m30b34 power loss/shutdown - Need thoughts on where to start

[ThisOldJeep]

So...Happy Thanksgiving. The e28 (1988 535is, manual swap, Turner Conforti chip) is in the shop where a lot of head scratching is taking place after the car decided to shutdown completely on a lovely, cold Colorado morning. Here's what happened:
- car started from cold and I headed out to a meeting. About 15 minutes into the drive (a little bit of traffic in rural Boulder), the car died while I was just starting to drive from a stop headed downhill. Didn't exhibit any issues prior to the shut down. Instrument cluster dead, no electrics running (radio, OBD II, etc). The only new thing was the battery light came on and the car would not crank.
- After about 15 minutes of trying to get the car to crank, and disconnecting/reconnecting the battery a couple of times, I managed to get the car started. As I went to pull out of the parking lot I'd rolled into, it died exactly as described above: all electrical stuff dead, and the battery light illuminated. I gave up, got the car towed, and made it to my client meeting with a couple minutes to spare.

For background, the car had some issues with bucking and dying until I undid some crappy splicing a PO did on the CPS and Speed Reference sensor wires. After that, no issues and ran smooth.

I've asked the shop (classic BMW specialists) to see what they can make of it, but would love some thoughts on where to start. My list is:
1) Main relay (doubt this is it, but hey, easy place to start)
2) Voltage regulator
3) Battery (it's three years old, never had an issue but who knows?)
4) The stupid OBD/immobilizer relay thing that chimes when it feels like it for no apparent reason
5) DME

Any other thoughts?

[Mike W.]

Sure,

the battery light came on and the car would not crank

says to me it's battery cable/connection related. It might or might not be the main battery cable, it might be the little one on top, maybe where it goes, but somewhere in that neighborhood. If it was electronics or engine management it would still crank. You could disconnect the ECU and it would still crank. But that's all good, it suggests a simple fix.

[ThisOldJeep]

Thanks, Mike. That is a great place to start. The cables are old and at times I've had trouble getting good enough contact (the battery terminal connections are far from smooth and a lot of dielectric grease was the fix). I'll bug the guys on Monday and see if simple is the solution.

Much appreciated.

[4DSC]

Correct me if I'm wrong on this, but your mention of dielectric grease made me wonder where you applied it? My understanding is it's not conductive so you should not apply it between connections, ie. On battery terminals under the clamp, but it should be applied on top of connections which are already electronically joined to prevent corrosion and arcing from water exposure.

[ThisOldJeep]

Well how about that? 4DSC, you are correct! I guess I was misled by my father in law years ago. Anyway, that would explain my current issue if I had any on the terminals at present. I guess the grease didn’t help me a couple years ago even though I thought it did. I suppose retightening the battery clamps was enough to get a good connection back then.

Either way, step one will be to check the connections, test the battery and hope it’s simple.

[Notae30]

I've had issues like this where I've had bad grounds. cars went dead on me while i was in the twistys due to a bad ground just stopped. would flicker wouldnt turn on. i redid some grounds but the main one is the one on the battery terminal. the negative to the chassis i redid in 0 gauge OFC. never had a problem since

[gwb72tii]

I've had issues like this where I've had bad grounds. cars went dead on me while i was in the twistys due to a bad ground just stopped. would flicker wouldnt turn on. i redid some grounds but the main one is the one on the battery terminal. the negative to the chassis i redid in 0 gauge OFC. never had a problem since

Nearly every electrical issue I've ever had on any BMW I've owned came down to a bad ground.

[ThisOldJeep]

Well, it wasn't a bad ground, and it wasn't even a "bad" battery cable. Merely a misbehaving battery connector at the end of a fine battery cable. The positive terminal gave the impression of being connected, but the pitting and age-related issues with them meant that the connection wasn't perfect. A bit of grinding and polishing gave them a better surface to connect to the battery terminals meaning the electrons can flow more happily.

I think what was actually happening was as the car got hot, the cable connectors loosened leading to the power dropping out, the battery light coming on etc. Then, once everything cooled off, it could start, but vibrations, etc kept it from being a nice, tight, solid connection.

Thanks for the advice. Happy Motoring!