"Repaired" ECU solved engine running problem (long).

[M6csi]

For the past month or so, my 88 M6 had been afflicted with a very frustrating, annoying and potentially dangerous intermittent engine dying/no start problem. Luckily, all the stalling and no starts happened on city streets pretty close to home and not on the freeway. Usually after waiting a very short period, the car would start again with no further problems. Car does not misfire and buckle before dying, it simply just shuts off. Also, when the car is running, it is running perfectly, with no indications of any problems.

Initially, all symptoms pointed toward the classic no fuel or spark situation. So, new main and fuel pump relays were installed, still the intermittent motor dying/no starts persisted. Check the OBC relay and saw a possible cause, the two sockets were kind of loosely attached. Cleaned and securely fastened the sockets to the relay pins and... no fix! Just wanted to check and try the cheap and easy fixes first.

A bad ECU? Quite possibly, as back in 1996, after 4 years of ownership, a new ECU from BMW was installed due to the exact symptoms as now. Getting a rebuilt or new ECU is quite costly so I did not want to consider this until as a last resort. As a "non-wrencher" back then, when the mechanic says, "you will need a new ECU", then a new ECU it is. IIRC, a new Bosch M5/M6 Montronic box from BMW back in the mid 1990's was about $800. Looking back now, should have kept the original core (minimal core charge) instead of returning it back to BMW.

Next, replace the original 20 year old main fuel pump with a good spare, still no go, car would run or a short while and engine would abruptly cut off again as if I were to just shut of the engine by turning off the key. On to installing a brand new in-tank auxiliary pump, last replaced around 2001. Thought the car was finally "fixed" as the car ran without nary a problem for about 2 weeks, then the same problem reappeared. After about another week of this aggravating problem, the car finally failed to start period.

At this point, a bad ECU is almost 99 percent the "culprit". Considered the rebuilt ECU's from the various automotive electronics repair companies, but don't know about the quality or lack of for these $600+ units. Maybe a remanufactured or new ECU from BMW? At $1000 to $1500, it is quite a sum, to say the least. "Good used" ECU boxes from Ebay? The going rate is about $400-$500, if you can find them as they are in scarce supply. Also, as most are 20+ years old used units, they are probably prone to fail as soon as they are installed so there is no guarantee of longevity.

I'm no electronic wizard! From my small and limited experience with electrical components is this, they rarely if ever go bad, the problems usually lie with their soldered electrical connections. Throughout the years, I have heard and read about people re-flowing and resoldering the circuit boards in their ECU's. The reasons are that cracked/oxidized solder joints will cause various engine running problems. In most instances, they will say just resolder any cracked solder joints, in particular, the connection joints near the 2 heat sinks. The problem with this is that there are maybe cracks at a microscopic level that one cannot see, so this present a challenge as to what is a good or bad solder connection.

So, last night, I opened up the Motronic box and inspected it. Saw a couple of suspect solder joints and resolder them. One thing I noticed about the factory solder connection is that most of them are very thinly soldered in as some of the resistors, transistors and integrated circuit pins are barely soldered into the circuit board. I decided to add just a little more solder to some of the connections, so, one joint lead to another, 4 hours later, every one of the electrical connecting pins on the circuit boards (must be in the couple of hundreds) were reflowed/resoldered.

At that moment, which was about 2 AM, I cleaned up and reassembled the Motronic box to see whether the "repaired" unit would work or not. Reattached the wiring harness socket to the ECU, turned on the ignition and "viola", the engine fired up and ran normally and perfectly. Ran the car for quite a while and shut it off a few times and restarting the engine and still no problems. Pretty sure, this "sucker is fixed" now. Have my fingers crossed!

Some tips and tools for the job: A low wattage (15w) soldering iron with a ultra small tip so it does not overheat. Let iron heat up nice and hot, touch solder joints until it flows and if need be, add a small amount of solder. A spool of good quality thread thin rosin solder. A good size magnifier (preferably attached to a base) with a good light source to see and make soldering these ultra small connections a whole lot easier. Use a soft brush sprayed with a small amount of electrical cleaner to clean up the circuit boards and their solder connections and dry thoroughly. A can of aerosol air to blow and clean up any dust and old fragments/particles that may have accumulated in the electrical components and the circuit board. Making good solder joints is not hard, but it does requires steady hands, a little patience and a bit of practice.

