Diagnostics of MS43

Time ago, MS43 was a very popular control unit of petrol engines. In this entry, I will describe some nuances of diagnostics for MS43.

The menu of basic parameters of the engine:

Correct values.

Cold engine, running idle (around 0 oC):

  • consumed amount of air: 35 .. 40kg/h;
  • opening time for injectors: 5 .. 7ms.

The engine in work temperature:

  • consumed amount of air: around 12kg/h;
  • opening time for injectors: around 2ms.

The menu of electric accelerator and throttle:

PGWA: defined position of accelerator pedal;

PGW voltage 1: indication of the first potentiometer of accelerator pedal;

PGW voltage 2: an indication of the second potentiometer of the accelerator pedal.


To control the position of position of the accelerator pedal, two potentiometers with proportional indications (it means, that indication of the first potentiometer is two times higher than the indication of the second potentiometer) are used.

If the indications of sensors are correct:

V1 = 0 .. 4V

V2 = 0 .. 2V

V2 = V1/2

The position of PGW is proportional to indications of sensors.

At idle (correct values): V1=0,7V; V2=0,35V


In case, if the indications of sensors don’t correspond, an appropriate error message is registered, and thereafter the position of the accelerator is ignored.


DKP: indicated position of throttle;

MDK voltage 1: indication of the first sensor of throttle position;

MDK voltage 2: an indication of the second sensor of throttle position.

The position of the throttle is controlled by two position sensors with the cross-type curve, it means, those indications are inversely proportional.

V1 = 0 .. 5V

V2 = 5 .. 0V

V2 = 5 – V1

The opening of the throttle is proportional to indications of the first sensor. In idle, the first sensor indicates 0,5V; when the throttle is fully open: 4,5V.

If the proportion of sensor indications is not complied with, MSD43 records an error message regarding throttle and in future don’t use these indications (even, if the status of the error message is passive).


How to check the accelerator pedal:

smoothly press and release the accelerator pedal, indications of sensors should be proportional to the angle of pressure, additionally, the indication of the first sensor should be two times higher than an indication of the second sensor. Example: 0,5/0,25V; 1,5/0,75V, etc.


How to check throttle sensors:

observe the indications of sensors, while driving. Indications should be inversely proportional, for example, 1,0/4,0V; 1,5/3,5V, etc., the position of throttle  – proportional to indications of the first sensor.

In the situation, when the fault of throttle valve indicator is detected, MSD43 stops to use data from these sensors. From this moment, MS43 defines the position of the throttle, using measurements of air mass meter, for control of fuel mixture – Lambda probes.

We have to understand, that in case of a cold engine, for first few minutes:

  • Lambda probes are cold, no data from them are available;
  • air mass meter data also are not available – it has to be heated up.

As a result, for first few minutes, the engine runs in practically uncontrollable mode!

Moreover, if the error messages regarding throttle valve are present, both offset and multiplicative fuel adaptations are switched off, also individual adaptations of cylinders in idle are not available.

As a result, error messages and real misfires are very common, the abnormal performance of the engine (especially during first minutes). When the engine has warmed up, it continues to vibrate in idle (because the equalization of cylinder efficiency does not works), also fake error messages regarding fuel trim, Lambda probes, CO catalytic converters are registered. Of course, the replacing of these components does not solve the situation.

VANOS menu. I believe, particular explanations are not needed, regarding hidden defects of VANOS read here.


Lambda probes, menu No.1:

two bottom lines: heating of probes. MS43 performs the heating very simply, the temperature of probes is not being stabilized.

Two upper lines: probe voltages.

The left side of the menu: probes before CO catalytic converters, right side: probes after CO converters.

For the warmed up engine, idle or driving smoothly – voltage of probes (before catalytic converters) approximately once in a second has to change from:

  • voltage less than 0,1V;
  • voltage more than 0,7V.

If changes of voltage are significantly less often – the probe has aged.

If voltage persists 0,1V for a long time – the fuel mixture is lean.

If voltage persists 0,7V for a long time – the fuel mixture is rich.

The right side of the menu – probes after CO catalytic converters. Correct voltage values of these probes are equal to probes described before. Differences:

  • if CO catalytic converters are with low efficiency, these probes repeat (with very small time delay) signals of probes, located before CO catalytic converters, of the corresponding bank;
  • if CO catalytic converters are with good efficiency, control probes mainly perform in area 0,7 .. 0,8V.

Min/max voltage of control probes can be evaluated conveniently:

  • voltage has to be at least 0,7V when accelerating sharply (if the enriched fuel mixture with a voltage around 0,7V is confirmed before CO probe);
  • voltage has to be not higher than 0,1V in overrun mode (performing braking with the engine).


Menu for fuel adaptations:

From the top, both banks:

  • integrators;
  • offset type adaptations (idle);
  • multiplicative adaptations (partial/full load).

More about STFT and LTFT read here.

Important: if for the warmed up engine, any of the menu data groups is 0, it means: MS43 work in EMERGENCY mode and does not perform normal fuel correction!

To improve the situation: problems, mentioned in the error message list, has to be eliminated, the error message memory has to be deleted (cleared). 

After this, when new driving session will be started (MS43 power supply voltage: on/off), with the condition, that MS43 has not registered new error messages, it immediately (in first few minutes) creates offset adaptations for idle, integrators will correct fuel mixture: performing driving sessions, multiplicative adaptations will be created, but in idle – the performance of cylinders will be gradually equalized (rough run menu will display smaller and smaller cylinder performance unevenness).


Rough run menu:

These ARE NOT data of cylinder adaptation! These are measurements of mechanical efficiency of cylinders (measured, detecting changes in flywheel speed).

The larger the number, the relatively smaller the cylinder mechanical efficiency. 0 reference value – most efficient of all cylinders.

Notice: if the engine runs in EMERGENCY mode, it (also) doesn’t equalize the cylinder performance in idle. Accordingly – large unevenness of cylinder performance (which is not being equalized MS43) shows that the engine performs in emergency mode.


Menu for Lambda probe test:

../F6/F3/F5 (Activate/system test DK/lambda probe system test)

Two bottom lines: Nernst cells resistance of Lambda probes.

Correct values: starting from 15 .. 20 Ohm for probes of old releases to 200 .. 300 Ohm for probes of newer (modification) releases.

Unfortunately, increased (around 200 .. 300 Ohm) Nernst internal resistance can indicate a probe that has aged or is from the old release.

Two upper lines: min/max value for each probe, which is reported after opening the menu.

Correct values:

Umax 0,7V (at least);

Umin 0,1V (not more than).

More about Nernst resistance of Lambda probes, diagnostics of probes you can read here and here.


General relations for Siemens managing units:

  • if the ONLY functional problem, for example, misfires, fuel trim problems, is detected, the engine will restore normal performance, when the status of the error message changes to passive (typically: after starting the new driving session);
  • if abnormal performance of any sensor or mechanical actuator (for example, VANOS, throttle etc.) and appropriate error message is recorded, managing units of MS series try to “bypass” current know, the engine performs in emergency mode regardless of error message status, and normal performance of engine is restored ONLY after solving the problem AND deleting error messages. Besides, according to the status of the error message (passive or active) “emergency” mode can vary (and it does vary)!

Note – managing units of MS series have not only one “emergency” mode, but many alternative performance modes, which try to ensure much more comfortable driving even if any knot is damaged. More about error messages you can read here.

Important to comply – in case of problems, at first all problems of knots/sensors should be eliminated, obligatory: delete the error message memory after fixing the problem – restore normal performance mode. Only then evaluate the functionality fails/problems (misfire, fuel trim problems etc.).