Alternator

[fixitflav]

I do have an amp gauge but haven't a clue how that's wired

It should be wired between the alternator output terminal and the battery input (+ve) terminal. Does it read anything? If not it may not be connected, or faulty.

The regulator also had two one blue and one red on terminals marked I & F
And it says negative ground I'm assuming that's trough the metal case

The I is input from the alternator, F is to the alternator rotating field, via one of the brushes. The output from F varies, that's how the regulation is done. The regulator also has to be earthed, in order to detect voltage from I (which is the same volts as the main output).

It could go to the charging light on the dashboard - does your system have such a thing?

No I only have two lights one says hot and the other oil but I have never seen them on

Usually alternators need the warning light, as the current through it gives initial rotor magnetisation. Unlike a dynamo, which has sufficient residual magnetism in the stator. At least, that used to be the case, maybe the latest alternators have a way round it. Does the blue wire go anywhere now? That terminal is almost certainly for a warning light, and it would be useful to have. You can test it by running a wire from the battery +ve via a 5W bulb, to the terminal. The light should come on when the engine is not running, and go out above a moderate speed. Then you could wire it in permanently, from a switched live point.

Just noticed some of this was covered in earlier posts

 

[johnad]

" initial rotor magnetisation " ?
If the warning light is nessasary it could explain a problem I have noticed
I started the engine today and the amp guage wasn't working
Checked battery and was only getting around 12.4 volts
I then gave it a good hard rev up and all was well again ? Amp guage working and around 13.8v at the battery.
This happens each time I start the engine

 

[Burnerman]

To get a voltage out of an alternator, you need to have a voltage going in....this is done by electricity going through the warning light and then into the alternator.
Alternators do have a 'cut in' speed....the 1970s Lucas ACR was actually 1200 rpm. However, once charging, the output didn't fail until the engine was switched off - in otherwise it would still be charging below 1200 rpm.
I think your system needs a rewire - there's probably some info on the web if you fancy giving it a go!
John

 

[johnad]

Yes I agree I wish I would have done that at the beginning
It shouldn't be to bad being that the engine is in a boat it is only the basic ignition and charge wiring
As a matter of interest if the engine is ticking over for a few minuets it suddenly dies ?
Could this also be due to the alternator getting to a point where is stops charging ?
It restarts perfectly after and has only been happening since fitting the new alternator

 

[Burnerman]

For sure, if an alternator is producing a lot of current then it can be more difficult to turn and it could be stalling the engine.....I guess the only real way to test it is to run the engine minus the alternator belt to see if it continues to happen.
One other reason is that so much current is being absorbed elsewhere then the coil gets starved and the engine stops - but I can't see why in your case!
I wouldn't think a rewire would be too difficult - basically you'd need a standard original type ignition switch with the 'on' and spring loaded 'start' action. With the ignition switch on, power is applied first to the warning light and then continues to the small alternator terminal.....naturally it would be a good move to check my reasoning out!
John

 

[fixitflav]

" initial rotor magnetisation " ?

Yes, the alternator has a rotating magnetic field, produced by the field current fed from the sliprings, 3-4 amp max. Output current is from the stator via the rectifier (in contrast to a dynamo, where the field is stationary and the output from the rotor via the commutator). Either needs magnetisation to produce initial voltage, which is fed to the field, so it takes off. A dynamo always has enough residual magnetism to get it started (you could polarise it for +ve or -ve earth by flicking a wire from the battery on to the field terminal). An alternator often doesn't, and the initial current is provided via the warning lamp. Some have a resistor in parallel with the warning lamp, so if the bulb fails, it still works. If you haven't already, I think you need to install a warning llamp.

As a matter of interest if the engine is ticking over for a few minuets it suddenly dies ?

Many years ago I used to recon alternators as a hobby, in the days when you could swap a recon one onto the old Mk 3 Cortina for testing in about 10 minutes, as opposed to a morning's work on my present Mondeo. One test was to run the engine a bit above a fast tickover, then bypass the regulator by connecting the earth side brush (which is normally earthed via the regulator) direct to earth. If the alternator is OK you hear the engine labour. No risk of overloading anything as the alternator is self-limiting on current, unlike a dynamo. In your case, it's possible that if the battery is low the alternator load could be stalling the engine, specially if it's a big alternator relative to engine size, and 75 amp is fairly hefty. But it seems rather unlikely. If it is that, keeping a few more revs on till the battery is more charged would get round it.

One other reason is that so much current is being absorbed elsewhere then the coil gets starved and the engine stops - but I can't see why in your case!

No, neither can I.