Sudden cut off

[russellrh72]

I wanted to be careful with the title here. My car is not having "stalling" issues, or "loss of power" as in horsepower issues. I think it is actually powering down, or cutting off completely at random points during a drive. Out of nowhere, I suddenly have no acceleration and the gas pedal is dead. You can here the silence of the car having cut off. The RPM slowly dive, and then the car will either kick back in on its own, or I have had some success with slowing down some and then popping the clutch to kick it back in. Around the same time, I began having difficulty starting the car about 1 out of 5 times I get in it. But it always eventually cranks. Not sure if that's related.

The car is a 1988 535is. Manual swapped back in the spring. I have researched here but I get topics about slow stall outs or loss of horsepower, but nothing that describes my issue. I have tried the following to correct the issue with no luck.

Replaced coil
Replaced both reference sensors with known good ones.
Replaced main relay with known working relay
Cleaned and checked grounds in engine bay
Cleaned and checked connections at battery terminals
Checked for vacuum leaks and replaced rubber boots (although that doesn't seem to be something that would cause this).

Next I'm thinking ECU maybe? But I don't have one to test. Is there a trick? I did read where an ECU "tap test" was mentioned but it did not explain the process. Any ideas or help would be appreciated. Thanks all

[kojo96]

I had a 533i w/ an automatic transmission that would randomly shut off, while driving, while sitting at idle, while moving slow or fast and sometimes just at a stop sign. I chased that issue for months, to no avail. It didn't matter if it was hot/cold out, or even if the engine was hot or cold.
Did lots of trouble shooting, changed parts that tested marginally and the problem persisted.
One day, I was determined, so hood up, car ideling and I started touching things, wiring, connectors mostly, and wiggling just about everything..
Here's what I found, when I wiggled the C101 connector, at the harness end and by the wires,the car would stall... unplugging the connector it was visibly obvious that the female pins had spread open. After going through them all and closing them up, that issue was solved. The Green wire on that plug feed the coil.
I kept going, same thing, hood up, car at idle. Tapping the main/fuel pump relay didn't do anything, but... grabbing them amd moving them around made the car shut down. Tested the relays, all good, looking at the connectors (harness end) it looked like the prongs were all there and in place. Tried again, wiggled the main relay and the car cut off. This time I pulled the relays, and took the connector bodies off of the fuse box and started to take a close look. A couple of the female spade connectors were spread out, to the point of allowing the relay tabs to loose contact. Also, the 12v battery wire was bad, corrosion and barley hanging on. Trying to splice in another relay body didn't work well, wires were too short. I ended up replacing the entire harness for both the fuel and main relay using another harness that I removed form another car.
Stalling issues fixed, the car never stalled again.

[russellrh72]

I had a 533i w/ an automatic transmission that would randomly shut off, while driving, while sitting at idle, while moving slow or fast and sometimes just at a stop sign. I chased that issue for months, to no avail. It didn't matter if it was hot/cold out, or even if the engine was hot or cold.
Did lots of trouble shooting, changed parts that tested marginally and the problem persisted.
One day, I was determined, so hood up, car ideling and I started touching things, wiring, connectors mostly, and wiggling just about everything..
Here's what I found, when I wiggled the C101 connector, at the harness end and by the wires,the car would stall... unplugging the connector it was visibly obvious that the female pins had spread open. After going through them all and closing them up, that issue was solved. The Green wire on that plug feed the coil.
I kept going, same thing, hood up, car at idle. Tapping the main/fuel pump relay didn't do anything, but... grabbing them amd moving them around made the car shut down. Tested the relays, all good, looking at the connectors (harness end) it looked like the prongs were all there and in place. Tried again, wiggled the main relay and the car cut off. This time I pulled the relays, and took the connector bodies off of the fuse box and started to take a close look. A couple of the female spade connectors were spread out, to the point of allowing the relay tabs to loose contact. Also, the 12v battery wire was bad, corrosion and barley hanging on. Trying to splice in another relay body didn't work well, wires were too short. I ended up replacing the entire harness for both the fuel and main relay using another harness that I removed form another car.
Stalling issues fixed, the car never stalled again.

This is super helpful and encouraging advice. I'll get to wriggling one day this week.

