Alternator question

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Charles Morgan

MB Enthusiast
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Mercedes 250CE W114, Alfa Romeo GT Coupe 3.2 V6
My BMW M535i has always had issues with batteries - regularly flat and in need of replacement. On one occasion the car was stuck in heavy traffic on the M40 after an accident and it died - the garage decided it was a faulty alternator and changed it. Recently I've been prepping it ready for sale and have been out and about a lot, sometimes in heavy traffic, and the battery has been playing up. The other day the engine died at idle and the battery was as flat as a pancake.

Having replaced the battery I thought I'd check alternator function. Battery was showing 12.6v in the car then when the engine was started the voltage across the battery was down to 12v when it should have been 14v or so.

Revving the engine hard then letting it slip back to idle at the voltage was then 14.6v attenuating to about 14.3v after several minutes and remaining at that level.

What could be causing this? The alternator is new as is the battery.
 
Only thing I can think of off the top of my heard would a problem with the generator earth return which might point to a dodgy engine earth strap or its mounting on the body.
 
Charles, what is the condition of the engine earth strap and battery earth cable?
 
Battery earth strap a bit tired but acceptable, engine earth strap looks original - so 33 years old. Something to change.
 
My BMW M535i has always had issues with batteries - regularly flat and in need of replacement. On one occasion the car was stuck in heavy traffic on the M40 after an accident and it died - the garage decided it was a faulty alternator and changed it. Recently I've been prepping it ready for sale and have been out and about a lot, sometimes in heavy traffic, and the battery has been playing up. The other day the engine died at idle and the battery was as flat as a pancake.

Having replaced the battery I thought I'd check alternator function. Battery was showing 12.6v in the car then when the engine was started the voltage across the battery was down to 12v when it should have been 14v or so.

Revving the engine hard then letting it slip back to idle at the voltage was then 14.6v attenuating to about 14.3v after several minutes and remaining at that level.

What could be causing this? The alternator is new as is the battery.

Hi,
I take it from being 33 years old it is a an E28 M535i, I had 2 of these and 2 of the earlier E12 Body shape M535i, all dog leg manual gearboxes, and 1980 635i exactly the same running gear and platform as the 5 series, cracking cars loved them and bloomin quick for the day especially as a 23 year old :rock::bannana:
I had an issue with three of these cars with the charging light glowing, I always put it down to a failing diode or regulator, despite this I never suffered a flat battery. Though I replaced the alternator it didn't fix the issue.
I did fix it finally on one of them which I replaced/upgraded the earth straps, and replaced the charging light bulb, which one fixed it I am not sure.
Apparently the resistance of the warning light bulb can effect the efficiency of the charging, as the bulb is in the excitation circuit of the alternator, though I always thought that if the bulb blew, the excitation circuit would still work due to a resistor across the bulb.
 
Thanks Andy - oddly there is no charging bulb showing (I wonder if it has blown?) but the news about the earth straps confirm that is the way to go. All these little model specific quirks entertain as long as they get sorted!
 
My alternator looks to be producing enough not to trigger the charging light.
 
the ignition lamp in your instrument cluster is the lamp in question,the one that flickers on

when you have an alternator failing on you,this lamp is crucial as already mentioned.
 
That thread mentioned certain replacement alternators having flexible mounting bushes effectively insulating the generator return path to the engine block?
In certain set ups theres a built in link in the actual connector plug from the ind terminal to the excitation terminal might be worth checking if this is the case- what's the alternator make and model?
 
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I'll check all this tomorrow. A bit dark and cold now!
 
The alternator warning light is part of the circuit for the initial excitation of the field coils (rotor).
If for whatever reason this is blown, this does not happen, and the excitation is generated from the residual magnetism in the rotor. This needs to be revved quite highly to start charging, but once done, should charge normally until the engine is switched off. This sounds to be what is happening to you.
You can test the bulb (and circuit) by grounding this wire at the alternator - it is usually blue, and a thin wire - personally I would ground it via a testlamp as there is then no danger of shorting anything out - both testlamp and charge lamps should then be on dim (ignition on).
When working correctly the lamp comes on as there is 12v across it -ignition feed, and return via the rotor circuit to ground.
It goes off as there is 12v supplied to the rotor and therefore the warning light by the warning light diodes in the alternator, thus giving 0 volts across the lamp.

Hope that makes sense - not explained it too well.

In short - test the warning light and rectify.
If the warning light is all ok suspect the regulator iirc the warning light diodes are encapsulated in that.
 
(scratches head, pretending to understand but realising he's well out of his normal shallows).

Makes perfect sense!
 
Ted is spot on if the alternator circuit is of that type. The same simple Bosch system was used on BMW motorcycles of a similar era as this circuit shows.

+12v from the battery passes through the charge lamp and via the voltage regulator to energise the rotor windings. This flow of current illuminates the charge lamp. Once the alternator is producing output the charge lamp has the same voltage on both sides so the lamp is extinguished.


Charging_System3.jpg
 
Quick iphone vid (hence execrable quality and sound) - hopefully self explanatory. The charging light is the one on the right. I realise that it has been like this for some time and I have always revved it to clear the warning lights. As it was sleeting and very chill and damp outside I'll do some more explorations later. Still showing 12v at the battery before blipping the throttle and around 14v after.

[YOUTUBE]xvXYJBoGQxg[/YOUTUBE]
 
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Still showing 12v at the battery before blipping the throttle and around 14v after.

I'd conclude that Ted (among others) is spot-on, and stop looking now. Make sure you blip the throttle on each trip....
 
That doesn't look too bad - at least the warning light is working, however I would expect it to have gone out at that 1k rpm tickover.
Given that, I would get a full test of the charging and battery, rather than advice from a bloke on the internet, just to ensure we haven't missed anything - the battery going flat is puzzling as from your voltages, it all seems to be charging pretty well (although 14.6 is a tad high for an older vehicle) - we were taught it should be 13.7-14.3.
I HAVE seen something similar in the dim and distant past where the wrong size (too big) pulley was fitted to an alternator which meant that it was rotating more slowly than it should. Highly unlikely to be this but just a thought.
It may be also worth getting a simple voltmeter that plugs into the cigarette (or in your case, finest MonteCristo) lighter might give a good indication of what is going on - I have one in the Jeep, and it is surprisingly accurate as measured against my trusty Fluke 75
It would also be worth getting the battery tested as all may be ok in the charging department but the battery isn't holding the charge.
 
I like that little gauge. My Allard had an ammeter in the cockpit which meant I could measure when it wasn't generating (oddly it had a similar issue - the voltage regulator didn't click over until you'd rev the car furiously and then the ammeter would move from showing a drain to charging). Modern cars are just so deliberately opaque.
 

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