The automotive repair company approached me with a question: “which MOSFET corresponds to which injector”? And sent me this picture:
The picture shows the bottom part of the below (smallest) PCB of the MSD81 management module. As we see, they have already given indicated the transistors A to F.
In addition, pins (Gate and Source) of the transistor C is already pinched – the repair on “full throttle”.
Well, I had to upset them. These transistors are performing completely other functions.
Transistors A/B and D/E create the PWM cascades of the high-side output of the injector management (A and E are switches of negative polarity; B and D are switches of positive polarity). These PWM outputs manage the groups of the injectors (cylinders 1 .. 3, and 4 .. 6).
C and F are management switches for the power supply units, which create a +130V power supply from +12V onboard voltage.
Individual low-side transistors of the injectors are “hiding” on the other side of the board:
Low-side switches – in the sector, circled with red, on the opposite side of the PCB.
In the region marked with red in this image – low-side MOSFET switches for injector management.
On PCB, we see also:
on both sides of the low-side switches: inductors and capacitors (blue color) for LC filters of PWM modules; slightly closer to the middle part of the board – transformers for high-side PWM output MOSFET (positive polarity switches) power supply (+130V).
Let’s come back to today’s situation. I asked them why the decision – to start to do something with the MSD81 was made, and I got the answer – the error message 29D2 was recorded.
29D2 means: misfiring, cylinder 6.
This error message refers to the IGNITION system, not the management of the injectors!
In the case of the ignition system (if there are suspicions that the DME is faulty), the transistor of the ignition coil management should be checked, not the injector management switches!
The management transistors of the ignition coils are placed on the second (large/central) PCB.
Nuance: with a probability of 99.9%, in case of a defect of ignition coil management MOSFET, there will be a short circuit of the transistor (in the D/S path if MOSFET is used or C/E path if IGBT is used), and the ignition coil will release HIGH heat output (the ignition coil will “inflate” and will smoke/stink). If the error message misfires (29CC .. 29D2; 29D9) are recorded in the DME error message memory, but previously mentioned defects are not present – the cause of the problem should be searched for in the fuel mixture or somewhere else, not in DME.
This time – men took out plugins of the smallest PCB without reason; cut off the power supply MOSFET transistor outputs. I hope my advice to look for a problem not in DME but in another place will be heard. Otherwise – further “repair” will only make the situation worse.
