Jetronic

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Jetronic is a form of engine management, responsible for controlling the fuelling and occasionally the ignition of early E30 engines. It was superceded by Motronic in 1987.

Overview

Jetronic is an analogue engine management system which uses a number of sensors to measure the condition and performance of the engine in order to manage fuelling, all controlled by a simple ECU. Ignition timing is governed by a mechanical distributor, which was regulated by a number of mechanisms to advance or retard timing. The ignition timing is completely separate from the fuelling, unlike the later Motronic management which combined these two features.

A number of Jetronic versions were fitted to the E30 during its build. The earliest models, from late 1982 to early 1983, were equipped with K-Jetronic; a purely mechanical fuel injection system with a rudimentary transistor-based ignition. This technology had been carried over from the E21, who shared the same M10 and M20 engines with the E30. However, none of these models were available in RHD, so very few are seen on UK roads.

LE-Jetronic quickly replaced K-Jetronic, and is the most commonly used Jetronic system in the E30. Often referred to as simply L-Jetronic, the E stands for Economy and highlights the system's focus on fuel efficiency over power. The system went through a revision during its production years, meaning that a variety of components were used to solve issues such as idling and cold starting.

While Jetronic was originally a very practical system, decades of use is rendering Jetronic setups more and more inefficient and unreliable due to their reliance on mechanical and vacuum-driven components which can wear out over time. For that reason, we recommend converting to Motronic 1.3.

Versions

K-Jetronic

M60 engine with K-Jetronic

K-Jetronic is a mechanical fuel distribution system, and does not need an ECU to control it.

The basic concept is that a fuel distributor is charged with fuel from the fuel pump, up to 5 bar. From there, individual pipes run to each cylinder where a spring-loaded nozzle acts as injectors. The fuel distributor is directly connect to an air intake meter; an air flap that rises and falls with the flow of air into the engine. As the flap rises due to increased air flow, it also opens up the flow through the fuel distributor allowing more fuel to be pumped through to the spray nozzles. When the fuel pressure is high enough to overcome the springs in the nozzles, fuel is sprayed into the engine.

K-Jetronic systems can therefore be identified by the snake of pipework running over the intake manifold, and the large mechanical fuel distributor bolted to the left hand inner wing in the engine bay.

M20 engines fitted with K-Jetronic were known as M60 engines, not to be confused with the later M60 V8 engines.

LE-Jetronic

LE-Jetronic was the first engine management system for the E30 to feature an ECU. The analogue brain's primary function was to take readings from the air flow meter, and to control the fuel injection accordingly. Unlike K-Jetronic, this system took readings from sensors around the engine and made its fuelling accordingly. However, ignition was still driven by a mechanical distributor, which depended on vacuum power to advance or retard the timing depending on the speed of the engine.

LE-Jetronic is found on all 323i models as well as some early 320i cars; it was replaced by Motronic with the introduction of the 325i in 1986, but could still be found on M10-equipped 318i models until 1987.

Components

The following components are specific to Jetronic-equipped vehicles. The information here supercedes any other details of components outlined in the Wiki, especially regarding Ignition, Fuel and Intake.

Distributor

All Jetronic versions used a mechanical distributor for ignition timing, which was driven directly from the crankshaft via an intermediary shaft. The distributor is placed on the engine block on the intake side.

Ignition timing was set via vacuum. Hoses from the throttle body fed a timing regulator connected to the distributor. As the vacuum changed with engine speed, the rotor arm would be pulled on a spring to a different position, advancing or retarding the spark as required. For vehicles with automatic gearboxes, the system was slightly simpler as no retard function was fitted.

Along with the air slide valve, the distributor is responsible for most problems with the Jetronic system related to poor running. Specifically, it's quite common for the rotor spindle to be seized with rust, so ensure the rotor arm can be turned a few degrees before springing back.

