Motronic Re-Soldering -- How Often Have You Had to Do So?

E28 technical advice asked and given! Troubleshooting, modifications and more.
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Shawn D.
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Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

Y'all,

My 535i has developed a strangely familiar intermittent instantaneous shut-off issue. After checking the other usual suspects (relays, flywheel sensors, fuel pump, etc.), I'm left with the ECU power transistor solder joints. This would be the 3rd or more time doing this.

So, when you've fixed this issue, has it been "one and done" forever, or have you had to repeatedly do the fix?

Thanks,
Shawn
Mike W.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Mike W. »

I never had an ECU issue, but an AFM one that could have been mistaken for one, and for which I swapped ECUs, to no avail.

First, it would randomly shut off. Rarely, but when you're in New Mexico and live in Calif it gives you a queasy feeling as you coast to the side of the road. Instant restart as soon as I turned the key. Later it got a little crankier, the idle was just a touch rough, and occasionally it would just die at idle with no warning. Finally I changed out the AFM and bam, as soon as it started I could tell the difference at idle. Not idle speed, and still the standard M30 hunting not quite smooth, but better. And none of the issues ever reappeared.

So if you haven't swapped one in, don't rule out the AFM.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Blue Shadow »

I have soldered a few but never had to solder one a second time.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

Mike W. wrote: Sep 24, 2022 4:02 PM So if you haven't swapped one in, don't rule out the AFM.
I haven't -- thanks for the suggestion.

The next thing I was going to look at was the coolant temperature sensor.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Blue Shadow »

I had intermittent complete shutdown. Driving along, tach drops to zero and I'm coasting.

It was one of the connectors on the main relay had backed out of the socket enough to be loose when the relay was plugged in but the compression of the wire made enough contact that the car usually started right up again.

I do not think that relay will come out without tools now the spade clip is so tight.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

Blue Shadow wrote: Sep 26, 2022 1:11 AM I had intermittent complete shutdown. Driving along, tach drops to zero and I'm coasting.

It was one of the connectors on the main relay had backed out of the socket enough to be loose when the relay was plugged in but the compression of the wire made enough contact that the car usually started right up again.

I do not think that relay will come out without tools now the spade clip is so tight.
Good suggestion. However, I had jumped the main relay terminals with a test wire setup, so I know it wasn't intermittent.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by RetiredDoc »

325K on my owned since new ‘86 eta and still running fine on the original ecu. I did swap the original chip for a bavauto chip at around 180K. So I have never had to do any internal ecu fixing. I do have a backup ecu bought maybe 20 years ago from ProgRama but the box has never been opened. So in my research with an n=1 the ecu is quite dependable at 36 years and still working.

Results may vary depending upon your local driving conditions.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Panici »

Shawn D. wrote: Sep 24, 2022 2:00 PM I'm left with the ECU power transistor solder joints. This would be the 3rd or more time doing this.
What type of solder are you using for the repair?
Could be an issue with a "lead free" type of solder?
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by gadget73 »

generally when solder joints fail, it was because the initial prep was bad, it gets too hot, or its got some sort of mechanical problem that flexes the joint.

Re-work its more often not cleaning the crust from the original failed joint, or not getting it flowed through properly on a double sided board. Joints need to be clean, you need flux core solder, and a sufficiently hot iron to get the job done. If it balls up instead of flowing its either too cold or dirty. I'm not a fan of unleaded solder but it does work if your iron is hot enough. If its non-fluxed solder or some type not intended for electronics it won't mix properly with whatever is on the board either. Usually need 60/40 or 63/37 for this.

things that get too hot and de-solder themselves need a heat sink of some form. If its something already on a heat sink, replace the thermal goop. It dries out after decades, or someone was already in there and didn't replace it or just buttered it up like toast. A thin layer of the proper stuff is all it wants. I spread it with a razor blade after removing the old with alcohol. If there are burrs or something else that keeps the device from sitting flat and making good contact, file it off. If there are any insulator washers or spacers, be sure to replace those too.

