Motronic 5.2 (M44)
by Brian Brown
The DME unit (the 'brain box') for '96 and newer 318's with the 1.9 liter M44 engine is a Motronic version 5.2 system.

The main functions of the Motronic system are to control the fuel injectors and spark plug firing. It also regulates the idle speed, intake manifold crossover selection, evaporative emission controls, air conditioning compressor, and performs OBD II and other diagnostic functions. Additionally, it communicates and interacts with the traction control system (ASC + T), automatic transmission control module (EGS), dashboard instrument cluster, electronic immobilizer control module (EWS II), cruise control module (TEMPOMAT), on board computer (OBC), and integrated heating and climate control module.

Basically, the system takes in information about the environment and the operating conditions of the engine, takes its best guess at the correct amount of fuel and spark timing for the situation, and listens for engine knock and sniffs for unburned gas in the exhaust so that it can make instant corrections to the amount of fuel and spark timing. While all of this is happening, it also keeps track of how good its 'guesses' were. If the 'guess' for that particular situation was off, it readjusts the guess that it will try for the next time that situation comes up.


The amount of air that enters the engine is regulated by the throttle, which is basically a plate shaped valve that opens and closes to restrict entering air. The throttle is directly connected by a cable to the throttle pedal (commonly known as the 'gas' pedal - it probably would be more accurate to call it an 'air' pedal). This is the fundamental control that the driver has over the entire engine 'system'.

The other factors that determine how much air enters the engine are: Air pressure. This is what 'pushes' the air past the throttle and into the engine (less at higher altitudes). Engine RPM. The faster it spins, the more air it will try to displace. Engine displacement. (A fixed value). Controls how much air the engine will try to displace with each revolution. Intake impedance. This represents the restriction to the air pulses passing through the intake manifold, cylinder head ports and valves as the pulses try to enter the cylinder. (varies with RPM and throttle position).

Supplemental air regulation:

Idle control valve: This is an electrically controlled valve that bypasses the main throttle. The Motronic system uses this to self adjust the idle speed.

Intake air resonance changeover valve: The intake impedance is affected by the length and diameter of the passages of the intake manifold (this is touching on another subject). Longer, narrower passages favor low RPM performance. Shorter, wider passages favor high RPM performance. To help performance over a wider RPM range, the M44 intake manifold has two separate primary passages: one longer and narrower, one shorter and wider. The intake air resonance changeover valve switches from one to the other. The changeover occurs on the M44 at about 4200 RPM.


The Motronic's job is to match the amount of air that enters the engine with the correct amount of fuel. (More on how it figures out below).

To actually control the fuel, it can turn on the fuel pump (actually two pumps in series - one to get the gas out of the tank, and one to build up pressure) and pulse the injectors.

The fuel pump is turned on briefly when attempting a start. For the pump to continue to be powered, the Motronic system must detect that the engine is running (the pump will otherwise be disabled for safety).

The fuel goes from the pump through the fuel filter to the fuel rail, which is basically a rectangular pipe that the fuel injectors connect to. At the end of the fuel rail is a pressure regulator that bleeds off excess pressure and sends fuel back to the tank (this circulation helps keep the gas cool and well mixed). The pressure regulator has a vacuum hose from the intake manifold that helps the regulator fine tune the fuel pressure. The idea is to keep the pressure differential between the fuel and the inside of the intake manifold constant so that the Motronic system doesn't have to worry about it. Otherwise, more fuel would be sprayed when the manifold was at a higher vacuum.

An injector is either on or off. This is a 'sequential' injection system - meaning that the injector spray corresponds to the opening of the intake valve, rather than spraying continuously. When it's time for the injector to spray fuel, the Motronic system rapidly opens and closes it at a high fixed frequency. The rate that the injector sprays fuel is determine by the ratio of the time that it's held open during this high frequency operation, vs the time that it's held closed (also referred to as the 'duty-cycle'). The greater the percentage of open time, the more fuel will be sprayed.


There are four separate ignition coils (integrated into a single module), one for each spark plug. This eliminates the need for a distributor cap and rotor (less to wear out, and more powerful currents can be used). The Motronic system can adjust the timing of each spark plug separately.


Evaporative Emissions Valve (allows gas vapors to be drawn into the engine and burned off when the engine is running). Oxygen Sensor heaters (preheats oxygen sensors so they will be operational sooner). At Speed relay (unloads accessories from the engine when starting). Air conditioning compressor relay.


To observe the outside environment and the general operating condition of the engine, the Motronic has the following sensors available: Intake air temperature sensor. Engine coolant temperature sensor. Air conditioning pressure switch. Engine temperature switch.