[1st 5er]

Not E28, not even Bimmer, but rather wifey's XJ.
Same issue to yours with the occasional no-start hot or cold. After much fumbling around and parts swapping, I found female wire connectors under the fuse/relay box to the fuel pump relay and the ASD relay both had dislodged themselves and were only making contact with the male relay prongs by touching rather than by being actually inserted.
Reinserting those connectors solved the problem, for now. If either of those relays fail I'll have to pull the fuse box to securely reinstall those relays because the tabs in the fuse box that secure the wire connectors are nonexistent at this point.

Good luck with you fix.

[russellrh72]

Not E28, not even Bimmer, but rather wifey's XJ.
Same issue to yours with the occasional no-start hot or cold. After much fumbling around and parts swapping, I found female wire connectors under the fuse/relay box to the fuel pump relay and the ASD relay both had dislodged themselves and were only making contact with the male relay prongs by touching rather than by being actually inserted.
Reinserting those connectors solved the problem, for now. If either of those relays fail I'll have to pull the fuse box to securely reinstall those relays because the tabs in the fuse box that secure the wire connectors are nonexistent at this point.

Good luck with you fix.

This is also helpful! I will do some digging

[K-jetFTW]

I'll second what everyone else said.

I bought my 533i and it died several times on my 100 mile trip. Hitting bumps would do it, complete shut down each time. I nursed it home.

After some fumbling and replacing relays/fuses in vein, I noticed just like others that wiggling wires would energize or cut off power. I started probing near the relay harnesses and noticed the "hot" wires weren't always hot. I think it was terminal 30 on the fuel pump relay where i noticed it first. The 12v would magically reappear and the main and fuel pump relays would click to life by moving red wires.

Ended up tracing it back to one of the main 12v leads directly from the battery. It had several shoddy splices from PO.

Good luck and make sure your hot wires are hot !

Kjet

[Shawn D.]

Next I'm thinking ECU maybe? But I don't have one to test. Is there a trick? I did read where an ECU "tap test" was mentioned but it did not explain the process. Any ideas or help would be appreciated. Thanks all

Where did you read about this "tap test"? I don't recall that as ever been discussed here.

My money is on bad solder joints at the ECU power transistor. Search for "ECU solder" (as keywords, not as a phrase).

[Mike W.]

Next I'm thinking ECU maybe? But I don't have one to test. Is there a trick? I did read where an ECU "tap test" was mentioned but it did not explain the process. Any ideas or help would be appreciated. Thanks all

Where did you read about this "tap test"? I don't recall that as ever been discussed here.

My money is on bad solder joints at the ECU power transistor. Search for "ECU solder" (as keywords, not as a phrase).

I've not heard of it before either, but my guess is it's tapping/hitting the ECU is what is being referred to, and a rap of the knuckles might briefly remake the broken connection.

But I would look towards other diagnostic info, what does the tach do? Does it drop to zero or does it just accompany engine RPM? Any lights on the cluster light up? Or does it go blank? IIRC without looking at it, power to the main relay comes right off the battery. Easy for the main positive/negative terminals to be good, but the secondary not so good.

Now it seems I always had weird stuff happen to me, but on my '85, 535i, I had it oddly dying at times. Rarely, the most was on vacation one time, 4X during 4K miles, I'd coast to the side of the road, take a deep breath, turn the key off, then back to start. And it did, every time. Later I had a problem, idle was off just a touch, not high, not low, but I could tell something was off. Occasionally it would die at a stoplight. Just die, like someone shut the key off. Same time period it would occasionally just kind of become disconnected, pushing on the gas pedal did nothing. Didn't die then, but nothing. I would coast for a bit, then all of the sudden it would be ok again. Eventually I tried a different AFM. Bam, at first start I could tell that was the problem. It was and it never came back. Odd, I've never heard of it before, or since, so long shot, but keep it in mind.

[russellrh72]

Where did you read about this "tap test"? I don't recall that as ever been discussed here.

My money is on bad solder joints at the ECU power transistor. Search for "ECU solder" (as keywords, not as a phrase).

I've not heard of it before either, but my guess is it's tapping/hitting the ECU is what is being referred to, and a rap of the knuckles might briefly remake the broken connection.

But I would look towards other diagnostic info, what does the tach do? Does it drop to zero or does it just accompany engine RPM? Any lights on the cluster light up? Or does it go blank? IIRC without looking at it, power to the main relay comes right off the battery. Easy for the main positive/negative terminals to be good, but the secondary not so good.

