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Old 09-14-2021, 03:47 PM   #1
CapLeaker
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Default Catapillar ECU repair, need N548AB replacement?

Iíve got a Catapillar ECU here from an 18 wheeler. Complaint: Effer doesnít start! Nice!
Well here we go: open up the ECU and you get greeted with 70 pins of Catapillar madness! It takes a bit of time to desolder 70 pins and some prying to get it out.
Underneath that main board is another board with all the Power goodies. You thing the 70 pins are hard? This bottom board is a real pain to figure out what pins to unsolder in order to get it out. However I did get that power board out too. Only took me almost 3 hours to get those 2 boards off and that is with using a heat gun and a desoldering at the same time. Had to empty the desoldering gun 5 times! Itís probably close to 100 pins to desolder before one gets anywhere in this thing.

So whatís the problem again? No start. The victims this time are 2 Fairchild N548AB in a TO247 SHT package. Dead short.

Now to my problem: With what do I replace these? I canít find anything much useful about them besides that they are discontinued.

Here is some ECU porn.
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Old 09-15-2021, 05:00 AM   #2
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Default Re: Catapillar ECU repair, need N548AB replacement?

HMMMM.......
is that a "suicide battery" i see!!!
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Old 09-15-2021, 04:33 PM   #3
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Default Re: Catapillar ECU repair, need N548AB replacement?

I was surprised to see that in there too.
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Old 09-15-2021, 05:27 PM   #4
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Default Re: Catapillar ECU repair, need N548AB replacement?

can you find a datasheet ? i sure couldn't .
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Old 09-15-2021, 06:04 PM   #5
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Default Re: Catapillar ECU repair, need N548AB replacement?

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can you find a datasheet ? i sure couldn't .
No Sir! I tried my darnedest to find a spec sheet on that N548AB and found diddly squat! There are not even any markings on these 3 legged caps. But I did figure out that this ECU came out of a 2007 Catapillar MXS engine.
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Old 09-15-2021, 06:19 PM   #6
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Default Re: Catapillar ECU repair, need N548AB replacement?

Great job desoldering all that, that's a lot of work

I would beep out the ECU connector pins to see what they end up connecting to. Because of the big TO-247 package and heatsinking, wild guess is the fuel rack solenoid, as a high current load.
I'll check with someone in the business, but there might be a HV DC-DC converter sometimes to make say 100VDC for driving big solenoid loads and the mosfets might be for that. I'd think they get power from the big caps, not from 24VDC.
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Old 09-15-2021, 06:24 PM   #7
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Default Re: Catapillar ECU repair, need N548AB replacement?

If it really is a suicide battery, I wonder what got lost...
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Old 09-15-2021, 07:04 PM   #8
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Default Re: Catapillar ECU repair, need N548AB replacement?

Quote:
Originally Posted by CapLeaker View Post
No Sir! I tried my darnedest to find a spec sheet on that N548AB and found diddly squat! There are not even any markings on these 3 legged caps. But I did figure out that this ECU came out of a 2007 Catapillar MXS engine.
wondering what it is
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Old 09-15-2021, 08:17 PM   #9
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Default Re: Catapillar ECU repair, need N548AB replacement?

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Great job desoldering all that, that's a lot of work

I would beep out the ECU connector pins to see what they end up connecting to. Because of the big TO-247 package and heatsinking, wild guess is the fuel rack solenoid, as a high current load.
I'll check with someone in the business, but there might be a HV DC-DC converter sometimes to make say 100VDC for driving big solenoid loads and the mosfets might be for that. I'd think they get power from the big caps, not from 24VDC.
That is what I was thinking. The truck doesnít start, because it didnít get any fuel, because the solenoids not firing. The truck would run on ether, but then die. The TO247 mosfets are in parallel, both are shorted drain - source. I also think that these mosfets should be a HV range and not get 24v.
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Old 09-15-2021, 11:11 PM   #10
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Default Re: Catapillar ECU repair, need N548AB replacement?

