Battery light

[Rocket356]

Hi.
I've bought an E21 316 (UK model) and, on the whole, am very happy with it indeed. It has a couple of niggles that I would like to sort out before I can really enjoy it.

When the battery is connected, the battery light on the dash comes on (even with the ignition off). When I started it to bring the car home, the light went off. Then about half way through the journey, it came back on again, then went off, the back on for the rest of the journey. Once the engine was switched off, it stayed illuminated. I tried again today and had the same - light comes on when battery is connected and goes out when started - but didn't take it for a spin to see if it would come back on again.

I've done a little searching and found this thread:
viewtopic.php?f=3&t=20129&start=0&hilit=battery+light

It sounds like he had a similar problem, but the fix was a strange one (ie stripped wires inside the heat shrink). Somebody mentions bad earths.

Today I went out and cleaned up the main battery earthing point, plus that odd (to me - I'm used to older British stuff like Hillman Imps) earthing point that mounts behind the L/H suspension top under the bonnet. I cleaned up the little brown alternator earth cable, too. Still no luck, light still on.

So what other earths are there to clean up - does each light unit have a separate earth, for example? In that thread somebody mentions cleaning up the earths behind the dash, but I don't know how to get to those.

One final point that may be relevant: my car needs a new hazard switch (it's currently wedged in, like most people have had to do at some point). Could this be causing the battery light issue? And if so, can I isolate it until I have time to fix the switch?

Thanks.

[Jeroen]

Main chassis ground point underneath the battery tray? Also see our FAQ section. No relation to the hazard switch.

[Rocket356]

Do you mean the earthing strap that runs between the engine block and the L/H chassis rail? If so, I just tried cleaning it up the mountings and putting it back on - no luck.

Anyone have an idea of how I can trace the fault, short of going through the wiring diagram, finding every single earthing point and cleaning it up?

[nas80]

Are all the relays in the fuse box original ones? If someone replaced but used a generic one it can cause the issue as the pins are the other way.

Check the engine to body earth and make sure the alternator is ground to the block.

[Rocket356]

Not sure on the relays, I'll check.

Engine to chassis ground has been checked but I only had time to get to one end of the alternator ground, so I'll try that next.

Thanks.

[Rocket356]

Three relays have a BMW symbol on them, one is just a Bosch. I'll look further into that.

The problem has now reversed itself. On the drive home last night, the light came on while buzzing along the M4. When I got home and turned it off, the light went out. Now, when the battery is connected, the light is off, but once it's started up the light comes on again. So it's now doing the opposite of what it was doing before. Strangely, even driving home at night for an hour with the lights on, it doesn't seem to have run the battery down despite the light showing.

[Rocket356]

Update:
So the light now goes off when the ignition is turned off, but remains on while the engine is running. Revving it only makes the light brighter. Just because the light isn't on when the ignition is off, the problem clearly isn't solved.

Main battery ground has been checked
Block to chassis ground checked
Relays looked at - one is a Bosch brand instead of BMW but that doesn't explain the way this problem seems to swap itself around at random
Alternator brushes looked at - appear to have enough meat left on them

I'm going to put a new fan belt on as I don't seem to be able to adjust it any further out (although that doesn't explain the light being on when the ignition is off).

According to the wiring diagram, there should be an earth from the back of the alternator direct to the battery. Mine doesn't have that, it just has a small brown wire going from the back to the alternator mounting bolt next to it.

[Jeroen]

Hmmm that still leaves us all puzzled I guess! Could it be a corrosion issue? Did you do a fuse rolling session?

[Rocket356]

Indeed. Not sure about the corrosion, but when it was still lighting up with the ignition off, I did try removing some fuses. Maybe I'll try it again with the car running.

[Reck]

Is the alternator itself grounded properly? Battery lights are usually earthing or charging issues (presume the amount of driving you have done has ruled out the latter?).

[opal green]

I had this issue with an intermittent battery light. The light is telling you the truth, your battery is discharging. I went so far as to remove every fuse, and when I would connect the battery, the light would still illuminate. It wasn't until I cut back most of the heat shrink that I found my battery (red wire) was chafed through to the field wire (I think it was yellow striped), therefore, with the ignition off, the battery wire was trying to feed the alternator, thus showing a discharge. Hope this helps.

[mcjjordan]

Diode packs gone inside alternator.