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I'm at the end of my rope guys and only you can help.
With keys in my hand, my battery discharge light remains illuminated. I isolated the circuit to the alternator by disconnecting the main battery lead and touching the positive battery post with just the wire that comes from the alternator. No light. I pulled every fuse and relay in the fuse box trying to isolate the circuit, with the battery connected, yet the light remains illuminated. To add to the mystery, somehow my fuel guage is reading (it has power going to it). Going through the wiring diagram, I believe it can be my service interval relay that will not relax. I would love to pull the wires for it but I cannot locate it. Can anyone give me an exact location of this relay? It's an 82 320i manual trans with factory air left hand drive. Oh, more info, I put a meter on the wire that illuminates the discharge light and it's reading around 2 1/2 volts. Also, this problem is intermittent and is not affected by temperature, humidity or length of driving time. Come on guys, who can pull a rabbit out of their a**? Thanks for any help and advice!

I had a similar thing and it was bad earths. Might be worth you cleaning yours up?

Yeah, that was one of the first things I did. But that is not the problem. You see somehow, even my fuel guage is reading with the keys out and the charge light illuminated. This is not right. But following the wiring diagram, still leads me to suspect this service interval/ seat belt relay.
I'll be digging into it this weekend. Thanks for the advice! But again, can anyone tell me where this relay lives?
Cheers!

I didnt know the e21 had a service or seatbelt light?

Looks like a specific US issue as only these models had Check Engine / Seat Belt warning lights. That defo places the relay and relevant wiring under suspicion.

Sounds like someone put in new wiring to me.
Maybe a previous owner had an electrical problem en decided to bypass the original system so now it has power going to it at all times ?
Have you checked if there are any wires that are not original ?

Greetings.

mark has a uk car and he had this issue - so i dont think its us specific issue.

id clean up ALL your earths even the ones under the dash. then check the relays (maybe replace?) failing that remove any wiring not original and id replace the hazard swtich a known working one and see if that makes a difference.

Thank you all for the advice.
As I stated above, after having every fuse and relay out, battery connected and key off, my charge light remained illuminated.
Enough was enough, I had to start digging.
Started cutting back the heat shrink on the harness leading from the alternator.
After cutting back about 10 cm, I found the problem.
After 31 years of vibration........
One field wire had about 5 cm of insulation missing, just bare wire.
The second field wire was burnt almost completely through in one spot, maybe three to four wires intact.
The large alternator output wire had it's insulation worn through.
So my field wires kept getting a voltage because they were in contact with the alternator output, which is connected to the battery thus telling the truth about a battery discharge.
I was never so happy to find a chaffed wire before!
Repairs completed in an hour or so and all is well.
Beware guys and gals, after over 30 years of running, these things will bite us in the you know what!
Thanks again everyone!
Oh, bought myself a "new" 82 320i with 107,000 original kms. Dark blue, original paint, I'm in love!
Now just have to transfer all the stuff from the old to the new.
Rear sway bar, air conditioning, fog lights, hazard light switch, glove box hinges, jack, etc.......
Cheers!

Not a common failure, glad that's sorted now! And your new toy, do we get more info and pics in the Owner's Gallery?

Of course! Let me wash and wax it first!

I have a 1982 BMW 320i US Model, and had this issue.
Go and check the back of your alternator. There is a blue wire with a clip at the end. This is the wire that sends the signal to you battery warning light on the dash board. (FIRST DISCONNECT BOTH SIDES OF THE BATTERY). You will have to crawl under front driver side of the car to fix this. You also may have to loosen the pivot bolt, and top bolt so you can rock the alternator down, and it will be easier to get at. There are two prongs which will support the blue wire clip. one is protruding from a small square opening. The other is next to the positive cable coming from your battery. I am almost betting that this blue wire clip is connected to the prong in the small square opening. Unplug the blue wire from that prong and connect it to the other in the larger opening. The battery light should be off now. Please let me know if this worked for you.

So, my problem. Please anyone with the same issue, I'm at my wits end!
One evening I noticed the battery warning lamp glowing in the dash board. I took the car home, and tested with a multimeter on the battery. I was not getting 14+ voltage from the alternator, so I replaced the voltage regulator. Still no fix. I re tested and under load I was getting 12.2 volts. Took alternator out, and had it rebuilt (probably didn't even need it now). Put it back in, and tested at 14.2 volts, check under load and 13.9. took for a drive, and tested again, and under load (everything on) 12.4 volts and battery warning lamp on again. I have checked all connections (ground etc), and cant figure this out.

Could this be a battery refusing a charge issue, or a belt issue? The belt seems tight, and no noise or squeeking. Like I said I'm at my wit's end

Thanks for your contribution on the earlier issue, hope that helps!

As for your charging issue... when the engine is running you don't need the battery at all. So if the voltage drops dramatically with the engine running, the battery should not be the cause, the alternator should output 13.8V +/- 0.5V. With a decent tension on the belt and things running freely that's good enough as well. The only think would be when the battery is really dead causing a short circuit but that's not the case I guess?

Lemme think, I suspect a ground problem. How are the main ground wires running? Alternator to engine, engine to chassis (on LH side), chassis to battery, right? Could it be a bad ground connection, especially at the chassis, corroded perhaps?

Thank you Jeroen.

It was the ground. There is a wire attached to the ground strap under the metal sleeve, which also connects to the back of the alternator. It must have not been making good contact. I made a second ground wire, and attached it to the frame under the opposite side of the ground strap, and then to the back of the alternator as a test. Well, it worked. It was a bad ground the whole time, and now fixed. 30 years of wear and tear took it's toll on the old wire.

Thank you for your response.

Mike

You're welcome, glad it's solved!