SMD component identification

[jackW]

Hi everyone,

This is a special ECU for an older race car I am trying to get working. The bodge wire was from a previous blow up and was added by the ECU manufacturer as a repair. Now a similar fault has happened (in the car engine harness external to the board).

You can see in the photos the component has split. I'm just struggling to identify the component as its writing is not quite readable and what I can make out doesn't come up with any searches...

I presume it must be a diode? only two leads but no clear markings of which end is which etc? How would someone usually proceed from this situation?

Jack

[Whales]

Yes it looks like a diode. Still could be some other magic however.

I recommend trying to trace out the nearby components, find out what the part was being used for.  That will give you hints as to what direction it should face and what type is needed; and whether or not a diode makes sense there.  Paper, pen and a multimeter are the tools of the trade; as well as overlaying photos of the top & bottom of the board (if you have any image editor skills).

Where does the bodge wire go?

[SMdude]

It has the marking DX and from that I get SMBJ90 when I do a marking search.
It is a diode.
It would be handy to know what it hooks up to.

[gamalot]

It could be a Transil (TVS) diode, model SM6T15A. 

https://www.st.com/resource/en/datasheet/sm6t.pdf

[Whales]

A TVS diode would explain a few things:

• No direction marking
• No PSU parts shown nearby
• Death related to a fault in the wiring harness

Daft question: any other boards in the car have this part?  Or is this part anywhere else on the same board?

N.B. if this is a TVS then "technically speaking" the board may work fine without it; however I do not recommend using the board without it.  If it is being used as a protection clamp then it's there to protect the whole board: replacing one part is generally nicer than a whole ECU.

[bob91343]

1N734 isn't likely.  But that's what I read.  Maybe 4N734.  If the latter, it might be an optical device but two leads aren't enough.

[fzabkar]

The SM6T15A (marking DX) is unidirectional, so its polarity will be important. The SM6T15CA (marking LX) is bidirectional.

[jackW]

Very interesting,

It does seem like it was being used as a type of protection Diode hence it failing when the improper loom was connected to it.

So the consensus is to maybe throw a SM6T15A into the mix and see what happens? How do they usually denote the polarity on these types of packages as the pins don't seem to be labeled?

Do we all agree that the DX marking is for a unidirectional diode?

Unfortunately, the board is very rare now so its quite difficult to find one to compare this to.

Appreciate everyone's input!

Jack

[fzabkar]

A 15V TVS diode will have a standoff voltage of 15V and a breakdown voltage that is 1V or so higher. I would think that it would make sense for such a diode to be connected in parallel with a 14.4V automotive supply. If the supply is reversed, then a unidirectional diode presents a forward voltage drop of 0.6V, thereby taking out the fuse. OTOH, a bidirectional diode would afford no reverse polarity protection. An overvoltage, such as an alternator load dump, would cause either diode to go S/C and blow the fuse.