Is this a diode?

[TheGenesis]

I try to repair a Transmission-ECU from an Audi Quattro which has been under water for a few days.
Most parts have survivied, but this one on the picture is gone and I dont know what it is.
Can soneone identify this part?

[jmelson]

Looks like it might be a Tantalum capacitor.

Jon

[lordvader88]

well does it look like a diode to a multimeter ?

I'm very new to SMD stuff but usually that stripe at 1 end is the cathode

[james_s]

That's almost certainly a tantalum capacitor.

[janoc]

The question is - how do you know it is bad when you don't know what it is?

[james_s]

The terminals look all nasty and corroded, that's a pretty good indication that it should be replaced.

[chris_leyson]

The package looks more like diode than a tant cap, the sides look as if they are tapered whereas I would expect straight sides on a plastic mold tant package. Maybe DO214AC or SMA package, some dimensions would help.

[Zero999]

well does it look like a diode to a multimeter ?

I'm very new to SMD stuff but usually that stripe at 1 end is the cathode

Tantalum capacitors exhibit diode like behaviour, which can be used to determine the anode from cathode. When connected backwards, a tantalum capacitor will conduct like a diode, dropping a small voltage. When connected correctly, the capacitor will charge, then block the voltage when fully charged. The same is true for other polarised capacitors. The aluminium capacitor was developed from the electrolytic rectifier.

Now in this case, if it's dropping about 600mV, when forward biased, then it's most likely a silicon diode. If you're unsure, you could try it on a capacitance tester: it should read below 100pF. If it were a tantalum capacitor, then the capacitance would be much higher >100nF.

[TheGenesis]

The dimensions are 4.5mm x 2.6mm.
I know that it is faulty because one terminal is completely gone.
Found another two of the same item on the pcb (see pic) and measured them in-circuit. Resistance is slowly increasing regardless of the polarity. Then it should be a capacitor and not a diode, right?
How do I find out there specification?

[james_s]

You could reverse engineer enough of the circuit around the part to try to figure out what it does, the value may not be all that critical. You could also remove one of the good ones and test it.

[janoc]

The terminals look all nasty and corroded, that's a pretty good indication that it should be replaced.

Given the "potato quality" of that photo, those "nasty and corroded" terminals could well be a flux residue or some other contamination, especially given that it is a desoldered component.

[TheGenesis]

I have desoldered one of the "good" ones and China says it's a diode (see pic).
And no, I haven't just laid the part on the contacts while measuring. Instead I pressed it down with a tweezer and also checked the opposite polarity. The china thing always comes to the same conclusion.
But dont know how much voltage this thing can/must take and why does the china thing told me a capacitance of 15pF?

[Gyro]

That's the junction capacitance it's showing. It is indeed a diode - an ordinary Silicon (not Schottky) one too.

Edit: There's a small chance that it might still be a zener or TVS with a breakdown voltage greater than the tester uses (about 5V). If you have a variable bench psu and a series resistor (~1k) you could wind it up, reverse biased with a DMM across it and see if it clamps at some higher voltage.

[gamalot]

Looks like 1SR154-400 from Rohm

https://www.rohm.com/datasheet/1SR154-400

(picture from RS)

[Gyro]

Well spotted 

[james_s]

Hey look at that, yep, definitely a diode. I've seen some capacitors that looked very similar.

[TheGenesis]

Thank you all ... I'll try and post the results

[hydrolisk1792]

Hey look at that, yep, definitely a diode. I've seen some capacitors that looked very similar.

Aren't the caps in that package usually orange?

[james_s]

I've seen orange, yellow and black. I've got some black ones I used on some boards I built, they do have obvious voltage and uF ratings printed though.

[TheGenesis]

Haha ... I scraped a bit of the podding around the point where the terminal was attached to and managed to solder a jumper wire on to it.

Today we tried the module on the car and everything is working fine again

Thanks again for your suppoert.

Cheers