[dm635]

Wow, I wouldn't have the patience to do every connection. But every old 059 ECU I've opened has benefited from resoldering the large transistor. I've seen some that I wouldn't even bother with if they had water damage.

[FernandoBunster]

WTG!!!! I love this story.

[carsnplanes]

WTG!!!! I love this story.

I know YOU do Fernando... and so do I.

[tlister67]

Wow, I wouldn't have the patience to do every connection. But every old 059 ECU I've opened has benefited from resoldering the large transistor. I've seen some that I wouldn't even bother with if they had water damage.

I thought I was going to flow all of mine until I saw how many. There was also seemed to be a lacquer coat on the board making the job quite difficult. Then I just did the ones that looked oxidized. Cleaned the baord with IP alcohol. No sign of water damage in mine.

[CW6er]

Great write-up!

Here are some pictures to go with it:
http://e28-535i.com/technical/e28-ECU_Repair.php
http://www.teamdfl.com/bmw/e28/ecu_repair/index.html
http://forums.bimmerforums.com/forum...3#post16490553


The large power transistor in the middle of the aluminum heat sink operates the coil, and have 6 pins to solder, 3 on the other side of the board and 3 on the transistor. The other 2 on either side are for the 2 banks of 3 fuel injectors.

[M6csi]

I've severely underestimated the number of solder connections on both circuit boards. Looking at the #2 link that CWer provided, there are maybe 200+ connections per board. So, in total, approximately 400-500 joints will need to be re-flowed. Seems like an unmanageable number, but after you have done a few solders and gained enough experience, each connectiion can be re-solder pretty fast. >>>

[Flash635]

While we're on this subject, does anyone have a spare 059 that they could look into and get a part number of a transistor for me?

I want to repair this 059 I have here but the transistor is so damaged I can't read the numbers on it.

[bimmerjo]

Its funny how the older BMW's test the area of your area of skill, where you are not-so-skilled.

[M6csi]

http://www.teamdfl.com/bmw/e28/ecu_repair/index.html

Not mine! The two 079 M5/M6 ECU pics are from the above link. In all probability, the Motronic box looks like the original unit that came with the car.

Top side board. Original solder joints on this board look really good and clean. Shiny solder joints indicate that they were re-flowed or additional solder was added. Board and solder connections look pretty much liked my 2 boards.


Bottom board and connections appear real suspect. Cold solders and cracked/oxidized joints, in addition to board condition may indicate some possible water exposure.


Chris, thanks for the pics and links!

[BigHeed87]

same thing with my L6, just fixed it today, Thank you so much for this post!!!!!

[bimas]

While we're on this subject, does anyone have a spare 059 that they could look into and get a part number of a transistor for me?

I want to repair this 059 I have here but the transistor is so damaged I can't read the numbers on it.

Can you post a picture of what you need? I have a 059 apart right now. I can clearly see the number on the large coil transistor if that's what you need -- 1012 followed by 6/1 bellow. I think the '1012' number should be good enough for cross-reference. If you need anything else, I have to dig deeper since the 2 boards are not separated at the moment. Just let me know.

[S14]

Bumping an old thread, great post OP!

I think I'm having a ecu/solder problem however I took the ECU apart today and didn't see any cracked solder joints. I did see plenty of unoriginal solder additions, and I'm thinking that maybe these repairs might be the cause of my problems.

Here are my symptoms:
-car stalled 3 times in the last 2 days while coming up to a stop light
-engine cuts out then immediately starts back up. Almost like the motor is blinking... very fast cutout/hiccup.
-I noticed the cutout/hiccup often happened with the car rolling over a decent crack in the road
-runs fine at idle, but cuts out/hickups at part throttle and cruising.
-pulls hard to redline making me think that it's not a fuel delivery issue.

Here is a GIGANTIC photo of one of the solder repairs, does this look good or bad? More photo's can be seen on my photobucket album... I don't want to kill the bandwidth. http://s98.photobucket.com/albums/l2.../M5/?start=all



Thanks for the help!

[dbc011]

Do the ECU's differ for the 635 & the M6?

