3
\$\begingroup\$

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.

enter image description here

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?

enter image description here

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?

enter image description here

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?

\$\endgroup\$
6
  • 7
    \$\begingroup\$ 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?" Honestly, you're not. \$\endgroup\$
    – DKNguyen
    Feb 21, 2022 at 15:30
  • 5
    \$\begingroup\$ Every component will have some parasitic capacitance, you can't just measure it (especially in circuit) and draw conclusions like that. U1D is a on semi MURS120 \$\endgroup\$ Feb 21, 2022 at 15:36
  • 2
    \$\begingroup\$ The markings are mostly there to confirm assemblies and such like, they’re not intended to be a 1:1 mapping back to an actual part number. Many smaller parts are not marked at all these days, we just hope the P&P wallah loads the correct reel. \$\endgroup\$ Feb 21, 2022 at 16:18
  • \$\begingroup\$ Small light grey SMD components could also be ferrite beads. Transistors are these assets.nexperia.com/documents/data-sheet/BUK9277-55A.pdf \$\endgroup\$ Feb 21, 2022 at 17:53
  • \$\begingroup\$ If you need to figure out parts from their marking codes, this is a good resource. Not exhaustive, but good for common parts: smd.yooneed.one \$\endgroup\$
    – Polynomial
    Feb 21, 2022 at 18:46

3 Answers 3

8
\$\begingroup\$

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.

\$\endgroup\$
1
  • \$\begingroup\$ 1.9V drop is observed with black probe on the large "foot", and the red probe on the small foot on the left. with those positions reversed, the measurement is around 2.9V \$\endgroup\$
    – Dportology
    Feb 21, 2022 at 16:10
6
\$\begingroup\$

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?

\$\endgroup\$
6
  • \$\begingroup\$ You're probably right :/. I was just hopeful I could avoid the $3000 OEM replacement part. It's seeming like I'll have to start checking around junk yards or something \$\endgroup\$
    – Dportology
    Feb 21, 2022 at 16:24
  • \$\begingroup\$ Yes, that’s probably your best bet. \$\endgroup\$ Feb 21, 2022 at 16:49
  • \$\begingroup\$ I did a search for ‘remanufactured transmission control module sprinter van’ and they show up for less than $200. \$\endgroup\$ Feb 21, 2022 at 16:55
  • \$\begingroup\$ Try yourself here: duckduckgo.com/… \$\endgroup\$ Feb 21, 2022 at 16:56
  • \$\begingroup\$ wow thanks so much. I totally missed this option! Looks very promising \$\endgroup\$
    – Dportology
    Feb 21, 2022 at 17:07
0
\$\begingroup\$

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.

\$\endgroup\$
1
  • \$\begingroup\$ ”That board is probably rated at 150 C or higher depending on where it is mounted.” I doubt it. The FR4 will turn into a brown cracker at that temperature. \$\endgroup\$
    – winny
    Jun 1, 2022 at 16:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.