Alternator/Battery warning light question

[IamWedgend]

Hi all,

Back again with my delightfully temperamental 280i.

My battery warning light has been playing up recently. When I turn the car on, it lights on key turn, but recently has started staying on while idling.
Once I pull out and start to accelerate, it goes off and stays off.
Additionally, if I rev the engine up once, even while stationary, the light will go off and stay off.

I've run some tests. I turned it on and disconnected the battery while the warning light was still on. This killed the engine dead. Dead alternator, right? Not so simple.
I then turned it on, and revved it till the light went off. Then, allowed it to go back to idling, disconnected battery again. This time the engine did not die, but stayed idling.

It's a bit weird, because if the alternator was bad, you'd expect it to stay bad when at idle, not fail to work for a bit but then magically work after you've driven it a bit. Anyone else had any experience with this?

I think i might just replace the alternator. Anyone can confirm for me that this is the right one? Attached a picture of mine below (think mine is missing the back casing).
https://www.ebay.co.uk/itm/202924477422?ul_noapp=t...

PS - Also noticed a wire was disconnected from the engine. Included pictures below. Bit weird, but didn't seem to do anything at all. Anyone know what it is? Doubt it's related but just in case.

Back of my alternator


The wire (previously disconnected)

[Penelope Stopit]

Not meaning to come across as being a smart ass or anything but, shouldn't be disconnecting the battery with alternator running, can cause all sorts of problems due to uncontrolled voltage, could pop the alternator diodes or knock out a high voltage to most circuits

Anyway

Disconnecting the battery...........

When the light was on with engine running, the alternator hadn't cut-in hence wasn't charging and ignition coil had no supply

When revving the engine the light went out, alternator had cut-in and stayed cut-in and kept charging when going back to idle, hence the ignition coil still had a supply

All of the above is what's to be expected

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Moving on to your problem, as long as the warning light bulb is good and its cable to the alternator is good

Alternator light used to go out at idle but now doesn't

Idle has dropped (very likely would have noticed if it had)

Loose fan belt

Bad alternator earth

Bad alternator positive

If they're all ok

Alternator diodes are failing

Brushes and or slip-ring are worn out or covered in oil

Voltage regulator is playing up

[colin mee]

I agree check the fan belt.colin

[IamWedgend]

Hey Penelope Stopit

First off thanks for the warning - good to know, I'll not disconnect it while running again. Last time I follow random instructions off the internet.

Re the feedback on the problem.
The light seems in good order as it is coming on and turning off with a half turn. I've recently replaced and tightened the belt. The earth and positive i'll check tomorrow, but otherwise sounds like I just need a new alternator eh. I'll pop one in just to to be safe - got an MOT coming up.

Cheers for the advice!
Best,
J

[Penelope Stopit]

Turning off with half a turn?

If you mean a quick blip on the key to turn the engine a half turn, the light shouldn't go out

If light is going out with a blip on the key, brushes are playing up, slip-ring could be badly worn

[IamWedgend]

Ah sorry - turning on with a half turn and turning off when the key is turned back. Blip it and it stays on.

[adam quantrill]

Out of Penny's list my money is on the voltage regulator.

Diodes tend to either work or they don't (sometimes they don't when hot).

If you want a whole new alternator try ebay item 221214811186

I bought one 6 years ago and it's still going strong, although I put new brushes in after 4 years running.

Or if you want to try a regulator first (with new brushes) ebay item 311909551662, they are dead easy to fit.

[Penelope Stopit]

IamWedgend said:

Ah sorry - turning on with a half turn and turning off when the key is turned back. Blip it and it stays on.

Ok then, understood

[IamWedgend]

Thanks all for the advice - i've ordered a replacement. As far as I know the one on there now could be 40 years old so it's time for a change.

As a post script - did anyone know what the wire that I photographed? The plug has started to disintegrate so i'll need to replace that too, but I have no idea what to start looking for.

Cheers,
J

[adam quantrill]

On a 280i you need a V6 chap. But it's probably a sensor of some sort. Those plugs are bog standard, even get them on V8's, you could snip it off once you get the replacement, and sleeve the wire with heatshrink before putting the new one on.

[IamWedgend]

Hi Adam,

When you say 'on a 280i you need a V6 chap' i'm not entirely sure what you mean. Do you mean I need a different alternator - or are you talking about the plug? If it's just bog standard that's great though, thanks - sensor makes sense, could be the temp sensor as it goes in by the coolant pipe.

[rev-erend]

Think Adam means a V6 expert..

[adam quantrill]

Indeed I only do V8's! But we use the same plug, and yes that's what I was on about.

Now you mention it though you probably have a different alternator to the one I recommended.

[KelWedge]

The Battery warning light started flickering after start up and at low revs, at night it seems to glow red.
Sorted it, dont drive at night and after about 11 years have got used to the flicker!
Hope that helps.

[adam quantrill]

This looks like the right plug, but the price is exhorbitant....

https://www.ebay.co.uk/itm/DURITE-0-012-71-2-WAY-2...

or if you have an RS account, but add the crimp terminals to this too

https://uk.rs-online.com/web/p/automotive-connecto...

Edited by adam quantrill on Friday 9th October 17:06

[Penelope Stopit]

Posted the following to another topic a few weeks back, might be of some use

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Should have posted this earlier but thought others had got you sorted

https://www.autoelectricsupplies.co.uk/delivery

Goods will be despatched as soon as possible, subject to stock levels and cleared payment. Postage will be charged at £1.50 plus cost subject to a minimum charge of £4.50. The method chosen for despatch is dependent on the weight and/or value of the goods ordered, and will either be by Airmail or by Courier (Interlink).

https://www.autoelectricsupplies.co.uk/product/680...



Any car scrap yard will also have plenty of them

[adam quantrill]

+1 for the scrap yards.

However the connector posted just above by Penny looks like the "shallower" female one (sic!), which might work, but possibly won't be as secure as the others.