[Flash635]

Can you post a picture of what you need? I have a 059 apart right now. I can clearly see the number on the large coil transistor if that's what you need -- 1012 followed by 6/1 bellow. I think the '1012' number should be good enough for cross-reference. If you need anything else, I have to dig deeper since the 2 boards are not separated at the moment. Just let me know.

Sorry I didn't reply to this post, one thing that does annoy me is if someone asks for help and it's given that the asker doesn't acknowledge the effort and concern that a replyer has given to the asker.

I did end up finding the number of the transistor, a google search shows that they are not available. The 4 beside are but not the one I wanted.

Surely another type of transistor would work but I have no idea how to cross reference it.

Right at this point the ECU isn't my greatest concern but I will get back to it one day, there isn't a bottomless resource of these ECUs.

[M6csi]

Do the ECU's differ for the 635 & the M6?

E24/E28 M5/M6 NA S38 engines use 079 designated ECUs and standard 535/635 engines use 059 ECUs among others. Euro M88 cars utilize 055 ECUs.

Status report on my re-flowed ECU: Nary a problem since repair of 2 years ago. Without a doubt, the most satisfying E24 repair projects for me ever. Anyone who is experiencing abrupt motor cutoffs and intermittent starting problems should look into re-soldering the 2 ECU circuit boards. Definitely beats paying hundreds or possibly thousand+ dollars for a rebuilt or new ECU from BMW.

[Steenland]

If anyone is still monitoring this thread, I have a question. What type of "no start" do you get with a bad ECU? In other words, will the engine crank but not catch? Solenoid clicks but starter doesn't turn? Nothing whatsoever? Thanks!

[Flash635]

That's more likely to be a faulty starter, bad battery leads or flat/bad battery.

[Steenland]

Thanks for the reply Flash. However, my question is, IF one has a faulty ECU, like those described above, causing a no-start, how does the no-start manifest? Does the engine crank at all? In other words, I know a faulty ECU will prevent fuel from getting to the injectors, but can it also inhibit the operation of the starter?

[stoner]

From what I have found a faulty ECU does not prevent the engine from cranking, that is the sole responsibility of the ignition switch and starter providing you have the voltage required to run the starter. An ECU no start situation will allow you crank the engine as many time as your battery will last. It may even start at one point but when you turn the car off it won't start again, it will just crank. Now, if you have a bad chip or socket you car may not start at all but it will just turn over and over. I had this happen when I upgraded my socket to 24 pin and my new chip got wet where it was stored. I ordered a new ECU and rather than order a new 24 pin socket I saved a few pennies by hacking up two old sockets to make 24 pins to solder into the new board. When I put the chip back in and it still didn't start and all of my solders and wiring changes were checked and rechecked I had to suspect that I needed a new chip which eventually solved my issues. I do get intermittent no starts and I am suspecting my latest board may need to be reflowed as well.

[Steenland]

Thanks Stoner. I have a vexing no-start problem with my 88 635CSi, but it's not even cranking, so I guess I can rule out the ECU.

[stoner]

Thanks Stoner. I have a vexing no-start problem with my 88 635CSi, but it's not even cranking, so I guess I can rule out the ECU.

Try this page.
http://pwp.att.net/p/s/community.dll...pid=160912&ck=

[e24mpwr]

I'm bumping this thread to say that a long-running series of balkiness under throttle and several years of a strange runs-great-until-it shuts-off-and-will-only-idle has been fixed by swapping in an known-good ECU. It feels great to have a car that is running well!

I'm going to have someone look at my original and see if it can be recovered...

[Bert Poliakoff]

Try fuel Injection Corp of America. This is a small one guy operation. IIRC he is an engineer at Livermore Radiation Lab in CA. IIRC when I called him several years ago about my very early Motronic ECU, he said don't worry about it as the early units are just about indestructible. He also said if was worried about it he would check it out entirely for $75. I have seen positive mention of him on the E28 boards as well. He may be worth a phone call as he seems to be a straight shooter.

[e24mpwr]

Try fuel Injection Corp of America. This is a small one guy operation. IIRC he is an engineer at Livermore Radiation Lab in CA. IIRC when I called him several years ago about my very early Motronic ECU, he said don't worry about it as the early units are just about indestructible. He also said if was worried about it he would check it out entirely for $75. I have seen positive mention of him on the E28 boards as well. He may be worth a phone call as he seems to be a straight shooter.

Thanks!