Recent battery drain issue

[Russell]

I recently had a battery drain on my 1995 525i that seemed difficult to find. It would drain a good battery in 24 hours. My Indy scanned the body and engine electrical system checked the gm and assorted relays.

Ultimately they found the OBC unit was responsible for the battery drain. They said they have never seen this problem before. There was a hint as the problem. The OBC would click every 10 seconds or so. You could hear it on if there was little noise. I did not know there was anything mechanical in an OBC that could make such a sound.

Everything is fine and I have another OBC on the way. Anyway this is an oddity. In over 30 years, this shop has never seen this issue. Just thought I would share.

[632 Regal]

Must be a small relay in there? Open it up and check it out, interesting indeed.

[shogun]

disassembly OBC part 1: Disassembly (http://evansweb.info/2003/02/22/obc-part-1-disassembly)
OBC part 2: Changing the backlight colour (http://evansweb.info/2003/02/24/obc-part-2-changing-the-backlight-colour)

assembly OBC part 3: Reassembly (http://evansweb.info/2003/02/24/obc-part-3-reassembly)

[Russell]

632 Regal, I opened it up. Nothing obvious yet. Three double sided circuit boards and three ribbon cables. Overdesigned/constructed in typical German fashion. Bet the Japanese build the same capability in a tiny chip now.

[genphreak]

AFAIK OBC is on the constant power circuit and has several uses when the car is locked;

- Immobilizer function
- Parked air circulation
- Clock

If there is a faulty relay (probably just a hairline joint needs soldering) then it may be simple to fix what you have. I've seen relays go bad and constantly go on and off, causing a small drain just keeping the relay on or having it fire all the time. In these cases the relay gets warm as it is constantly on/switching and over a few days will drain a good battery.

However just because the OBC ticks doesn't mean that it is faulty; root cause could well be elsewhere. For example, it might be getting 'woken' by a remote feed/signal that is incorrectly high, grounded or suffering from an open circuit.

Perhaps try unplugging your OBC before doing anything and see if the current drain after 15 mins is still high.

PS. The Japanese production stuff these days benefited from the Germans having done it all well in several ways (few of which were optimum) beforehand. All they had to do was dissect one of each and go from there in order to learn themselves. Of course none of us are perfect, and they may have occasionally screwed things up on a new model. Rather than be too late for producing a new model they could simply fall back on a last moment deal with desperate salespeople at the likes of Bosch (who'd help or even provide external engineers to implement systems where internal teams struck too much trouble.). But from what I've seen of Japanese production they'd only use the systems for the first model of a given production run before having local suppliers provide 'equivalents'.

Nowadays (read non-e34days) cars are designed with Canbus signalling in their circuits so there is no need for 30kg of body electronics wiring, everything can be run from literally 3 main circuits and be built using solid state/Surface Mount Tech.

[Russell]

Informative post and entertaining as well. :)

Car seem to run fine without an obc. Does not seem to be any battery drain. That said, I WILL NOT say the problem is fixed until I put another OBC in the car and everything works fine. Should have it in a couple of days.

[genphreak]

:) No probs! Yep it will work without no trouble you just miss a few 'useless features'. I just re-edited that post as it wasn't too clear, but anyway, I', sure you'll have your trouble nailed soon.

I'm working on e34 electricals a lot ATM and am constantly in awe over how involved they were, however back then things were pretty rudimentary on all other cars. What they did with the smart logic in the e32/34 platform by implementing the GM-controlled body electronics with nothing but pretty basic tech of the day was really incredible, esp. when you realise how incredibly reliable they got it (let alone the amount of wires and components, and the fact that it was all being done for the first-time).

All in all it isn't that hard to troubleshoot either- once you get used to the wiring diagrams and all the ingenious ways Hannes, Hans and Hansie were at hiding modules and snaking wires in and out of every cavity in such a way as to never let it come out again.

Another example; under the rear seat there are probably over 20 large ground wires on perhaps 6 grounding posts. That's not counting the module wires, just the grounds for the relays and main components!

[Russell]

The battery was weak after installing the used obc. In fact so low the car would not start. Before we say the drain is still there. note that I have only used the car for short trips (1-3 miles) in the past few weeks So I doubt if it was well charged. I bought a Deltran Battery Tender Plus to charge it up and help me though the issue.
Again, thanks for your comments, they are most informative.

[genphreak]

Cool. Thanks for posting that update. Do let us know if the OBC ticking goes away with the new battery (are you sure its not the fan blower or an IHKA unit actuator?).

Bad batteries are a commons source of faults on older cars. The electronics in an e34 does not like sub-par batteries, and one can get as little as a year or two and sometimes 7 years out of a battery, it just depends on the weather, how you use the car, the accessories and how well the alternator/regulator maintains charge. As a result, none of us know when it is the battery at fault. I reckon what we need is a standard testing procedure that can be done without specialised gear- ie where the battery is removed, charged, tested and then measured... (we all spend weeks fiddling with these kind of symptoms only to find out in the end it was the damn battery after all!)

[Russell]

Installed replacement OBC a couple of weeks ago and all seems to be fine. Battery seems to have not been damaged by severe discharge.

[genphreak]

Ah-ha! Good work Russell.

It'd be good to test and compare it against the ones (I have some here) to isolate the internal cause. These are litle nigglies it's worth knowing about as e34s 'mature' :-)