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Old 09-19-2021, 08:36 AM   #41
eccerr0r
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Default Re: Catapillar ECU repair, need N548AB replacement?

Do you have an avr transistortester or capacitance tester? How many pF capacitance do you see on the gate terminal of the "fried" transistor?
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Old 09-19-2021, 05:59 PM   #42
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Default Re: Catapillar ECU repair, need N548AB replacement?

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Do you have an avr transistortester or capacitance tester? How many pF capacitance do you see on the gate terminal of the "fried" transistor?
About 3.3pF. Identical on both. Measured with my LCR meter.
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Old 09-19-2021, 06:20 PM   #43
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Default Re: Catapillar ECU repair, need N548AB replacement?

I'm now 90% sure that's a dummy device.

---

oh...forgot to refactor the possibility of a dead diode... hmm. Maybe 45% dummy device, 45% diode, 10% I'm talking out of my ass. I have to put in that chance because I'm only guessing...

---

No nevermind, make that 70% dummy, 20% diode, 10% no clue. Once again yes I have to roll the dice, and based on the circuitry available...

Last edited by eccerr0r; 09-19-2021 at 06:33 PM..
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Old 09-19-2021, 06:38 PM   #44
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Default Re: Catapillar ECU repair, need N548AB replacement?

Well, I am not so sure on that. I found a post in a different language somebody asking someone about that exact same FET on a caterpillar ECU too, but never got an answer. I was hoping that someone would have a schematic for that thing, but I guess that's not happening. Even if that thing isn't repairable, I am thinking about grinding the board down to the middle layer and see where that effin trace goes. Makes a nice winter storm project.
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Old 09-19-2021, 07:22 PM   #45
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Default Re: Catapillar ECU repair, need N548AB replacement?

i had a thought they might be fuses . but couldn't find anything to back that up . maybe dual diodes makes more sense . do the tops pop off easily like they do on them 3 leg fuses ?
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Old 09-19-2021, 07:44 PM   #46
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Default Re: Catapillar ECU repair, need N548AB replacement?

Why are people assuming they are MOSFETs? Just thinking they could be MOSFETs doesn't make them MOSFETs.
Can't make an assumption, the clues are pointing away from MOSFETs too.

I'd say grind down/dissolve the shorted devices. You have nothing to lose:
1. if they are really dead (trash anyway)
2. if they are dummy devices. Replace with a jumper
3. if they are diodes, diodes can be easily found.
and we'll know for sure if they truly are MOSFETs or whatnot...
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Old 09-19-2021, 11:36 PM   #47
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Default Re: Catapillar ECU repair, need N548AB replacement?

BTW this will make people go
Look at the part number: TO247SHorT



(Now replace the "or" with an "i") ...
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Old 09-20-2021, 07:18 AM   #48
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Default Re: Catapillar ECU repair, need N548AB replacement?

gotta say, if i was designing an injector driver then those would be darlingtons
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Old 09-20-2021, 09:59 AM   #49
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Default Re: Catapillar ECU repair, need N548AB replacement?

I sure wouldn't use a jumper as a current amplification device.
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Old 09-21-2021, 11:35 AM   #50
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Default Re: Catapillar ECU repair, need N548AB replacement?

I am still intrigued on how this thing is supposed to work. Let’s say it is a MOSFET with the gate left unconnected (floating). Wouldn’t the MOSFET be turned on if the source and drain have power? Basically working or shorted MOSFET wouldn’t make any difference in this case? More sense it makes if the MOSFET has a body diode, it would be a diode in disguise marked as a transistor rather than a diode.
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Old 09-21-2021, 11:53 AM   #51
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Default Re: Catapillar ECU repair, need N548AB replacement?

If gate is unconnected, it simply cannot be a MOSFET.
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Old 09-21-2021, 03:01 PM   #52
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Default Re: Catapillar ECU repair, need N548AB replacement?