To determine the current operational state, the Motronic uses the following: Throttle position sensor (can be used in conjunction with RPM's to calculate engine vacuum and load). Hot Film Air Mass Meter (measures air entering engine - compensates for varying densities and temperatures). Camshaft position sensor (used to synchronize the injectors and spark plugs to the engine). Crankshaft sensor (sends pulses to determine engine speed and acceleration - one pulse is missing to indicate top dead center).


The following sensors are used to determine how well the system is doing and to make fine-tune adjustments:

Oxygen Sensors: (two - one before and one after the catalytic converter. This allows monitoring of the catalytic converter performance.) The presence of oxygen in the exhaust indicates that there is no unburned fuel. The lack of oxygen in the exhaust indicates that there is unburned fuel. To get a precise balance of air and fuel, the Motronic system rapidly tweaks the amount of fuel up and down trying to center it around the point that the oxygen signal barely comes and goes away.

Knock Sensors: (two - one for cylinders 1 and 2, the other for cylinders 3 and 4). These are basically microphones. The Motronic system uses these to 'listen' to the sounds inside the engine. It is able to distinguish the sound of ignition knock from all of the other bumps, clanks and rattles going on. It can detect knock at very low threshold levels. It uses this information to tweak the spark timing just below the point that knock occurs, optimizing performance and economy.

The crankshaft sensor also falls under this category because it can be used to measure the acceleration after every cylinder firing.


I described these in a previous post.


It is desired that the correct amount of fuel to match the incoming air be sprayed into the engine. Generally there shouldn't be any extra air, or any extra fuel. Under light loads, the system will try to go slightly lean (extra air). Under heavy loads, the system will try to go slightly rich (extra fuel).


The sooner the spark is fired before the piston reaches the top (spark advance), the higher the resulting combustion temperature and pressure will be (a similar effect to increasing compression ratio). This will also allow a slightly greater time for the pressure to be applied to the piston. All of this results in more power and better fuel efficiency. CO and HC emissions are reduced because combustion is more complete. If the timing is advanced too much, however, an uncontrolled explosion will result (engine knock). This can cause engine damage (piston melting) and will increase NOx emissions because the extremely high temperatures will start to oxidize the normally unreactive nitrogen in the air. It is very desirable from performance, economy, and emissions standpoints that spark timing be advanced to just below the point where knock occurs.


When trying to determine the spark timing and fuel requirements, the Motronic system reads information from the general and operating sensors and then goes to some big look-up tables (or Control Maps) that indicate the best guess for spark and fuel at that moment. The feedback sensors are then used to tweak the spark and fuel values towards an optimum. If a big 'tweak' was needed, then the firmware will change the value in the look-up table with the hope that it will be closer the next time that data point is used (Adaptive Tuning). All of this process is continuous, and it cycles and repeats at a high frequency.

The Motronic system contains a 'base map' that is copied over to the active control map when the system loses power or is otherwise re-initialized. This is useful if there was a malfunctioning component or something else that caused the active map to get messed up.

How long does it take for adaption to fully readjust the map after a change? Hard to say.

Some points on the map will be updated fairly quickly. Others might take a long time.

Remember that the map is fairly big. For a data point to be updated, it has to be used. To go through the whole map would require the engine to go through all the combinations of temperatures, loads, warm-up cycles, etc.


Motronic 5.2 is a FLASH memory based system. Unlike earlier systems that stored their code on an EPROM chip that needed to be physically replaced, this system can be downloaded through the diagnostic connector.


Generally speaking, a performance upgrade doesn't involve changing the software algorithms, only the data base. Usually an algorithm change would only come from BMW as a bug fix or an operational enhancement.

Changes to the data base include changes to the points in the base map, extending the limits of how far away from the base point a parameter can be adaptively changed, and the rate that adaption will adjust things.

There are three potentially useful changes that can be made for an otherwise stock engine: Increasing the amount of spark advance (requiring the use of premium fuel). Extending the rev limit (there's some more power available at the top end, but it will increase stress on the engine somewhat to use it). Making a *slight* adjustment to the fuel curve.

An engine that has any significant mechanical modifications should have custom mapped firmware.


Between OBD - II security issues, and because BMW regards the actual code for this system to be highly proprietary, aftermarket resources for Motronic 5.2 have been slow in coming. It appears that Jim C. will have well reverse-engineered offerings for this system (hopefully soon). The actual performance and control that this system is capable of is excellent, we just need some more flexibility to reconfigure it.

Regards, Brian Brown. BMWCCA #130878 '96 318tiS
September 17, 1999