensor, a signal is simulated by the controlmodule and the system functions on the assump-tion that engine temperature is +20C (68F).The throttle position switch tells the control mod-ule whetherthe throttle butterfly is in the fully openor idling position.On M1991 and later cars with a 8212 engine, thethrottle position switch has been replaced by athrottle position sensor which continuously andsteplessly informs the control module of the currentthrottle butterfly angle.The auxiliary air valve compensates for losses dueto friction when the engine is started from cold byallowing air to bypass the throttle butterfly.On cars equipped with a catalytic converter, theauxiliary valve has been replaced by anidle air control (lAC) valve which also compen-sates for momentary increases in the load when theengine is idling. On M1991 and later cars with a8212 engine, this valve is of new design, see under"Component s" .

Saab 900

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200-40 Technical descriptionLH 2.4 fuel injection systemThe LH 2.4 fuel injection system has been intro-duced on all M1988 and later cars with a B202i en-gine and on all M1998B and later Turbo cars. Thissystem is a further development of the earlier LH 2.2fuel injection system. The improvement consists pri-marily of an expanded memory capacity for the con-trol module, as a result of which the module is fittedwith a 35-pin connector.Special LH 2.4 functions: Control module with expanded memory lAC valve with integral Limp-Home system Adaptive (intelligent) idling control system Adaptive Lambda system Integral deceleration function (fuel shut-off) Improved function for shift-up indication New CP valve Integrated fault-diagnosis system Pressure-switch function integrated in control

moduleM1990 and later cars have additional LH 2.4 func-tions: Wider scope for diagnosis with the integratedfault- diagnosis system

new control module Possibility of carrying out fault diagnosis and fault

tracing using an ISAT ( I ntelligent SA ab Tester)On M1991 and later cars a number of new diagnos-tic trouble codes, which can be read on an ISAT,have been included in the fault diagnosis system.LH 2.4.2 fuel injection systemThe LH 2.4.2 fuel injection system was introducedon M1991 cars with all versions of the B212 engine.The most important changes are the wider scope forfault diagnosis made possible by a new control mod-ule and the substitution of a throttle position sensorfor the throttle posit ion switch.The lAC valve is also of new design.

Saab 900