I would think either the parts are Schottky rectifier diodes with oddball pinout, or the gate-drive pcb trace fused somewhere and is open-circuit? I'll see a little black mark on an inner layer if this happened, using a bright light and magnifier to inspect the board.
Or, a gate-drive resistor got hot and fell off the board once the solder melted.
I keep thinking the gate connects somewhere to something and just have to find it.
Does any lead on the mosfets connect to GND? As a low-side switch. I'd also follow the mosfet pins whichever connects to the ECU 70-pin connector, to find out what solenoids etc. the parts are driving and who the fuse is.

The model number of the ECU would help, sometimes there are schematics out there.

edit: usually a shorted mosfet fuses all three pins together as a blob. So the old parts should have continuity to the gate, unless the bonding wires fused open circuit.

Last edited by redwire; 09-21-2021 at 03:11 PM..
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Old 09-21-2021, 04:05 PM   #53
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Default Re: Catapillar ECU repair, need N548AB replacement?

It's very unlikely both the MOSFET gate and driver both fused at the same time, one will give way first. Also there are two MOSFETs in parallel and what's the chances both failed the same way. I still think there are too many depop devices on the board for that to be a real MOSFET there. Plus two shorted diodes in parallel? What's the chances of that?

How many ohms is it from the "apparent" D to S pins? Dead 0 ohm short or have a few tenths of an ohm?

Still think it's a SHORT dummy device and barking up the wrong tree. Anyway curious of what the part number is for the other devices on the board, are they typical or also caterpillar corrupted?
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Old 09-22-2021, 05:20 AM   #54
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Default Re: Catapillar ECU repair, need N548AB replacement?

The chances of 2 Mosfets in parallel shorting out are pretty good. Actually that is one reason I don't like that method.

The short on the device measures 0.2 Ohm, same as touching the 2 probes together, like a dead short.

Can't see the inner layer of the board. The outer layers are made that way, I cannot shine a 1000 lumens Olight through it in order to see anything of the middle layer.

The cover has maybe an interesting number printed on it: MXS69947
The truck is a 2007 pre emissions. Some other numbers: BB55U and 248-5546.

Working on the rest to figure out later today.
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Old 09-22-2021, 09:16 AM   #55
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Default Re: Catapillar ECU repair, need N548AB replacement?

Not sure they would frequently both short out at the same time when in parallel either. And surprised there's not a heat stress mark on the PCB.

I still would not discount it's a dummy device. Not enough clues to point to it being a MOSFET in my books at least.

Is this another case of Topcat's No-Nos, "got board to fix, no way to test" (i.e. you don't have access to the rest of the TV/truck?) Or at least a FSM?
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Old 09-22-2021, 03:12 PM   #56
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Default Re: Catapillar ECU repair, need N548AB replacement?

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Not sure they would frequently both short out at the same time when in parallel either. And surprised there's not a heat stress mark on the PCB.

I still would not discount it's a dummy device. Not enough clues to point to it being a MOSFET in my books at least.

Is this another case of Topcat's No-Nos, "got board to fix, no way to test" (i.e. you don't have access to the rest of the TV/truck?) Or at least a FSM?
Nope, I donít have the truck. I still have to do some more tracing in order to see if I can find something new.
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Old 09-22-2021, 04:37 PM   #57
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Default Re: Catapillar ECU repair, need N548AB replacement?

put the part in a vise and bend the tab so the resin cracks away from it - see if it actually has a square of silicon in it.
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Old 09-23-2021, 04:58 PM   #58
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Default Re: Catapillar ECU repair, need N548AB replacement?

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put the part in a vise and bend the tab so the resin cracks away from it - see if it actually has a square of silicon in it.
Iíll try that tomorrow.
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Old 09-24-2021, 12:14 PM   #59
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Default Re: Catapillar ECU repair, need N548AB replacement?

should be able to also tell from the silicon a diode from a transistor. Photograph of the "decapsulation" would be nice.

Even if there's just a piece of metal that bridges two pins...
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Old 09-25-2021, 04:57 AM   #60
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Default Re: Catapillar ECU repair, need N548AB replacement?

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should be able to also tell from the silicon a diode from a transistor. Photograph of the "decapsulation" would be nice.

Even if there's just a piece of metal that bridges two pins...
Sorry for the late reply, things got a bit too busy at work. So here is what I did: I took the FET off that cement paste and jammed it in the vice side ways. That took the top off very nicely!
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