Now it seems I always had weird stuff happen to me, but on my '85, 535i, I had it oddly dying at times. Rarely, the most was on vacation one time, 4X during 4K miles, I'd coast to the side of the road, take a deep breath, turn the key off, then back to start. And it did, every time. Later I had a problem, idle was off just a touch, not high, not low, but I could tell something was off. Occasionally it would die at a stoplight. Just die, like someone shut the key off. Same time period it would occasionally just kind of become disconnected, pushing on the gas pedal did nothing. Didn't die then, but nothing. I would coast for a bit, then all of the sudden it would be ok again. Eventually I tried a different AFM. Bam, at first start I could tell that was the problem. It was and it never came back. Odd, I've never heard of it before, or since, so long shot, but keep it in mind.

So I got my audio and visual remembrance mixed up. I didn't read about the tap test, but instead was told about it by my friend who has worked at the BMW dealership here in town since 1994. He told me that one of the things they were taught at the dealership was to hold the ECU flat in your hand with the car running and tap on it with the handle of a screw driver or something similar. If the ECU was bad or had bad points it would start causing problems in the car. Sounds simple enough but I haven't done it yet.

To elaborate more on my issue, the the RPM stay where they were when the car cut out. Normally I'm cruising down the road and merely feathering the gas pedal and can all the sudden tell that the car has cut off because when I push the gas pedal nothing happens. The RPM slowly come down as the car starts to slow down obviously. But the lights on the dash do not come on until the RPM get low enough for the car to stall out or i push the clutch pedal in. I have had one or two instances where the car has cut out while sitting at a stoplight but normally it is while driving down the road. Sometimes I can make five or six trips in the car without this happening, and other times it happens multiple times in one trip. I can feel the car kick back in on its own half the time or other times I feel like popping the clutch brings it back to life. The symptom that is showing itself more frequently now is the no start. Seems like once a day now I have to sit in my garage or in a parking lot and try for 2 or 3 minutes to crank the car. It always does. More often than not it cranks right up and runs fine. Since I bought the car in October, I've had sort of a funny idle. When I first bought the car, it was a very constant surging idol. Bouncing between 1000 and 1500 but always high. Stop lights were maddening because it would just fluctuate in rhythm up and down up and down. That has gotten better on its own over the last 10 months. The idle does not surge anymore and it it only idols slightly high at about 1,000 or $1,100 RPM at stop lights half the time. The other half of the time it idles lower where it should. Most of the time when the car is idling around 1000 RPM, I can rev the engine slightly and it will bring the RPM down to normal. It's odd to me that the idling issue has continued to get better over time on its own. I know these cars like to be driven and I was told by the previous owner that it was not driven very much. Perhaps just loving on the car over the last 10 months has made it run better. But the cutting off issue seems to be separate obviously

[stuartinmn]

It doesn't sound like this is the problem but I'll put it out there anyway as a possibility...some years ago, I was having problems with my car where it sometimes wouldn't start, and sometimes would just momentarily quit running. After a lot of fooling around I figured out it was the onboard security function - the deal where you can set a code on the keyboard of the onboard computer to disable the car. That feature controls a relay that's in a metal enclosure under the dash, located about where your left knee is when you're driving. That relay supplies power to the main relay, so if it opens up the car won't start or run. I removed the enclosure and put in a jumper wire to bypass the relay's function, and it fixed the problem.

[russellrh72]

It doesn't sound like this is the problem but I'll put it out there anyway as a possibility...some years ago, I was having problems with my car where it sometimes wouldn't start, and sometimes would just momentarily quit running. After a lot of fooling around I figured out it was the onboard security function - the deal where you can set a code on the keyboard of the onboard computer to disable the car. That feature controls a relay that's in a metal enclosure under the dash, located about where your left knee is when you're driving. That relay supplies power to the main relay, so if it opens up the car won't start or run. I removed the enclosure and put in a jumper wire to bypass the relay's function, and it fixed the problem.

Wowzers! Didn't even know that existed. How did you determine that was the issue or ever think to consider that?

[stuartinmn]

Wowzers! Didn't even know that existed. How did you determine that was the issue or ever think to consider that?

I spent a lot of time studying the repair manual and probing with my multimeter (it probably also helps that I'm an electrical engineer. )
If you have a Bentley manual, it's shown in the wiring diagrams.

