Intermittent no-start with 635 Motronic 1.0

[Dave Rutkowski]

Hi. I have an '81 635 (e12-based) with early Motronic. Car has run well for years, mostly driving schools believe it or not. But the last couple of months I've experienced an intermittent no-start situation. Car is garaged in Chicago but just sits over the winter months, on a battery tender. Starter turns the motor over very strong, but sometimes it doesn't even try to catch (other times it fires on the first crank). Fuel pressure is good, and if I spray starting fluid in the intake that doesn't help - so I assume the fuel system is working and the problem is spark.

Sometimes the engine will start on the first turn, and then 10 minutes later it won't. Engine has never died while running (and it runs fine), it just won't start sometimes. I have verified no spark (pulled a wire, put a spare spark plug in it and grounded it - nothing). Verified Main Relay is functioning, and the ignition coil has >12V at both posts while starting or running. One other weird thing - a couple of times I have started to drain the battery trying to start it, and have put a 10A battery charger on there. Both times I did this, the car started immediately, so thinking maybe something in the system needs a threshold voltage in order to fire?

Thinking ignition coil at this point, and I just took a close look at it and it appears to be original. I know I have not replaced it in 25 years anyway. But it seems odd that the coil would fail intermittently, and would never fail once running. Any ideas on this?

[canada karl]

How old is the battery?

[528i-1981]

Have you eliminated the injectors as a source of the problem? A spark tester will allow you to verify is spark is the problem or not.

[Dave Rutkowski]

Fair question on the battery - I checked my files and looks like it's ~14 years old so due for replacement anyway. But it turns the starter over really strongly, and that doesn't explain why it's intermittent.

[528i-1981]

Sorry, Dan I should have read your post more carefully. I think you've properly narrowed in on the spark as the culprit.

I did have a strong-crank no-start situation that was due to a partially discharged battery. Once charged, it started right up. So I think Karl might be on the right track. Have you ever charged the battery but still had a no-start?

[Dave Rutkowski]

Sorry, Dan I should have read your post more carefully. I think you've properly narrowed in on the spark as the culprit.

I did have a strong-crank no-start situation that was due to a partially discharged battery. Once charged, it started right up. So I think Karl might be on the right track. Have you ever charged the battery but still had a no-start?

That's interesting, and a good question - I don't think I've had the no-start on a fully charged battery. But I have had a case where it wouldn't start, I was troubleshooting for awhile (not doing anything to the battery), and then tried again and it started right up. So it's inconsistent.

I picked up a new coil (at 39 years old, it's probably worth replacing just in case). I'll swap that and see if I still get the no-start after the battery is no longer fully charged, just to satisfy my own curiosity. But will probably replace the 14 year old battery either way as well. Of course when I went to start the car tonite without having changed anything, it fired right up on the first crank...

[canada karl]

Further to batteries. 5 years takes a lot of life out of a battery up here in Canada with our winters. 14 years might be be pushing it. Other thing is that once a battery has been run dead once it's never quite the same. Might take a charge but just wont have the power.

[Dspoljaric]

Dave, I'd swap the ECU with a know good one to determine if it is a ECU that is beginning to fail. An ECU that is on its way out will start to cause intermittent start issues.

Here is a link to an eBay seller that could help you identify a replacement. I'd start with pulling your ECU out far enough to grab the Bosch part number off the sticker on the housing.

Good luck - cheers

[Dspoljaric]

Sorry, forgot the link:

https://www.ebay.com/itm/0280001118-E12 ... 2994583136

[528i-1981]

That's interesting, and a good question - I don't think I've had the no-start on a fully charged battery. But I have had a case where it wouldn't start, I was troubleshooting for awhile (not doing anything to the battery), and then tried again and it started right up. So it's inconsistent.

It takes a fair amount of juice to fire up everything, recalling the ignition and fuel delivery are all booted up together, and it's all analog. I think there are likely edge cases where a certain charge level on the battery may work sometimes and sometimes not.

This is just spitballing, but I had some cold-start issues at one point because the idle switch on the throttle body was stupid-dirty and wasn't always closing. After some no-starts, the movement of the throttle linkage would eventually close the switch and the L-Jet ECU would understand it was a cold-start. Not sure if Motronic 1.0 works the same, but I think you do have a limit switch for idle on the throttle body.

[keehn]

Dave,

I went on realoem.com and couldn't find any 1981 635's. However, I found 633i for the 1981 year; Is it possible your bimmer is mislabeled?

Anyways, the part number on the 1981 633i ECU is "12141466072". Google "bmw 12141466072" and it will narrow down any possible sellers. Please be aware that the ECU's around this timeframe (mid 70's to early 80's) were being continuously updated, and that an ECU from another year might not work with your vehicle.

Here's the link to the ECU I was able to look up:

https://www.realoem.com/bmw/enUS/showpa ... Id=12_0018

Mike

[Dave Rutkowski]

Dave,

I went on realoem.com and couldn't find any 1981 635's. However, I found 633i for the 1981 year; Is it possible your bimmer is mislabeled?

Anyways, the part number on the 1981 633i ECU is "12141466072". Google "bmw 12141466072" and it will narrow down any possible sellers. Please be aware that the ECU's around this timeframe (mid 70's to early 80's) were being continuously updated, and that an ECU from another year might not work with your vehicle.

Here's the link to the ECU I was able to look up:

https://www.realoem.com/bmw/enUS/showpa ... Id=12_0018

Mike

It's a Euro car, definitely an '81 635 with M90 engine. ECU is Bosch 0 261 200 002. I'm trying to eliminate the easy (cheap) causes before I tackle replacing the ECU, but thanks for that link.

[Dave Rutkowski]

Further to batteries. 5 years takes a lot of life out of a battery up here in Canada with our winters. 14 years might be be pushing it. Other thing is that once a battery has been run dead once it's never quite the same. Might take a charge but just wont have the power.

Looks like it was the 14 year old battery. I let the battery draw down to 11V, it cranked strongly but would not start. Interestingly, while cranking there was only ~6V available to the ignition coil - not enough to get it to fire. I swapped in a new coil, no change. Then I hooked up the battery charger and tried again, and now there was ~8V to the coil and that was enough for it to fire right up.

So I guess I replaced the 39 year old coil as preventative maintenance, and now I'm off to pick up a new DieHard. Thanks Karl for the lead on the battery!

[canada karl]

Glad you got it going. Let's keep those 5's on the road.