Help Identifying SMD components + How is one meant to know what these are?

Question

This board is a transmission control module from a mercedes van which I have been told by a garage needs to be replaced because of a "short". For the cost of the part, it's worth taking a look at it to see if the fix may be easy, but I can't identify a few components on the board.

Firstly, are a number of these small surface mount boxes which are a light grey color. I have tried a number of multimeter tests on them. They do provide 0.5nF capacitance, so does that mean its a capacitor?

Above is a component labeled "934 U1D." (branding looks to say "ON"). I can find no information about this component online. There are several of these components on the board, and the only measurement I can get is a resistance reading, which for some of these parts is 214 Kohm, and for others only 2Mohm, but I think this variance is just due to the fact that I was measuring them in circuit. What is this component?

Lastly, there are 9 of these transistors. I tested them all thoroughly to the best of my ability using my multimeter's diode test mode. All displayed the same readings for all tests (I tried every combination of probe positions on every leg), except for one test on ONE transistor alone, which when placing one probe on the large "heatsink" part (sorry I don't know what to call it. the metal leg which runs the length of the component at the top) and the other on the bottom left leg, it displayed a voltage drop of 1.9V, where the rest of the transistors for the same test had displayed an open circuit. Could this be an indication of a faulty transistor, or might this be a problem with testing these components in circuit?

How is one meant to identify components like these when they are not labeled on the board, and or, there is seemingly no datasheets online?

Answer 1

Firstly, are a number of these small surface mount boxes which are a light grey color. I have tried a number of multimeter tests on them. They do provide 0.5nF capacitance, so does that mean its a capacitor?

By the looks of it, its either a capacitor or a fuse. You may be measuring other capacitors in parallel, and you THINK this is a capacitor.

Bear in mind: If you measure them anything while they are on the board, you might be measuring parallel capacitors, or resistors in parallel to the IC. Its really hard/impossible to know exactly that what you are reading on a IC (capacitance or resistance) is correct if its still on the PCB.

Above is a component labeled "934 U1D." (branding looks to say "ON")

Looks like a diode (more probably, I can also tell by the way it's placed on the traces), but it could be a capacitor. You can test it using the diode function of your Multimeter. Don't forget, while the IC is on the PCB, it might read parallel resistors/capacitors so you might get wrong readings. Diode testing has polarity. If you see a ~0.7 to ~0.2 V drop when testing this IC, it is a diode.

it displayed a voltage drop of 1.9 V, where the rest of the transistors for the same test had displayed an open circuit. Could this be an indication of a faulty transistor, or might this be a problem with testing these components in circuit?

Again, it might be a parallel diode. Can you edit the question to tell me what was your probe polarity when you saw the 1.9 V drop?

How is one meant to identify components like these when they are not labeled on the board, and or, there is seemingly no datasheets online?

It's really difficult. Your best bet is someone who has seen this part before and he/she remembers the reading on the IC.

Answer 2

Given that this is a power train component, it may be one where it’s best to swap it out rather than try to repair it yourself if you don’t have the knowledge and support. If it malfunctions it could lead to a stalled vehicle or a crash.

Even if you did possess electronics knowledge, component-level debug and repair of an assembly like this needs more information than just the board itself. You need, at the very least, a schematic and if possible a troubleshoot + test procedure.

I understand the frustration - these modules can be heinously expensive, and manufacturers are loath to give out information. It shouldn’t be that way, which is why you see more ‘right to repair’ initiatives being promoted these days. But until that comes about you’re better off getting a new module. Maybe they can credit you for a core?

Answer 3

That is a proprietary mercedes board as are most boards used by the OEMs in automotive. To keep it that way many of the components are either custom or marked with the OEM part number. The parts are specifically qualified for the application which the OEM pays for. That board is probably rated at 150 C or higher depending on where it is mounted. It would probably be high Tg FR4 PCB. Those component part numbers are not normally cross indexed or listed. Because of liability, warranty, and other problems they do not want anybody they do not approve making any changes.

New boards are not normally sold however remanufactured boards are available from approved remanufacturers. The OEM give them the appropriate information to properly repair them and a source or sell them the components.