[russellrh72]

Okay everyone....I'm still dealing with this. I drove the car a lot today because it was so beautiful here in Winston Salem, NC, and the damn thing cut off on me 4 times. The most since the problem began. 3 times while driving, and once at a stop light. Prior to today I found a couple of links when researching ECU issue that describe my symptoms. The one from bimmerforums describes it perfectly. Both talk about ECU repair, so I decided to pull my ECU and open it up. This thing looks terrible to my admittedly untrained eye. Pictures I have seen of ECUs have solder everywhere, and on mine it appears that about a third of the pins don't have any. Could someone with more knowledge than me chime in on whether or not this ECU looks problematic. If you look at the board, you can see many pins that literally have zero solder them, which has to mean that virtually all of the other ones have at least weak or cracked material on them. There seems to be a few dark spots of possible corrosion, but most of the pictures I have viewed have at least some of that going on. Can anyone comment on the success rate of tackling this with a soldering gun and some thin solder? I am willing to give it a try, even though I have about the shakiest hands on earth. I could buy a known working ECU from someone if the price makes sense, put it in the car and see if it solves the problem before I spend hours trying to solder all these points. Rebuilt ECUs I've seen are kind of expensive, but if they are in fact rebuilt and good then I suppose it would be worth it. Any advise would be appreciated. Of course I still don't even know if this is causing my issues but it sure doesn't look good to me. Thanks everyone.

http://e28-535i.com/technical/e28-ECU_Repair.php

https://www.bimmerforums.com/forum/show ... g-problem-

[HayekFan]

Some of those pads do look kind of unsoldered. I wonder they really are or if the solder is just hidden under the conformal coating. You might try gently scraping away some of the coating around one and seeing if there's solder underneath.

FWIW, I was chasing a weird intermittent no-start and extreme sudden power loss problem for a while that turned out to just be a bad distributor cap and rotor. I checked all the things you've checked and replaced the things others have mentioned and was starting to suspect the ECU. Then I happened to read in the archives about worn distributor cap causing erratic symptoms. I had ruled that out because it seemed to me you'd get constant running problems, not sporadic ones, with a worn cap/rotor. But sure enough that was it. Made me feel kind of silly for skipping over something so basic...

[ThisOldJeep]

So, I had a similar issue that haunted me for the first few years of owning my ‘88 535is. Two things ended up being the issue: first, the fuel line from the tank to the main fuel pump was crap. Gummed up from breaking down leading to a massive constriction. So, fuel pressure would drop and the car starved until it settled and could work again. Sometimes this led to an no start situation which sucked. A new hose and a conversation to a single in tank fuel pump fixed that part of the issue. But I still had cut outs going over bumps, etc which were inexplicable.

Ultimately, i found a motronic diagnostic process write up that led me to the reference and crank position sensors. In my case the sensors are fine, but the wiring from the ECU to the sensors has seen better days. There’s a spot where the shielding or ground layer was damaged which would cause cut outs similar to what you’ve described. Diagnosis is easy. With the engine running, wiggle the wires starting down by the sensors and work your way up. If you kill the motor, you might have found the culprit.

Good luck!

[russellrh72]

Some of those pads do look kind of unsoldered. I wonder they really are or if the solder is just hidden under the conformal coating. You might try gently scraping away some of the coating around one and seeing if there's solder underneath.

FWIW, I was chasing a weird intermittent no-start and extreme sudden power loss problem for a while that turned out to just be a bad distributor cap and rotor. I checked all the things you've checked and replaced the things others have mentioned and was starting to suspect the ECU. Then I happened to read in the archives about worn distributor cap causing erratic symptoms. I had ruled that out because it seemed to me you'd get constant running problems, not sporadic ones, with a worn cap/rotor. But sure enough that was it. Made me feel kind of silly for skipping over something so basic...

I can easily to this as well. I did replace the coil as I mentioned above because I had one that I knew to be good so I swapped it in. Cap and rotor did cross my mind but I assumed, and maybe incorrectly, that if those were the problem I would be getting a missing or stumbling type of situation rather than an instantaneous shut off. I will try that when I get it buttoned back up

[russellrh72]

So, I had a similar issue that haunted me for the first few years of owning my ‘88 535is. Two things ended up being the issue: first, the fuel line from the tank to the main fuel pump was crap. Gummed up from breaking down leading to a massive constriction. So, fuel pressure would drop and the car starved until it settled and could work again. Sometimes this led to an no start situation which sucked. A new hose and a conversation to a single in tank fuel pump fixed that part of the issue. But I still had cut outs going over bumps, etc which were inexplicable.

