The V8 Owners Forum -

I recall that the connection of the charging lamp to the alternator is needed to excite the alternator at start-up.

Then I read this on your favourite encyclopedia.

The field windings are initially supplied power from the battery via the ignition switch and "charge" warning indicator (which is why the indicator is on when the ignition is on but the engine is not running).

Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off.

The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power.

Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning lamp burns out.

The driver should check that the warning indicator is on when the engine is stopped; otherwise, there might not be any indication of a failure of the belt which may also drive the cooling water pump.

Some alternators will self-excite when the engine reaches at a certain speed.

Its possible your alternator is not "self exciting".

shawser wrote:well i got a voltmeter, reading just over 12v, on idle the same. i connected thin wire to charge light on instrument cluster, started and engine wouldnt turn off with key.

How many small terminals has the alternator?

ok folks got it sorted. yes you do need the ignition light connected. Managed to find the old ignition wire on hilux loom, connected that up to small feed on alternator, i then checked the ignition light bulb and replaced it. all good now. 12.5 at battery, 13.5 idling and 14.7 when revving.

14.7v is too high. There's a danger of boiling the battery dry.

I think that Dave's figure is a tad low, my personal research told me the following.

Run the engine at 2000 rpm or more RPM for two minutes. Depending on the load and ambient temperature, the voltage should increase to between 13.0 and 15.1 volts for a good fully charged battery.

Most cars will measure between 14.4 and 14.8 volts on a warm day, depending on the battery type that the charging system was designed for.

If output voltage is above 15.1 volts with temperature above freezing, and the battery’s electrolyte level is frequently found to be low or the battery smells of “rotten eggs”, then it is probably being overcharged and the charging system requires testing

http://www.vintagemodelairplane.com/pag ... ery01.html

Those are the sort of figures you'll find in older textbooks on charging systems. My experience of modern ones says most stay around the magic 13.8v figure, with a good battery in a reasonable state of charge. Which the 12.5 v figure suggests shawser's one is.

i set the idle at 2000rpm and the voltmeter showed approx 14.12v

shawser wrote:i set the idle at 2000rpm and the voltmeter showed approx 14.12v

That's more like I'd expect to see if the battery is near enough fully charged.

its a brand new battery mate, just fitted yesterday.

If you leave the thin wire disconnected and check the voltage across the battery terminals you will still see the alternator output voltage wont you?

CHRIS??

I'm sure I've checked mine with this wire off and it shows 13.8 volts at idle and around 14.7 at high rpm ??

It depends on the design of the alternator. IMHO the majority of the types fitted to UK cars won't start charging if the warning light is blown or disconnected. But may continue charging if disconnected after the engine has started.

Try it and see - you won't damage anything.

mgbv8 wrote:If you leave the thin wire disconnected and check the voltage across the battery terminals you will still see the alternator output voltage wont you?

CHRIS??

I'm sure I've checked mine with this wire off and it shows 13.8 volts at idle and around 14.7 at high rpm ??

Yes, the alternator output is wired directly to the battery. Measure at either end as convenient.

Chris.