Alternator charge

[Choppski]

Just checked the charging output on my starter battery and it is steady at 13.85V when ticking over and only increases to 13.90V when revved up. Does this indicate that my alternator is on the way out as i thought it should be higher at around 14.2 to 14.5V (according to wiki)?

Thoughts?

The fan belt deflects about the right amount and isn't slipping. The light on the dash goes out ok, but only when i rev the engine a little when first starting up. the battery is in good health and holds charge at around 12.8V whne engine is off.

i'm just curious and not suffering any real electrical problems...yet.

[ghost123uk]

Your voltage readings are perfectly normal for a standard alternator, with the standard brush pack. You can buy an uprated brush pack (~£12 and 15 mins to fit) that will provide ~14 to 14.5 Volts, but if yours is working, that may be a job for when it stops working

I fitted a 14.5V brush pack to the van a while back, I still only get 14V though

E D I T, both my van and my 85 Scirocco do the "slight rev to extinguish the light" thing, always have done.

[itchyfeet]

The light on the dash goes out ok, but only when i rev the engine a little when first starting up.

Thats a sign something isn't quite right, I'd go for the brush pack upgrade.

[CJH]

The light on the dash goes out ok, but only when i rev the engine a little when first starting up.

Thats a sign something isn't quite right, I'd go for the brush pack upgrade.

I'm not so sure. When I swapped my 45A alternator for a 90A alternator (both nicely refurbed units with new regulators) I noticed that the dash light stays on longer with the 90A version (and my dash voltmeter backs this up). I can start the engine and reverse off the drive, with the light still on, but it only goes off when I pull away with slightly higher revs.

Something to do with 'self exciting' perhaps?

[ghost123uk]

Both my van and my Scirocco do it. Always have.

[itchyfeet]

Maybe the charge light/excitor wire isn't a very good connection.
If you remove it the engine needs to be revved before it self excites and starts producing charge.
Just because others do it doesn't mean it's right.

[CJH]

Yes - I've just been through that 'alternator fault' thread from last year again. Saw your post that seemed to explain it:

Thanks bigherb so we are getting to the bottom of the confusion by the sounds of it

blue wire needed for exitation at low revs, will self exite at high revs with blue wire off

But, if the blue wire isn't good, or not present, would the dashboard light ever go off?

[itchyfeet]

But, if the blue wire isn't good, or not present, would the dashboard light ever go off?

if I remember correctly wire not present and the dash light never lights in the first place, so a bad connection may ( just a guess) cause dim light and late excitation.

When my brushes were worn out I had the light dim at low revs

[CJH]

I must admit I don't fully understand what the blue wire does in the van, if it enables excitation at low revs. But in the buggy that I've been helping to rewire, its only function is to put 12V on the dashboard bulb terminal to cancel out the ignition-controlled 12V on the bulb's other terminal. Once the voltage difference across the bulb reduces to zero, the bulb goes out. So in the case of an alternator generating less than 12V, say 10V, then the potential difference across the bulb would be 12V minus 10V, and it might flicker.

[CJH]

I just checked. Without the blue wire, by dashboard light came on, but stayed on even at high revs. But surprisingly the alternator would not kick in either, even at high revs. So I don't know for sure if the light staying on was because the blue wire was missing or because the alternator wasn't generating. With the blue wire reconnected it returned to normal, with the output kicking in at what I might call 'pull-away' revs (sorry, don't have a rev counter in my van).

So it would appear that a) the blue wire is indeed crucial for exciting my 90A Bosch alternator, b) my alternator won't self-excite at any revs, and c) the revs at which the blue wire enables the alternator's output are higher (from memory) than the revs at which my old 45A alternator kicked in.

[itchyfeet]

With wire off need to rev quite high, some alternators don't need the connection, they selfexcite apparently.
confused about the light, thought it earthed via alternator until it produced voltage, can't test mine right now.
looking at the wiring diagram can't see how it lights with the wire disconnected.

Ok just checked the lucas currently in the van and it does the same as bigherb says, no power until you rev it a bit for say 20 seconds then it starts producing power, let the revs die down and it continues producing power.

It is not actually time, it is RPM dependant, you can rev all day but unless you reach the RPM threshold around 3000 RPM alternator shaft speed 1500-2000 RPM crank speed it won't self excite.

[bigherb]

I must admit I don't fully understand what the blue wire does in the van, if it enables excitation at low revs. But in the buggy that I've been helping to rewire, its only function is to put 12V on the dashboard bulb terminal to cancel out the ignition-controlled 12V on the bulb's other terminal. Once the voltage difference across the bulb reduces to zero, the bulb goes out. So in the case of an alternator generating less than 12V, say 10V, then the potential difference across the bulb would be 12V minus 10V, and it might flicker.

Depends on whether is has a dynamo or alternator. Dynamo all it does is compare the dynamo output voltage to the battery voltage, no excitation. Alternator it will excite the alternator, both will flicker or glow if the generator output voltage is lower and in some instances higher than the battery voltage.

[CJH]

With wire off need to rev quite high, some alternators don't need the connection, they selfexcite apparently.
confused about the light, thought it earthed via alternator until it produced voltage, can't test mine right now.
looking at the wiring diagram can't see how it lights with the wire disconnected.

Ok just checked the lucas currently in the van and it does the same as bigherb says, no power until you rev it a bit for say 20 seconds then it starts producing power, let the revs die down and it continues producing power.

It is not actually time, it is RPM dependant, you can rev all day but unless you reach the RPM threshold around 3000 RPM alternator shaft speed 1500-2000 RPM crank speed it won't self excite.

I took it as high as I dared - definitely above 3000rpm. I wish I'd never even looked at this thread now.

Depends on whether is has a dynamo or alternator. Dynamo all it does is compare the dynamo output voltage to the battery voltage, no excitation. Alternator it will excite the alternator, both will flicker or glow if the generator output voltage is lower and in some instances higher than the battery voltage.

In my mate's buggy it's a dynamo. The ignition-controlled side of the bulb is on the same supply as the coil, which should always see battery voltage, so yes, I can see how that might be lower than the voltage on the blue wire if, say, the battery was heavily discharged before starting the engine.

Can you explain how the blue wire excites the alternator? Does it provide a voltage TO the alternator, or a route to earth perhaps?

[itchyfeet]

Look at this
surely remove wire led can't illuminate?

[bigherb]

Can you explain how the blue wire excites the alternator? Does it provide a voltage TO the alternator, or a route to earth perhaps?

Save me writing it This is Lucas but essentially the same.