Late night panic... Alternator query...

[Morls]

I've spent tonight fitting, removing and refitting an alternator after the old one stopped charging and the redlight on the dash was on dull'ly all the time.

 

With the Halfrauds exchange item fitted the light is now on as bright as can be.

 

I've double checked the connections and they're OK (I'm a spark by trade so should be OK at fitting wires).

 

In desperation I tried a test lamp (Caterham wing repeater smile.gif ) across the live terminal and earth... wthout the battery connected there was nada nilch nowt there.

 

Basically I'm confused.gif.

 

Could these things need a battery voltage across them to "exite" the gubbinsquestion.gif

 

It's not on the seven but on Karens (crossflow 1300) Festa.

 

It has one live wire and one (alternator light from zener diode?) smaller wire.

 

Double thanks in advance for any help.

 

Mark

[Fred]

i think there is a link fuse wire on the battery that you have to have conected to get a feed have you got this conected to the battery

 

fredid=blue> eek.gif

[Morls]

Thanks Fred thumbsup.gif

 

 

I've got no fused bit visible?

 

The battery to alternator main red wire goes: Battery>Starter Solenoid>Alternator.

 

The other small wire comes out of the loom near the Alternator.

 

Oh bu**er... Still confused.gif

 

 

 

Mark

[EFA]

Mark,

 

First check the engine earth strap. You might get this odd symptom if the alternator casing is not earthing back to the battery -ve.

 

The connection overview is as follows:

 

The thick wire as you say should have continuity to the battery +ve

The casing of the alt should be at earth.

The small wire that drives the warning lamp (this is the small wire on the alternator) drives one side of the warning lamp, the other side of the warning lamp is conneted to battery +ve (via the ign switch)

 

When the alternator is not turning, the output on the small wire is a slightly resisitve earth so the lamp has a feed from the battery on one side and a slight resistance to earth on the other, hence it lights.

 

When the aternator is revolving the output of the small terminal goes to +12v and this is the same as the battery voltage hence the ign lamp cannot light with no voltage difference across it.

 

Other potential causes are that it could even be that the new alternator that does not have the coil grounds connected properly inside it!

 

The dim lamp you originally had is an indication of low battery voltage OR a low output from the alternator on the small wire - this can indicate a similar problem on the main alternator output, but not always. - It could be the battery!

 

Hope the above helps and does not confuse too much.

 

 

 

Fat Arn

Visit the K2 RUM siteid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

[jf_renard]

Hello Mark.

 

You wonder

>Could these things need a battery voltage across them to >"exite" the gubbins

 

Actually, yes they need an excitation voltage.

The output current of the alternator is induced in the stator by the rotation of a magnetic field inside.

This magnetic field comes from the current in the rotor, and this current comes from the battery, through the light on the dash.

So: no battery connected -> no current in the rotor -> no rotating magnetic field -> no induction in the stator -> no current output.

There is therefore sometimes an exception: there can be a remanent magnetic field in the steel of the rotor, which then acts slightly like a magnet, and it can be enough to initiate the output.

In this case, it is dangerous to run the alternator with no battery connected: the regulator can't measure the charging voltage and forces the alternator to deliver its maximum power and it can burn the diodes if it lasts too long.

 

An easy way te determine wether your alternator doesn't charge or your battery doesnt't accept the charge is to measure the battery voltage engine stoped and engine running.

Stopped it should be around 12V, and running it should be around 13.5V.

If it's still around 12V engine running, no charging current arrives to the battery -> investigate alternator and its wiring. If it's arond 13.5V and battery doesn't charge and/or warning light is on, charging current arrives but battery doesn't accept it -> investigate the battery.

 

Hope it helps

 

 

 

Jeff

Cosworth 1600 BDR

[Fred]

you cant go wrong fitting this one of one on i think i would try another unit

if it is a serviced unit from halfords.

 

fredid=blue> eek.gif

[Morls]

Thanks Arnie, Jeff & Fred

 

I've swapped it today at Halfrauds (albeit a dfferent branch) and will try to fit the new 'un tomorrow while hungover.

 

I've also splashed out on a new multimeter to help my approach be a little more scientific.

 

Cheers again thumbsup.gif

 

Mark

[Marius]

Mark, I´ve got an Alternator with two possible places to put the charging-light plug on. If you take the wrong one the light is on all the time. The right one was in my case just the one beside the + terminal´, not the one on the other side.

 

Not so scientific approach I know but it might help.

 

Marius

[Morls]

I went for the ulimate in non-scientific approaches.

 

Took the altenator back to Halfrauds and swapped it for another "new" one.

 

This worked perfectly and the person on the parts desk said, "Oh yeah, some of the new ones don't work", no apologies or offers of discount proffered.

 

Grrrrr...

 

So, all's well that ends well, and thanks all for the help thumbsup.gif .

 

Mark