the L-Jetronic is an electronically steered fuel injection system of a petrol engine.

Table of contents


fuel way

the fuel is promoted by the fuel pump by means of a filter to the fuel distribution pipe. The fuel distribution pipe runs parallel to the cylinder head. At this pipe the Einspritzventile and a pressure control valve are attached. The pressure control valve keeps the system pressure constantly, dependent 2,5 bar on the suction tube pressure. A vacuum piping, those behind the butterfly valve is attached passes on the pressure control valve this information. This serves to keep the difference of pressure between fuel pressure and suction tube pressure always constant. Thus the Einspritzventile can determine the amount of fuel only by their opening duration. The Einspritzventile are not purely mechanical, but this concerns electromagnetic valves, which are headed for by the controller. This connects through only mass, since the plus supply rests against the valves. The installation position of the valves is as with the K-Jetronic about 70 - 100 mm from the intake valve removes. The spraying cone has an angle of 25°. The opening duration amounts to with engine and idling speed at operating temperature about 2.5 ms. With older vehicles still another cold weather starting valve is attached to the fuel distribution pipe, which serves for the enrichment of the mixture with the cold weather starting and is steered via a Thermozeitschalter. This cold weather starting enrichment is taken over with newer vehicles by the controller. This leaves the Einspritzventile longer opened during a fixed period.

air intake flap amount of air measurer

of the air intake flap amount of air measurers is similar in his principle to the amount of air measurer seals of a KE-Jetronic. A spring-tensioned flap is opposite the air flow and must be shifted by this. A potentiometer attached on the flap wave is rotated thereby. With different flap positions the potentiometer delivers different tensions and indicates to the controller in such a way one the sucked in amount of air appropriate size. For the compensation of the pressure fluctuations produced by the sucking in document a compensation flap is inserted, which is likewise fastened on the wave described above. Both flaps form a construction unit. In the sucking in range of the amount of air measurer a temperature-dependent resistance is inserted, which measures the temperature of sucked in air. At the lower surface of the amount of air measurer a screw is attached with different, usually older vehicles. This screw tapers a channel in the amount of air measurer, that gives the possibility to not measured air to arrive in the suction tube. If this screw is rotated, then the cross section of the channel will change. If it becomes larger, then knows more air at the air intake flap past and the air intake flap falls more into the air flow. If the cross section becomes smaller, can go around less air the amount of air measurer and the air intake flap is more raised. Thus changes the voltage level, which the controller measures at the potentiometer and fuel will more or less measure out.

coming off the injecting signal

the controller receives number of revolutions information from clamp 1 of the distributor. Crankshaft angles of rotation of four injecting signals are present per 720° (4-Zylindermotor). In the impulse molder of the controller these four signals are converted into square wave signals usable for the controller. The frequency divider downstream halves the impulse number. The signals developed in such a way are transformed now in the division tax multivibrator into the injecting time base. With beginning signal lets a tension rise the DSMV. At the end of the signal the tension is slowly again diminished. This dismantling phase is affected by the tension of the amount of air measurer potentiometer. Depending upon sucked in amount of air the dismantling phase becomes longer or shorter. The injecting time base is worked on now in the multiplying stage. Here two correction times are in addition-given or taken off. These correction values depend once on the intake air temperature, the engine temperature and the butterfly valve position and on the other hand at the current battery voltage. The injecting signal developed in such a way is strengthened in an output stage and passed on to the Einspritzventile.

Summarized this means:

  • The clock frequency of the Einspritzventile is determined from the engine speed.
  • Number of revolutions and sucked in amount of air determine the injecting time base.
  • The multiplying stage extends the injecting time base according to information of the sensors and the battery voltage for the actual opening time of the Einspritzventils.

mixture correction

cold weather starting

  • with older vehicles over Thermozeitschalter and cold weather starting valve
  • with newer vehicles by means of controller and Einspritzventil

warming-up phase

  • the warming-up phase by the no-load operation trick plate realized

load changes

  • of the butterfly valve switches gives information to the controller and this extends the duration of injection

change of the intake air temperature

  • recognition over the intake air temperature resistance. The controller reacts with extension of the duration of injection.

number of revolutions delimitation

  • the controller gets the number of revolutions information from the distributor and takes the duration of injection back

to Schubabschaltung

  • recognition over number of revolutions and butterfly valve switch controller stops injection up to a certain number of revolutions

Lambda regulation

  • recognition by a Lambdasonde. Duration of injection is changed accordingly.


