Alternator charge

[CJH]

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

That's certainly how it looks. So either there's more to that circuit than is shown in the diagram, or my 1983 dash is wired differently.

OK, so it's the current flowing through the dashboard lamp, to earth via the alternator field winding, that excites the alternator.

But again, my dash must be slightly different, because current flows through my dashboard lamp even when the blue wire is disconnected.

[bigherb]

So either there's more to that circuit than is shown in the diagram,
.

Yes there is, but the same principle, there are two resistors in parallel for safety so if the ignition is accidentally left on they share the current path and won't overheat.

But again, my dash must be slightly different, because current flows through my dashboard lamp even when the blue wire is disconnected.

Yes there will be because the power is from the ignition switch via the warning lamp circuit down the blue wire to the alternator.

[itchyfeet]

Just tested my charging light and I can confirm does NOT work at all with the blue wire disconnected (ignition on but not running) reconnect and light comes on.
So if yours does then I can only assume a partial short ( or resistance) to ground of this wire powering the light and perhaps also reducing the exciting effect.
The blue goes via fusebox so you could try removing the crimp from the edge connector at dash and see if the light stays on.

this makes no sense nothing can flow in a disconnected circuit.

But again, my dash must be slightly different, because current flows through my dashboard lamp even when the blue wire is disconnected.

Yes there will be because the power is from the ignition switch via the warning lamp circuit down the blue wire to the alternator.

[MidLifeCrisis]

But again, my dash must be slightly different, because current flows through my dashboard lamp even when the blue wire is disconnected.

When you disconnected the blue wire at the alternator I bet your lamp was earthing through your split charge relay.

[itchyfeet]

But again, my dash must be slightly different, because current flows through my dashboard lamp even when the blue wire is disconnected.

When you disconnected the blue wire at the alternator I bet your lamp was earthing through your split charge relay.

Yes that may well be it, I have a voltage sensing relay.

[CJH]

When you disconnected the blue wire at the alternator I bet your lamp was earthing through your split charge relay.

Well spotted. I don't have a split charge relay, but I do have a fridge relay, and a couple of towbar electrics relays.

[itchyfeet]

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

So do they both have relays powered from the charge light?

[Oldiebut goodie]

My van and my boat both need a little rev to kickstart the alternator. No relays involved on either of the exciter wires.

[itchyfeet]

Lots of factors here but voltage from alternator warning light us mentioned as a cause along with batt voltage.

So maybe common but you don't see it in new vehicles so in my book thats an indication something is not quite right.

Alternator Output:

Often not well known is that alternators do not produce voltage/current as soon as they start to spin. There is a zero-ampere speed at which the alternator reaches its rated voltage before it can generate any current. This is typically 1000 rpm (500 engine rpm). I came up which the second figure because the alternator pulley on the bus is half the size of the engine fan pulley and so the alternator spins at twice the engine speed. Next in the series of important events during alternator spin up is the idling speed: typically at 1500 rpm (750 engine rpm) the alternator must be able to satisfy the needs for long term consumers (basic electrical demands) without running down the battery.

Just above the idle speed is the cut-in speed at which the alternator begins to deliver current for the first time. The exact speed depends on the battery voltage, voltage from the alternator warning light (pre-excitation power), the rate of change of rotational-speed and the magnetic remanence of the rotor's magnets. If you've started your engine and "heard" the alternator slow down the engine at the beginning of fast idle warm-up this is the cut-in speed heard in action.

There is also a speed rating for the maximum current output, typically 6000 rpm (3000 engine rpm) and there is a maximum rpm at which the alternator can spin which is dictated by the roller bearings and carbon brushes inside as well as the fan because it take power to pull air through. On average this limit is 20,000 rpm (10,000 engine rpm): much higher than any stock VW engine can reach and any that do are drag racers who have no need for an alternator (substitute a dry cell battery or magneto powered ignition system).

Finally, the alternator can't run all day long at 100% duty cycle (80% is more reasonable) so you should factor your current load requirements with that in mind.

.

[CJH]

Lots of factors here but voltage from alternator warning light us mentioned as a cause along with batt voltage.

So maybe common but you don't see it in new vehicles so in my book thats an indication something is not quite right.

I feel a test coming on. It should be easy enough to measure the voltage and current coming down the blue wire to the alternator (ignition on, engine not running). If I do the same with my fridge and towing relays disconnected that will tell me what effect those are having. And with them disconnected I might see the alternator kicking in at a lower rpm.

I don't feel this is a problem I need to solve - but just out of interest I wonder if there's a sensible way to provide that initial current to get the alternator going sooner. Given that I'd like to retain the alternator-sensing relays, a separate ignition controlled circuit might be needed, set up to give the right current and voltage. Perhaps a diode with the same characteristics as the dashboard LED, but on it's own dedicated circuit. Maybe those characteristics could even be tweaked to give an even earlier switch on.

[CJH]

Alternator Output:
The exact speed depends on the battery voltage, voltage from the alternator warning light (pre-excitation power), the rate of change of rotational-speed ...

Is that a typo, or does that mean it's not enough just to slowly raise the rpm, you actually need to blip the throttle?

[CJH]

Perhaps a diode with the same characteristics as the dashboard LED, but on it's own dedicated circuit.

I'm an eejit - it can't have its own dedicated circuit, since it'll be linked to the original blue wire at the alternator.

[bigherb]

Is that a typo, or does that mean it's not enough just to slowly raise the rpm, you actually need to blip the throttle?

No it is a threshold speed if you don't reach it it won't start charging.

[itchyfeet]

I don't feel this is a problem I need to solve - but just out of interest.....

OCD?
Voltage sensing relay if your test proves its the relay but may also be corroded wire/terminal, poor batt condition or low idle speed set.

My van has a new batt, is regularly charged and has a batt cut off switch so no drain when not used. The alt blue has been cut off and remade with a new ring terminal, relay is voltage sensing, light goes out imediately.
is your alt the push on connector type?

[CJH]

OCD?

Very possibly, yes. And the stupid thing is that this little test may well end up taking precedence this evening over the more pressing task of swapping my brake master cylinder.

is your alt the push on connector type?

It was, but when I swapped to the 90A alternator I now use a hefty cable on a ring terminal, straight to the starter. I retained the original wiring though, in case I needed to switch back, and I made a little fly lead to link up to the blue wire in the plug. This is it before I changed the two red cables for one fatter one.



My battery is always good too (topped up by the solar panel continuously), and all the wiring looks good, so IF my alternator fires up at higher rpm than it should I'm more inclined to think it's the relay drain. We must be talking about small voltages and currents given that it's the earth side of an LED, so I think cable losses will have a small effect - is that right?