if its a mechanical support issue, have to figure that one out depending on the specifics. Missing or loose hardware, or sometimes just poor initial design that doesn't put enough fasteners into the thing so the board and the thing attached to it don't move together.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by HayekFan »

Have you tried swapping in a good ECU just to 100% confirm that's the issue?
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Blue Shadow »

Panici wrote: Sep 27, 2022 6:56 AM
What type of solder are you using for the repair?
Could be an issue with a "lead free" type of solder?
Shawn would use the correct solder and Mil-spec soldering techniques. No one uses lead free unless they have to. Sorry €s.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

Panici wrote: Sep 27, 2022 6:56 AM What type of solder are you using for the repair?
Could be an issue with a "lead free" type of solder?
This is what I'm using (plus pic of the actual spool of solder):

https://www.diverseelectronics.com/prod ... ool-p6386/

Image
gadget73 wrote: Sep 27, 2022 9:16 AM generally when solder joints fail, it was because the initial prep was bad, it gets too hot, or its got some sort of mechanical problem that flexes the joint.

Re-work its more often not cleaning the crust from the original failed joint, or not getting it flowed through properly on a double sided board. Joints need to be clean, you need flux core solder, and a sufficiently hot iron to get the job done. If it balls up instead of flowing its either too cold or dirty. I'm not a fan of unleaded solder but it does work if your iron is hot enough. If its non-fluxed solder or some type not intended for electronics it won't mix properly with whatever is on the board either. Usually need 60/40 or 63/37 for this.

things that get too hot and de-solder themselves need a heat sink of some form. If its something already on a heat sink, replace the thermal goop. It dries out after decades, or someone was already in there and didn't replace it or just buttered it up like toast. A thin layer of the proper stuff is all it wants. I spread it with a razor blade after removing the old with alcohol. If there are burrs or something else that keeps the device from sitting flat and making good contact, file it off. If there are any insulator washers or spacers, be sure to replace those too.

if its a mechanical support issue, have to figure that one out depending on the specifics. Missing or loose hardware, or sometimes just poor initial design that doesn't put enough fasteners into the thing so the board and the thing attached to it don't move together.
I'm well aware of all that, but it's good information for others.

I cleaned the old stuff off properly the first time I did the repair, I am using appropriate solder, and the temperature is appropriate. I'm getting a nice, smooth wetting of everything I've soldered. There are no hardware/chemical bonding issues, and there's no heat sink paste in play.

IMPORTANT TO NOTE AND UNDERSTAND BEFORE RESPONDING FURTHER: My initial post was based on the symptoms which mimicked the exact symptoms I and others have experienced with this ECU solder failure previously. After re-soldering (including an additional joint at a capacitor) this time, the symptoms have not been alleviated. I am no longer looking at the ECU's solder joints as the cause of the problem(s). Further discussion is fine, of course, and will be useful for others, but I won't be participating WRT the ECU's solder joints being the issue for what I'm working on now.
Shawn D.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

HayekFan wrote: Sep 27, 2022 11:08 AM Have you tried swapping in a good ECU just to 100% confirm that's the issue?
Yes, there was no change in symptoms. This is after re-soldering.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by adam_poll »

I had a failing coil wire to the distributor that was shorting sometimes giving me very similar symptoms to an ECU with failed joints (been there, done that as well). Probably not your issue but very easy to cross off the list with a quick visual inspection looking for failed insulation and black marks.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

adam_poll wrote: Sep 27, 2022 9:32 PM I had a failing coil wire to the distributor that was shorting sometimes giving me very similar symptoms to an ECU with failed joints (been there, done that as well). Probably not your issue but very easy to cross off the list with a quick visual inspection looking for failed insulation and black marks.
Excellent suggestion, I had neglected to check that!
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by gadget73 »

Shawn D. wrote: Sep 27, 2022 3:46 PM
I'm well aware of all that, but it's good information for others.