the basic structure of a LH-Jetronic is equal to a L-Jetronic. Not as with the L-Jetronic the amount of air is measured to separate air mass. Further an activated charcoal filter is built in the tank ventilation, which reduces the Kohlenwasserstoffemission from the tank. Thus the LH-Jetronic fulfills the Euro-2-Norm. The fuel pump is integrated into the tank.

activated charcoal filters with regeneration valve

over the tank ventilation can arrive starting from a certain temperature volatile hydrocarbons into the atmosphere. In the Euro-2-Norm this emission was set on a certain limit value. In order to keep this, an activated charcoal filter was built into the tank ventilation. This absorbs the escaped hydrocarbons and lets only pure air into the atmosphere escape. A regeneration valve is platziert between the activated charcoal filter and the syphon of the engine. The valve opens under certain conditions and makes a regeneration possible of the activated charcoal filter. Will sucked fuel steams over the regeneration valve into the syphon, arrive then into the combustion chamber of the cylinders and to become there burned. Since the controller cannot recognize the composition of air sucked in over the regeneration valve, it is quite possible that the mixture becomes too fat or too lean. This is recognized over the Lambdasonde and by the controller balanced. When turning the engine off the regeneration valve is kept long closed some seconds, in order to prevent a Nachdieseln of the engine.

air mass measurer

with the LH-Jetronic can be blocked either a hot wire air mass measurer or a hot film air mass measurer.

basic principle

of 1000 litres of air have with 0° C and an air pressure of 1013 hPa a weight of 1,29 kg. This value corresponds to the mass of air. Changes of temperature, in addition, changes of air pressure affect the mass of air. This is because of the fact that gases fill out the area the available fully. The individual molecules and atoms have grösst possible distance from each other. Now if the temperature rises, the independent movements of the air particles rise and they repel themselves further from each other. The air mass of 1000 litres of air sinks. If the temperature sinks, then the particles move less and the distance among themselves is reduced. Air mass rises. If the pressure rises, then the particles are squeezed together on a smaller area and air mass rises. The air pressure sinks in such a way also the air mass of 1000 litres of air sinks. If one lets air at a beheitzten wire flow past, then the wire is cooled down depending upon air mass differently much. More particles cool more, fewer particles cool less. If one tries to keep now the temperature of the heating wire constant, then the necessary power input results in a measure for moving by air mass.

This principle is used in the hot wire air mass measurer.

function of the hot wire air mass measurer

behind the air cleaner box is appropriate a pipe, in which a measuring tube is attached. In this measuring tube a 0.07 mm thin platinum wire is so appropriate that it is washed around well by flowing through air. Over a Wheatstone bridge connection the temperature of the hot wire is kept constant 100° C over the sucked in air temperature. A compensation resistance measures thereby the intake air temperature. Those measured variable is actually the voltage drop at the laboratory resistor of the circuit. This laboratory resistor has a high accuracy. A further important construction unit of the bridge connection is an operation amplifier. The measurement voltage drop at the laboratory resistor takes place 1000 time per second via the controller. This computes then with consideration of the other sensors the exact duration of injection.

function hot film air mass measurers

with the hot film air mass measurer the platinum wire by a sensor plate was replaced. The plate consists of three different resistivity layers with different functions. These three layers consist of different materials, which became on a Keramikplatte in Microhybridtechnik vapour-deposited. It concerns

  • the balancing resistance. It serves behavior for the collection of the air temperature of sucked in air and shows PTC -.
  • the heating resistor. It does not serve only as heating and has a sensor function.
  • the sensor resistance. It serves sucked in air mass for the collection. Depending upon air mass it is heated more or less by the heating resistor and changes thus its resistance (NTC - behavior).

The hot film air mass measurer offers an advantage opposite the hot wire air mass measurer. The Keramikplatte is in such a way in the measuring tube arranged the fact that air does not hit on it but at it to past flow can. Smallest particles are not taken up no more by the sensor unit and cannot festbacken also there no more, as this was with the hot wire the case. The life span was increased thereby. However the measuring accuracy is no longer high as as with the hot wire air mass measurer. Nevertheless it is still high enough, in order to ensure an optimal functioning of the plant.

The hot wire and hot film air mass measurer have to do with the same problem, as the air intake flap amount of air measurer. By the Druckpulsationen of the air column in the no-load operation it is quite possible that the same air mass is several times measured. This e.g. becomes. by a bypass channel prevents. In this channel the measure is integrated. Also air mass measurers with current reversal recognition were blocked.


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