Ultimately, i found a motronic diagnostic process write up that led me to the reference and crank position sensors. In my case the sensors are fine, but the wiring from the ECU to the sensors has seen better days. There’s a spot where the shielding or ground layer was damaged which would cause cut outs similar to what you’ve described. Diagnosis is easy. With the engine running, wiggle the wires starting down by the sensors and work your way up. If you kill the motor, you might have found the culprit.

Good luck!

This is easy enough to try as well. I did sit in the passenger floor board holding the ECU after unbolting it from above the glovebox last night. With the car running I jiggled the ECU harness, tapped on it etc and nothing happened. I didn't go under the car, however.
Now the issue is that I have the ECU out of the car and opened up. I could throw it back in the car easy enough but I'm wondering if I should resolder all of these points while it's out. Is that something I could do more harm than good by trying? From looking at the picture does it seem possible that the condition of the ECU could be causing the problem? I feel like since I have it out I should try to improve it's condition but I don't want to ruin it either. This is the first ECU I've ever had open so I really don't know if it looks bad or not, but it sure looks different than any other I have viewed in pictures.

[russellrh72]

Also, just to help me rule it out, the ECU functionality would not have been effected by the manual swap would it? When looking at my car on real OEM, there are two control units present. One control unit DME 12141705619, and one AT-control unit 12141706194. Both are still in the car after the swap.

[Ohmess]

If you've never done any board soldering, I wouldn't start with this. You need to control the amount of heat you put onto the board to prevent damaging the traces that run throughout the board and to avoid flowing solder into nearby connections. Boards can go from nothing is happening to solder all over the place pretty quickly.

I would go back through the wiring changes you made when you did the tranny swap.

What did you do for the motronic full throttle input (pin 3) that previously came from the transmission control unit?

What about the motronic pin 28 transmission position input from the TCU?

[russellrh72]

If you've never done any board soldering, I wouldn't start with this. You need to control the amount of heat you put onto the board to prevent damaging the traces that run throughout the board and to avoid flowing solder into nearby connections. Boards can go from nothing is happening to solder all over the place pretty quickly.

I would go back through the wiring changes you made when you did the tranny swap.

What did you do for the motronic full throttle input (pin 3) that previously came from the transmission control unit?

What about the motronic pin 28 transmission position input from the TCU?

These are very good questions sir and the answer is nothing. I did very little wiring for the swap. I jumpered pins 2 and 3 at the auto wiring harness which I had read would needed to be done to get the car to start, by telling the car that it was in park. Turns out I may have done that unnecessarily because I then learned from this wonderful forum that I could just jump the starter relay, which I then did. The car drove for some time after the swap without this cutting out issue. I am willing to do some more necessary wiring for the swap even if it doesn't solve my current issue.

Also, I hear what you're saying about attempting to perform repairs on all the solders in the ECU. It would make things easier if I had a known working ECU to swap in to see if it even fixed my problem. Perhaps I'll post a WTB here on the forum for one

[Ohmess]

So, if I have this right, you jumped pins 2 and 3 at the "auto wiring harness", and that didn’t allow the car to start, so you then went in a different direction and bypassed the automatic transmission starter relay. OK.

Did you remove the jumper at pins 2 and 3, or leave it in?

And where exactly did you do this? Was it at the white 13 pin connector above and to the left of the glove box (connector C238)? Or at the harness connected to the transmission control unit?

[russellrh72]

So, if I have this right, you jumped pins 2 and 3 at the "auto wiring harness", and that didn’t allow the car to start, so you then went in a different direction and bypassed the automatic transmission starter relay. OK.

Did you remove the jumper at pins 2 and 3, or leave it in?

And where exactly did you do this? Was it at the white 13 pin connector above and to the left of the glove box (connector C238)? Or at the harness connected to the transmission control unit?

I jumpered pins 2 and 3 at the connector in the auto trans shifter harness per a couple of threads I read here, and I jumpered the starter relay under the driver kick panel at the left knee. Both remain that way now. The no-start from that issue ended up being the black connectors for I think the ignition that I didn't get plugged in all the way when I was putting the column back together. I didn't do anything under the glove box.

The car fired up and ran flawlessly after the swap from April 2nd until May 18th when I had the car cut out one time while making the drive to Vintage in Asheville. Since then it has done it regularly.

[1st 5er]

Anyone localish have a spare ECU you can try?

[russellrh72]

Anyone localish have a spare ECU you can try?

Not that I know of right off the top of my ol' noggin but that is a good plan. I will look around. There are a couple of very reputable shops in my area like korman and Motorsport connections. They might have one