meant no offense, having spent too much time helping people with various electronics repairs over the years I often tend to forget that some people do actually know how to solder.
Shawn D.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

gadget73 wrote: Sep 28, 2022 8:31 AM meant no offense, having spent too much time helping people with various electronics repairs over the years I often tend to forget that some people do actually know how to solder.
No offense taken! I could have worded it differently, though. I just didn't want to get bogged down into whether something was done correctly in this instance.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Blue Shadow »

Shawn D. wrote: Sep 28, 2022 9:37 AM
gadget73 wrote: Sep 28, 2022 8:31 AM meant no offense, having spent too much time helping people with various electronics repairs over the years I often tend to forget that some people do actually know how to solder.
No offense taken! I could have worded it differently, though. I just didn't want to get bogged down into whether something was done correctly in this instance.
Knowing both these guys, I see why Thain would talk about how to solder. He has amazing old tube amp skills and got my Luxman tube amps running just by looking at them and asking for a jumper...gonna stick that in an amp that has 500V running through it, I stepped back. It was a step in the modification of the amp that was not called out in the conversion manual and he knew where it needed to go just looking at the amp.

And Shawn, his discussion of the LED taillight construction with the soldering requirements for each individual LED when these were available was to most over-the-top. But probably standard operating procedure when he was with that military jet manufacturing company. Making them that way would be correct because of the constant vibrations the lamps get riding along in the car.

One just doesn't know someone's background on these boards sometimes.
Shawn D.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

Blue Shadow wrote: Sep 28, 2022 1:25 PM But probably standard operating procedure when he was with that military jet manufacturing company.
I was not anywhere near any hands-on manufacturing of electronics, though. ;)
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by roadstar »

adam_poll wrote: Sep 27, 2022 9:32 PM I had a failing coil wire to the distributor that was shorting sometimes giving me very similar symptoms to an ECU with failed joints (been there, done that as well). Probably not your issue but very easy to cross off the list with a quick visual inspection looking for failed insulation and black marks.
And check the spark plug wires as well with a multimeter. They turned out to be failing part in my car causing intermittent stalling.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by HayekFan »

Shawn D. wrote: Sep 27, 2022 3:47 PM
HayekFan wrote: Sep 27, 2022 11:08 AM Have you tried swapping in a good ECU just to 100% confirm that's the issue?
Yes, there was no change in symptoms. This is after re-soldering.
Okay, so if you swapped in a good ECU with good solder joints and the symptoms didn't change, then that rules out the solder joints as the issue.

I will mention that I was recently made to feel silly by what turned out to be a bad distributor cap. The symptoms came on so suddenly and were so glitchy in nature -- it would run fine one day and then terribly the next day or maybe wouldn't even start -- that I thought for sure it couldn't just be a worn distributor cap. But sure enough that's what it was. Put in a new cap and it's been running rock solid ever since. This was after teeing a pressure gauge into the fuel rail and swapping in an AFM and hooking up noid lights and the whole bit.

So FWIW that's my suggestion: distributor cap.
Shawn D.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by Shawn D. »

HayekFan wrote: Oct 10, 2022 9:10 PM So FWIW that's my suggestion: distributor cap.
I will absolutely look at that when I get back in the car. I have to admit I haven't gone through every basic.
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Re: Motronic Re-Soldering -- How Often Have You Had to Do So?

Post by garageboy »

The only thing in common is that all 3 E28s (4/1983 528e, 2/1987 528e, 11/1987 528e) were garaged their entire lives. None were ever in accidents that potentially impacted wiring. And I only used factory front windshields when they were replaced a few times over the years. First 2 E28s had 300k miles apiece by the end, both served winter (i.e. salt) duty, and most recent E28 has 150k miles (ECU since 5-speed conversion at 80k).

I have never experienced a failing ECU. I have never needed to open one up to re-solder lines. I only opened the 2/1987 528e 5-speed ECU to put in a Conforti chip. That's my data point.
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