Quote:
Originally Posted by CapLeaker View Post
shorted drain - source
Wouldn't that mean stuck on and fuel keeps going?
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Old 09-16-2021, 12:41 AM   #11
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Default Re: Catapillar ECU repair, need N548AB replacement?

Unless it took out the device that supplies power to the mosfets but yes you would think might be the case
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These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

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All of these had CAPs POOF
All of the mosfet that are taken out by bad caps
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Old 09-16-2021, 07:49 AM   #12
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Default Re: Catapillar ECU repair, need N548AB replacement?

Nope. No fuel, as there isn’t any HV produced. There is no short to the gate on both, so the gate driving circuit is fine.

Last edited by CapLeaker; 09-16-2021 at 07:52 AM..
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Old 09-16-2021, 08:19 AM   #13
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Default Re: Catapillar ECU repair, need N548AB replacement?

imobiliser active / service scam activated?
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Old 09-16-2021, 01:44 PM   #14
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Default Re: Catapillar ECU repair, need N548AB replacement?

Nope. 18 wheeler works fine on a other ECU.
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Old 09-16-2021, 02:00 PM   #15
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Default Re: Catapillar ECU repair, need N548AB replacement?

i meant in the ecu!

some fuckers - i know john deer is one, need fault codes cleared with a service tool or they wont run.
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Old 09-16-2021, 03:49 PM   #16
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Default Re: Catapillar ECU repair, need N548AB replacement?

Had no active immobalizer or related codes.
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Old 09-16-2021, 09:46 PM   #17
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Default Re: Catapillar ECU repair, need N548AB replacement?

Asked my friend who designs ECU's and this is what he mentioned:
You'll need to ID what the mosfets are driving and if they seem to be N-ch (low-side) or P-ch (high-side). If they are driving the "electronic unit injectors", it's a guess as to their voltage rating.
For a diesel engine, it's like a gasoline fuel injector - the amount (pulse-width) of the squirt is your fuel amount, but a diesel faces much more pressure and the timing of the squirt is your ignition timing, so you need speedy operation.
So it is common to hit the solenoid very hard with 50-200VDC pulse at 20A and then less drive PWM down to say 12VDC for hold. So it is a peak and hold drive. It can be a single "master output" that gets switched to each cylinder by smaller mosfets, that select which cylinder. There is no overlap between cylinders so one power switch can do all the hard work.
The ECU should have a boost-converter for this, what is the HV electrolytic cap rated at? It would give some idea of the mosfet's voltage. Not the 1,300uF 35V part but the other big blue one.

The lithium battery is likely for a real-time clock, this is for time-stamping trouble codes and also for time-stamping driver's habits. There can be GPS on the truck available through the SAE J1850 bus which would set the clock, or use a scantool. I guess you can track the driver's foot stomping with geolocation and time/date in some trucks.

The thermal adhesive is usuallty two-part, Henkel or Dow and glue for the enclosure is typically Dow 7091.
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Old 09-17-2021, 09:01 AM   #18
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Default 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?
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Old 09-17-2021, 09:29 AM   #19
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Default Re: Catapillar ECU repair, need N548AB replacement?

I was surprised to see that too.
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Old 09-17-2021, 11:32 AM   #20
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Default Re: Catapillar ECU repair, need N548AB replacement?

redwire:
Interesting indeed. I already had guessed the voltage should at 100V - 200V (from a different ECU) so that is inline with your buddies thinking. The 2 big, 3 legged blue caps are wired in series to each other. The drain and source pins of the fet are in parallel connected and go to the plug that solders into the main board.
Get this: The 2 gates are NOT parallel between the two fets and they do not go to that plug that solders to the main board either. So far I have not been able to figure out where the gates go. I measured gate to gate and nothing at all. Its again a multi layer board and can't see through it at all. Maybe I'll doodle up a picture later on and post that.
Matter effect I can't see any gate drive circuit on that power board. So I had ass/u/me/ed at one point that the gate drive maybe on the main board. Well that is not the case.
I must sound like I got mad cow disease or had my dogs kibbles for breakfast, lunch and supper?! Can it be that the gate has been just left floating? Oooff!
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