Caterpillar ECU repair, need N548AB replacement?

[eccerr0r]

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") ...

[eccerr0r]

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...

[petehall347]

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 ?

[CapLeaker]

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.

[eccerr0r]

Re: Catapillar ECU repair, need N548AB replacement?

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

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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...

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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...

[CapLeaker]

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?

About 3.3pF. Identical on both. Measured with my LCR meter.

[eccerr0r]

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?

[CapLeaker]

Re: Catapillar ECU repair, need N548AB replacement?

i might get in touch with these people . https://fairchildproducts.com/contact-us-information/

Already did and got nothing back.

Added latest drawings

[eccerr0r]

Re: Catapillar ECU repair, need N548AB replacement?

The way the circuit is hooked up, a fake transistor is not unreasonable... A diode is also possible. A mosfet being used for its body diode only is possible too but a floating gate is bad.

It's also quite possible Fairchild is under NDA to not need to disclose the contents of those chips, worth a shot but don't be surprised if you get shot down...

[petehall347]

Re: Catapillar ECU repair, need N548AB replacement?

i might get in touch with these people . https://fairchildproducts.com/contact-us-information/

[CapLeaker]

Re: Catapillar ECU repair, need N548AB replacement?

Here is the latest tracing of the board. Sure thing the gates of the TO247's are floating. It looks like if these conducting between drain and source, the drain pin (red) will send a voltage to the main board, goes through a little 130k resistor, a couple small SMD caps to gnd and goes to the back side of the board. Kinda like a sensing circuit.

[sam_sam_sam]

Re: Catapillar ECU repair, need N548AB replacement?

One possibly is that it goes to the micro controller board instead where you think it should go this is just an idea or somewhere else who knows where that could be another idea as well

[CapLeaker]

Re: Catapillar ECU repair, need N548AB replacement?

I know that what I am going to say sounds nuts, but is it possible to use a MOSFET as a reverse fuse? Good = open circuit, Bad = short? Maybe a over voltage condition occurred due to jumping the truck for example? But why use 2 Mosfets? Baah… this doesn’t make sense in my head! The drain and source pins do go to the main board.

[eccerr0r]

Re: Catapillar ECU repair, need N548AB replacement?

Well I gave you one possible explanation which is not completely far fetched albeit evil explanation of what's going on. Yes I hate buried vias/buried pads and I think they're also frowned upon for best practices, but can't discount this possibility when you don't know where the s/d pins go?

[CapLeaker]

Re: Catapillar ECU repair, need N548AB replacement?

Well, I don't think I am going to decap that. Why I am doing this? Because no one else can do that around here, money is always an issue, but for me it's just because it's interesting on how this is supposed to work. I've been very busy fixing a lot of stuff, but because of time constraints for the last little while I just post now the harder, interesting stuff. Brain stimulation can't hurt.

So far I haven't found what is driving the gate of the Mosfet. I can follow it to a through hole and then it disappears in the middle layer of the board. I made a makeshift copper brush and went a few times over both sides and got diddly squat. Matter effect unpopulated pads of Q16 and Q26 behave the exact same way then Q2 and Q19. The drain and source are all paralleled (and are connected to Q2 and Q19 as well). The gate pins do the same trick, go to a through hole and disappear in the middle layer of the board. I tried this morning to see what happens if I go hook up a cable tracer to these gate pads and sniff around with a inductive probe. Played around for 5 mins and my 9V battery died. Pfft... Now I need some batteries.

[sam_sam_sam]

Re: Catapillar ECU repair, need N548AB replacement?

actually some of us have experience with those specific batteries.
they will last about 20years.
then all fucking hell with break loose - literally!
https://www.youtube.com/watch?v=OT9CWbK-2HU

This was a mess that he had on his hands he is good at repairing this device

[eccerr0r]

Re: Catapillar ECU repair, need N548AB replacement?

Depends on how much the battery is used... and how much it's not connected to external power. Those Tadiran batteries have been around for a while and probably has really low self discharge but I've come across many expended Tadiran cells in old computers...

[stj]

Re: Catapillar ECU repair, need N548AB replacement?

So this is BS, catapillar trucks have a lifetime of 10 or years max because the battery is not a FRU - and one must get a new ECU every 10 years?

actually some of us have experience with those specific batteries.
they will last about 20years.
then all fucking hell with break loose - literally!
https://www.youtube.com/watch?v=OT9CWbK-2HU

[stj]

Re: Catapillar ECU repair, need N548AB replacement?

easy to find out - decap one using nitromores paint stripper or some other resin desolving substance.

[sam_sam_sam]

Re: Catapillar ECU repair, need N548AB replacement?

would be evil if caterpillar OEMed dummy parts that just short S-D and just used those as jumpers...

I would have to totally agree with you on this point