MSD85. Misfire detection

MSD85 DME was used for management of N63 engines and is common in models F01, F02, F03, F07, F12, and F13.

At first: the oscillogram – let’s see, did the Siemens + BMW AG has taken into account the problems, which are typical for High energy ignition system.


As we see – no they haven’t… In the image – closing moment of the IGTB of the ignition coil management. During several us, U at IGBT’s Collector reach 320V, transistor opens for several times (in self-protection mode) – it works in  voltage limiting mode. As a result – a large amount of energy is released in the heat (heating up the transistor, not creating a spark), the transistor regularly works in conditions of overload. The only aspect, which improves a situation a bit: the transformation coefficient is not 1:100 (as for N3/N53 series engines), but only 1:50. Accordingly – the voltage in the secondary winding is 2 times less – the overload of the ignition coils is smaller. But the overload of the low-voltage side IGBT switches – consistently high.


This DME has one and powerful feature: it can measure ignition time. As if nothing complicated – the resistor from the ignition IGBT Collector to MCU Input, and – it’s done. Yes, but each such measurement requires an additional resource of the processor. So – MCU measures, how long the arc discharge (the spark is “active”) of each cylinder lasts. This is a very efficient misfire detection algorithm, which works by high RPM, large required torque – in any driving conditions. Also – in fast variables!

The absolute values of the misfire counters are available with ISTA and a full set (both actual and middle session) – in OBD Mode 6. OBD Mode 6 – nothing new and/or interesting, everything works as it is supposed to. So a bit more in detail only regarding the misfire counters, which can be seen, using ISTA.

How to use these counters?
1. before the driving session memorize (write down) value of misfire counter of each cylinder;
2. perform a driving session;
3. compare the “new” and recorded values of the misfire counters.


An example.
Before driving session:


After driving session:


The result:

Conclusions: in this driving session, the cylinder No.6 dominates, it has to be paid attention to.


Nuances, which have to be taken in the account:
1. misfire counters, reported by ISTA, are not deleted between driving sessions (there is no information, when and how they are reduced/deleted). It has to be taken into account when repairing the engines, managed by these DME. Also after a successful repair, the misfire counters continue to indicate high (old one + diff.) values. So the increase of the misfire events has to be calculated for the exact driving session.
2. When recording the error message regarding misfire in any cylinder, DME takes in the account the internal counters, which are known only to the DME itself. They include also the long-term data. Accordingly – even after a successful repair the previously damaged cylinder repeatedly (unjustified) will be marked as damaged even in a situation, if/when it will perform perfectly (and it will have only several misfire events – less than for other cylinders). So you have to remember – the increase of the misfire events has to be checked, not follow blindly the error messages regarding misfires, recorded in the error message memory of the DME. After repair, the engine has to be re-adapted, several short driving sessions have to be performed (for the between-session counter to reduce to more truthful values); if the false error messages regarding repaired cylinders are still recorded – they have to be deleted, not trying to repair unexisting problems.