INJECTOR PROBLEMS IN THE 2007-2009 BMW 335i

There are already lots of articles on the Internet about this, but I just wanted to add my perspective.

MODEL AFFECTED

Several times, I have received ECMs from the 2007-2009 BMW 335i models. This is the MSD80 model by Siemens. Other numbers include 5WK93608 or 7568842 (7 568 842), depending on the year of manufacture.

ISSUE

The most common problem with these models, which is caused by the ECM, is injector issues. This car uses piezo-electric injectors, driven by crystals (like quartz) that move slightly when an electrical potential is applied, rather than by magnetic movement from coils, as in regular injectors.

CONCEPT

Piezoelectric crystals can be physically altered (slightly moved) by applying an electrical potential. In these injectors, a stack of crystals in series makes that movement more significant. The piezoelectric concept is not new. In fact, piezoelectric injectors are commonly used in many diesel engines.

Applying a voltage to the piezoelectric crystals will “move” them, but the opposite also happens: if they are hit hard and sharply in a short time, they will generate a voltage in turn.

A simple example of this voltage-generating property is the long handheld lighters that generate a spark when you push the trigger to start a gas stove. Those lighters have a piezoelectric piece inside that, when a spring-loaded small hammer hits it when you push the trigger, generates enough voltage to produce a spark in its output gap, igniting the contained butane gas.

For the piezoelectric injectors to work properly, because they consist of a stack of crystals rather than a single crystal, the 12-volt supply from the car battery is not enough to control the movement of the injector’s needle. That is why the ECM contains an internal inverter that generates 80 to 150 volts from the 12 Volts from the battery, which is applied to the injectors through electronic switching inside the ECM.

Here, the bulge in the top part of the ECM is where the internal inverter is located.

Here is where the same inverter is located inside. Shown here is the bottom part of the inverter’s circuit board.

And here it is showing the top part of the inverter’s circuit board

ECM (DME)

The most common failure causing this problem is one or more shorted MOSFETs in the ECM. Relatively high voltages and heat can cause one or more MOSFETs to blow or, more often, short-circuit. Some aftermarket modifications can do the same. To troubleshoot injector problems on these models, first check for aftermarket modifications that may conflict with the injection system, then check the ECM for damaged MOSFET transistors.

NOTE: Many owners opt to replace their MSD80 ECM with an MSD81. The MSD81 was apparently built with the current MSD80 issue in mind. It is completely up to owners whether they want to follow that route. I have no experience with the MSD81 so far, and cannot comment on what modifications or improvements were implemented to avoid this very common problem with these models.

The ECM uses eight MOSFET transistors to switch injectors, as shown in the simplified circuit below. Gossip has it that the MSD81 uses IGBT Transistors instead of MOSFETs. Back to the MSD80, six of those MOSFETs individually switch each of the six injectors to ground (low side), while the other two MOSFETs switch two banks of three injectors each to the supply voltage (high side).

As shown above, each bank of three MOSFETs (for 3 injectors) is monitored individually for proper functioning by a current-sensing resistor. This means that if something happens to any of the three MOSFETs of one bank, the overload will be detected by the current-sensing resistor of the corresponding bank, and the bank will be switched off completely by one of the two high-side MOSFETs, and consequently, the three corresponding injectors of that bank will stop working all at the same time. And yes, if two MOSFETs from different banks fail, none of the six injectors will work, causing a no-start condition.

In the picture below, you can easily identify the MOSFETs. You can see them as soon as you remove the ECM lid. Here is a guide for identification. The MOSFETs in the green square are the 6 low-side switching MOSFETs, the ones in the white square are the bank A and bank B high-side MOSFETs, and the ones in the red square are the ignition coil drivers, where IGBTs are used instead of MOSFETs.

In the image below, the assignment of the MOSFETs for the specific injectors is more detailed:

PREVENTING FUTURE FAILURES

Since this problem is so common, a good idea is to use MOSFETs with better specifications that can withstand the operating conditions. That was supposed to be done when engineering this ECM, but apparently something went wrong. You need MOSFETs with higher current ratings (amperes) and better voltage ratings. Also, you should consider the MOSFET “ON resistance” of the replacement. It can be lower, but never higher than the original MOSFET resistance. The original MOSFET transistor used in the MSD80 is the IRFW644B, and you can directly replace it with the ISL9V5045S3ST, as an example, or the FDB14N30, as suggested in an internet search shown below:

“The original MOSFETs in the BMW MSD80 DME (Digital Motor Electronics) for N54 engines were prone to failure, commonly identified as W644B or similar types, leading to misfires (DTCs 30BA/30BB). These are typically replaced with upgraded, higher-voltage MOSFETs like the FDB14N30 or F644NS, offering better heat/voltage tolerance for reliable injector and coil control in tuned or stock vehicles.”

If you have any aftermarket modifications, make sure they aren’t affecting or causing the problem.

INJECTORS

Check the injectors by measuring their resistance. This resistance measurement isn’t the ultimate test, but it can help you find a damaged injector.

If you measure the injectors on these BMW models with an ohmmeter, you should get a relatively high resistance reading, around 10 kilo-ohms (10,000 ohms) or more. If you see a relatively low reading, like 200-300 ohms, for example, that could mean that the injector(s) in question could be bad. Take this measurement directly at the injector terminal with the harness disconnected to avoid measuring any parallel resistance from the wiring or the ECM.

SUMMARIZING

When troubleshooting injector problems with your BMW 335i, with an MSD80, the first thing to work with is the ECM because in these cases, a shorted MOSFET(s) is the most common problem. Next, check the injectors’ resistance and wiring. Remember that these injectors are switched with relatively high voltage (80V to 150V), and the wiring and the injectors themselves are more susceptible to damage in the high temperature, high voltage, and moisture environments they work, let alone any modification that may conflict with the circuit, like using incorrect injectors.

If the ECM is repaired and everything works well, but the problem returns after a few days, you could have a failing injector(s) or injector wiring issue that wasn’t detectable in the previous resistance tests. In this case, replacing the injector(s) is the next step after repairing the ECM if it has been damaged a second time.