L-jetronic page

Let us begin...

This the block diagram of the functions inside an L. The rings on the lines are connections across the pcb's. The pictures of the lower and the upper boards will help in finding the resistors.

This all is based on my own experiments, everybody proceeds at their own risk. Blowing up an ECU is possible but not obligatory.

I'll start from the beginning. The ignition pulses are taken into the ECU from the primary side of the ignition coil. They are scaled so that the peak is ~7V.

These pulses are then modified so that the rev-limit can be imposed by R106. The effect of the resistor is seen in the picture below. If the time of the pulses remains constant, than at certain revs the voltage no longer gets up, and the rev-limit has been reached. If the R106 is smaller, the time of the pulse decreases and the rev-limit raises.

These pulses are then subjected to a divide-by-2 network, giving the timing frame after which the L times the injection pulses. One pulse/one revolution of the engine.

Then according to these pulses the low-rpm dwell-limit is formed. When the voltage reaches maximum, the basic injection time can't grow longer.

The smaller the R103, the higher the limitation on the low-rpm dwell.

After this, the basic injection time thus limited is formed.

It is made according to the voltage measured from the air-flow meter. This voltage is put on the rail of the meter by applying the battery voltage to the rail. The voltage is also corrected for fluctuating battery voltages. The voltage swing is limited during cranking by R237 and R239 to compensate the back-firing and sudden collapses in manifold vacuum.

This signal is then brought to the upper board of the L-jetronic. The off-idle enrichment, post-cranking enrichment and and coasting-cutoff are on this board. Off-idle and post-cranking only work below a R616 specified level of resistance on the coolant temperature NTC.

Some enrichment is also imposed by the temperature limit itself. All the effects of this board are added to the amplitude of the basic timing signal.

This signal is then brought back to the lower board and is used to charge a capacitor. The capasitor is then discharged according to coolant temperature, wot switch state and intake air temperature. The WOT enrichment is fixed at ~9%.

The injection time so gotten is then subjected to voltage correction needed to compensate the different injector opening times. This is governed by R345.

The timing has now been figured out and the injectors are opened with a ~4Amps/injector. After ~.5 mSecs the total current is held at a current level determined by R408 (Typ. ~2A).

The total is as follows:

Engine warm, no WOT, Y is Dwell, X revs and curves percentage of air-flow openings. The exponential behaviour corrects the logarithmic behaviour of the air-flow meter flap.

This page will keep on changing but it is always going to be the page for ECU mods. Any additional info or test results etc. you would like to discuss or share, email k124775@ee.tut.fi .

some mechanical data and mods is also available from yours truly, and the link back is here.