Fuel Distributor

The fuel distributor was a feature of K-Jetronic. Operating similarly to a diesel fuel pump, this pressurised mechanism pumped fuel to each injector in turn, based on the position of the air flow vane in the air flow meter. The distributor was pressurised up to 5bar by the external fuel pump.

Ignition Control Module

The ignition module is a primary Ignition component for L-Jetronic systems. Two units exist, made by either Siemens or Bosch.

The ignition coil sits between the coil and the distributor in terms of wiring, and is bolted to the right-hand inner wing within the engine bay. Its function is to control the timing of the spark for the distributor.

It is very rare for the ignition module to fail, although after all these years problems do occur. Because the module is a sealed unit, it is very difficult to check it thoroughly, but there are some basic tests you can do to ensure that it is getting a supply. On the Siemens unit, remove the large round 8 pin plug and check for voltage (ign on) between pins 3(+) and 6(-). Pins should be numbered inside the plug. On the Bosch unit, check between pins 4(+) and 2(-).

If you dont get voltage, you need to check your wiring. The negative pin ( 6/Siemens or 2/Bosch) should show continuity to ground and the positive pin (3/Siemens or 4/Bosch) should show continuity to the 15 (green wire) on the coil.

Temperature Time Switch

The Temperature Time Switch, or Thermo Time Switch, is an early predecessor of the Blue Temperature Sensor. Its job was to measure the temperature of the coolant with regard to the cold start injector. When the engine was cold, the thermo time switch would activate the cold start injector to enrich the mixture; once the coolant was warmed up, the switch turned off the injector to improve fuel economy.

Air Slide Valve

AirSlideValve.jpg

The air slide valve is a water-controlled valve located under the inlet manifold. Fed by two coolant pipes and two air pipes, it is meant to open when cold, and close when fully warmed up. The idea is to regulate the different air requirements of a cold and a hot engine when idling.

Because it is based around a sliding mechanism, it is not uncommon for the slide valve to become sticky over time, or to lock up completely. If this happens, your cold idle will suffer terribly although your warm idle will still be acceptable.

To test the air slide valve, clamp the long hose that comes across the inlet manifold to the idle bypass screw housing when the engine is hot. If the revs drop when you clamp the pipe, the valve is faulty. You can attempt to remove, clean and refit the component, but most often the unit will need replacing, as they are not repairable.

For M10 engines, an electrically-heated equivalent valve was used, which has proved to be more reliable.

Idle Valve

Jetronic cars were also equipped with an idle valve, used to stabilise the engine speed when the throttle is depressed. Unlike the electric ICV in Motronic systems, the Jetronic Idle valve is driven by engine vacuum to hold the throttle position at a fixed position when you let go of the accelerator pedal. On early systems this was 1200rpm, although it was later dropped to 1000rpm for economy reasons.

7th Injector

7th injector located in the middle of the intake manifold

On cars up to 1985, an extra injector was added to help with cold starting. This 7th Injector was located in the middle of the intake manifold, and would be triggered by the ECU if a low reading was given by the temperature time switch. The purpose was purely to enrich the intake mixture during cranking; the injector would be cut off once the engine was running.

ECU Codes

ECU Number System Models From/To
0280 000 310 LE-Jetronic 318i with cat. 1983 - 84
0280 000 318 LE-Jetronic 318i 1983 - 84
0280 000 330 LE-Jetronic 318i 1984 - 87
0280 001 301 LE-Jetronic 320i and 323i 1982 - 84
0280 001 309 LE-Jetronic Later 320i 1984 - 87
0280 001 308 LE-Jetronic Later 323i 1984 - 86

Common Upgrades

Unfortunately, Jetronic systems are not easily modified. As L-Jetronic is analogue based, it is not possible to install a chip to improve the performance of the ECU; for that you would have to convert to Motronic.

However, because of the mechanical distributor, Jetronic systems can more easily be adapted to run Megasquirt, offering more direct tuning for your engine, especially the M20.