This time, quite short entry about management unit of Siemens MS41. In case, if someone will get to work with this DME “grandpa”.


1. For DME, only 2 Lambda probes are intended. Narrowband. With the supply of 12 V for heating.

In the pre-heat phase: heating around 20 %, in warmed-up mode: 70 .. 100 %.

2. The representation of the voltage of the probes is more interesting. MS41, unlike the other DME’s, uses Lambda probes of titanium dioxide, which “behave” differently than ”regular” probes.

They are heated as a “regular” zirconium probes. The difference is in the active element: the element of titanium dioxide probe changes the internal resistance depending on fuel mixture (Lambda value).

  • if the mixture is lean, the internal resistance of the probe element is high;
  • if the mixture is rich, the internal resistance of the probe is low.


The internal resistance of (warmed-up) probe typically changes in the range from 100 K Ohm (lean mixture) to 100 Ohm (rich mixture).

To “see” the internal resistance of the probe, its output is connected to the power source is connected to +5.0 V supply voltage. Accordingly:

  • if the mixture is rich, the voltage will be close to 0.0 V (below 0.5 V);
  • if the mixture is lean, the voltage will be close to 5.0 V (above 4.0 V).


Accordingly – the “polarity” of the probe voltage is opposite as for zirconium probes.


Here, the dependence of internal resistance of titanium dioxide Lambda probe vs Lambda:


In case of the stoichiometric mixture (the same as in case of zirconium probe), the voltage in the input of DME periodically will switch from “rich” to ”lean” and back.

If DME uses titanium probes, you have to take into account the following nuances:

  • if/when the probe is cold and the engine is turned off (the oxygen is in the exhaust), voltage, measured by DME will be close to 5.0 V. If the voltage is decreased for a cold probe – it (or its wires, plug) has current leakage. Unfortunately, if the titan probes are used, DME has no way to check “health” of connection and chemical efficiency of the probe. If the probe (or its wires) has current leakage, DME will consider, that the fuel mixture is rich;
  • if the probe has poor chemical efficiency (internal resistance cannot “drop” to the necessary value), voltage, measured by DME, will stay increased (the probe will indicate lean mixture).

In the image above we see: DME has reduced the amount of fuel mixture, injected in the cylinders of the first bank very significantly (taking into account offset type LTFT and value of integrator), but the voltage of the probe is around 2.0 V (has not increased to value, confirming the lean mixture).

Also, for the cold probe condition (by the turned-off engine) periodically decreased voltage measured by DME was observed, which confirms – the probe (or in wires, plug) has a current leakage.


“Health” test of the titanium dioxide probe:

  • in overrun mode (braking with the engine, in time when no fuel is injected in cylinders), the voltage measured by DME should be close to 5.0 V;
  • in kick-down mode, the voltage of the probe has to drop close to 0.0 V. If the voltage doesn’t drop at all or don’t drop close to 0.0 V (by condition there is a reason to believe the fuel mixture is truly rich), the chemical efficiency of probe is insufficient.