Alternator not charging at low rpm.

tomstickland

I was driving along today when all the electrics decided to have a go slow.
Windows wound at half speed, indicators ran at odd speed, radio went off. After a while it just went. A few hours later it happened again. I tested the battery voltage and it was 6v! With the engine idling it was still only 6v, so the alternator clearly wasn't charging. I managed to nurse the car home by revving it a lot, and after 2 miles suddenly all the electrics came back to life.

So I guess that the alternator isn't charging at low rpm. Is this a classic knackered alternator sign?

ohreally

Can you get hold of an ammeter to measure any output? If not try disconnecting the negative lead from the battery momentarily at tick-over speed, if there is no alternator output the engine will cut out. Don't do this at higher than normal tick-over or for longer than a couple of seconds though.

If output is down, consider cleaning the commutator (between the segments - use a filed down hacksaw blade) and fit new brushes and springs and if you can get one a voltage regulator.

Cat695

earthing fault??? if possible try a different known to work battery or could have been a simple loose wire

tomstickland

The battery is known to be good. The issue seems to be that the alternator fails to charge it at low rpm.

tomstickland

ohreally wrote: »

If output is down, consider cleaning the commutator (between the segments - use a filed down hacksaw blade) and fit new brushes and springs and if you can get one a voltage regulator.

I've never bothered servicing alternators before. Is it worth me taking it apart and cleaning it out, even as a temporary measure?

ohreally

I've just noticed this....http://cgi.ebay.co.uk/BMW-316i-E30-ALTERNATOR-FROM-A-1988-VGC-GENUINE_W0QQitemZ290217664740QQihZ019QQcategoryZ10374QQrdZ1QQssPageNameZWD1VQQcmdZViewItemQQ_trksidZp1638Q2em118Q2el1247

I noticed from your other thread you had an E30 but i don't know what alternator fittment you require.

tomstickland

I think the 4 pots all shared the same alternator design.

I've just been told by someone on the E30zone forum to take the cover off the back of the alternator and tease the brushes. This normally gains a few more months life from them.

The funny thing is that I used to always drive the car at high revs. It's only the last few months where I've been driving sensibly that it's become a problem.

pavlovs_dog

we had a series of problems with the alternator and battery on our car. serviced the alternator, replaced it, went through countless batteries over 6 months... only to discover that the main culprit was a corroded cable that ought to connect the two.

check your wires, tommy lad!

ohreally

You appear to be fairly confident with hand-tools so working from the perspective that you've got a below par alternator and your time is free you have little to lose by taking the alternator out stripping/ cleaning and re-assembling with a new set of brushes - just take your time, work methodically and keep everything in a clean and orderly manner. Pay attention for small fibrous washers that may be present on some terminals - they are insulators, be sure they are refitted correctly.

As others have said, before spending money or doing unnecessary work, check you cabling first...start at the basics and work methodically.

tomstickland

I've done a quick inspection of the surrounding wiring and it all looks alright. I did have a problem many years ago when an earth strap broke. I'll do some continuity testing tomorrow.

Incisor

Loose fanbelt ??? Or battery some months before the end, when the plates corrode or something and short out temporarily [I think - I have had similar a couple of times]. I would look at these before diving into brushes.