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[lee4206]

Hope someone can help.
 Alternator stops charging at 4000 rpm checked battery, belt, connections, wires ect all fine.  Makes no difference if underload or freely reved  everything electrical swiched on or off  under 4000rpm fine charges at 14v over 4000rpm seems to discharge at 12v.
Never seen anything like this before and ive been in the motor trade for 14 years so any info would be helpfull.
TIA
lee

[Dave DND]

I`d try swapping the alternator over I think?

Seen something similarly confusing with a knackered cell on a battery, but it would vary with load, whereas if yours is constantly failing at 4000 rpm it kind of points at the regulator I guess?

Any one else?   :-/

[lee4206]

my first thought as well but it was only replaced last year and that was the second on ive put in it since ive owned it and there not the cheapest things to get

[Dave DND]

my first thought as well but it was only replaced last year and that was the second on ive put in it since ive owned it and there not the cheapest things to get

Do you have somewhere nearby that can check alternators on the test bench - quick look through Yellow pages, or a word with a local garage may point you in the right direction. Getting it checked for the cost of a few beers may be a cheaper diagnosis at this stage?

[Mike Collins]

Check your main alternator to battery lead for high resistance joints. Does any part of the cable feel hot?

Try measuring the voltage drop between alternator +ve and +ve battery terminal, that will show up any high resistance in the cable and joints.

[lee4206]

Thanks for the replies and sugestions. as soon as i get the car back(lent it to some one) i'l check out them out.  

Could worn or damaged bushes cause it?
lee

[Dave DND]

Thanks for the replies and sugestions. as soon as i get the car back(lent it to some one) i'l check out them out.  

Could worn or damaged bushes cause it?
lee

Certainly could, and worn bushes would be consistant with your observations of cutting out at a certain speed.

[Marks DTM Calib]

Not convinced on this one.

Brushes dont wear particularly on alternators as they never arc (they contact slip rings and not a commutator).

So, think we need to consider what could be happening

Its a reasonable assumption that at the higher revs the unit will be producing more amps, this in turn will require a higher field current.

As such, we could eb seeing a couple of possibles.

1) The regulator failing and the field collapsing at the higher current

2) I²R losses in the supply cable causinga  volt drop at the higher current

If number 1 was was likely I would expect to see a sudden drop in volatge.

If number 2 was likely it will be a more linear (ish) drop in voltage.

So my first port of call would be to look and check the easy things first, these being the battery clamp terminals, make sure they are clean, tight and the cable is ok into the clamp.

Next step for me would be to change the alternator, I have done this a fair few times and carried out basic testing and re-frub on them (cleaning, bearing changes, reg and rectifier testing) and to be honest, apart from VERY rare regulator failures, the only usual failure point is the beaings getting noisy!

Also, if you remove the alternator, then check out the maintenance section as there is a sprung contact that is worth checking and cleaning.

[Dave DND]

Not convinced on this one.

Brushes dont wear particularly on alternators as they never arc (they contact slip rings and not a commutator).

So, think we need to consider what could be happening

Its a reasonable assumption that at the higher revs the unit will be producing more amps, this in turn will require a higher field current.

As such, we could eb seeing a couple of possibles.

1) The regulator failing and the field collapsing at the higher current

2) I²R losses in the supply cable causinga  volt drop at the higher current

If number 1 was was likely I would expect to see a sudden drop in volatge.

If number 2 was likely it will be a more linear (ish) drop in voltage.

So my first port of call would be to look and check the easy things first, these being the battery clamp terminals, make sure they are clean, tight and the cable is ok into the clamp.

Next step for me would be to change the alternator, I have done this a fair few times and carried out basic testing and re-frub on them (cleaning, bearing changes, reg and rectifier testing) and to be honest, apart from VERY rare regulator failures, the only usual failure point is the beaings getting noisy!

Also, if you remove the alternator, then check out the maintenance section as there is a sprung contact that is worth checking and cleaning.

Cannot fault your theory, but I have seen a few times a much simpler problem. When the brushes are worn down to stubs, there is often not enough distance left on the spring to ensure that they remain in contact with the commutator with high revs. Agree the regulator seldom fails and bearings I would imagine is terminal for most people, but the bruches are usually available for a nominal few quid and so easily replaced, yet often overlooked.

 :-/

[Marks DTM Calib]

The Omega alternator is pretty bulletproof.

I recently re-built one off my car (bearings were getting noisey) at 158K miles.....the brushes had 3mm of wear evident on them when compared to the new reg pack I keep on the shelf (the Omega has a combinde brush and reg setup).

Hence wear (as you would expect with slip rings) is very small.

If it was brush related I would expect to see more issues such as flickering lights etc with more pronounced voltage fluctuations.

Similarly with the point I mentioned related to the maintenance guide, this more often shows as flickering lights also.

Hmmm....wonder if it would be useful if I did a knowledge thread for the greater group on alternator theory, setup, service etc.

[Dave DND]

I think a guide to testing, diagnosis, stripdown and service procedures would be an excellent idea that would certainly benefit the OOF community.

 

[nabsim]

Diagnostics and testing is always useful. I always seem to have an idea what to do until I come across something different then its like hitting a brick wall, funny stuff this lectrickery

I would have suspected regulator on this but would a broken battery post do similar. Had some strange problems a couple of years ago which turned out to be a broken battery post but it wasn't obviously broken, was quite tight in the casing. Wasn't an omega though and I dont know how/where volts dropped. Just had strange charging/draining problems

[RichB]

Hmmm....wonder if it would be useful if I did a knowledge thread for the greater group on alternator theory, setup, service etc.

Good idea Mark, but I think you should include how to remove the alternator aswell, as the way you did mine was about 5 times faster than the way I did it from the Haynes manual and a lot easier

Cheers,

Richard

[lee4206]

So my first port of call would be to look and check the easy things first, these being the battery clamp terminals, make sure they are clean, tight and the cable is ok into the clamp.

i checked these as soon as i first noticed it

Next step for me would be to change the alternator, I have done this a fair few times and carried out basic testing and re-frub on them (cleaning, bearing changes, reg and rectifier testing) and to be honest, apart from VERY rare regulator failures, the only usual failure point is the beaings getting noisy!

I'm trying to avoid changing it if poss because i've already changed it twice and both times it was due to it not charging(never had a noisy one) this one is only about a year old and only done about 2-3000 miles


mark how do you check the regulator???


Thanks
lee