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Old 23-05-2021, 16:47   #1
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Need help with Perkins 4108 alternator wiring know how

I have a 1990 Compass 47 with the original Perkins 4108 with what I'm guessing is the original alternator. The workshop manual says it is most likey a Lucas (although it isn't leaking any oil ). I say this mostly because it is painted the same aqua blue cooler as the rest of the engine and the heat exchanger, air cleaner housing etc. I want to understand the alternator before I go upgrade it with a fancy automagic voltage regulator.

Over the years, the wiring on this yacht has been violated so I don't trust anything without a lot of research.

The biggest area of concern is are the 3 wires that come off the connector on the back of it. 2 of the 3 are red and they are joined together and attached to a lead attached to the positive bus and then the battery switch. The 3rd is green and it is not connected to anything. How should these D+wires be connected to what?

It looks like there is a female spade terminal close to B+ that is not connected to anything. Thoughts on this one?

W has a wire attached as does D+. I'll assume they are connected correctly. For now.

Then there is a larger gauge wire (10 at least) with a ring terminal just hanging there. I'm not sure that that is related to the alternator, but it is close by. I haven't gone chasing the wires yet, because they go into a nicely covered wiring loom and I need a big dose of patience before I start into that. I attached a couple photos to help identify.
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Old 23-05-2021, 17:41   #2
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Re: Need help with Perkins 4108 alternator wiring know how

Looks like an original alternator. Most people replace them. I have a 4108 generator and use a GM single wire alternator on it. Too hard to get old Lucas parts and too expensive. You can get a better alternator in any junk yard.
Image is the only wiring diagram I have for it.
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Old 23-05-2021, 17:59   #3
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Re: Need help with Perkins 4108 alternator wiring know how

The function of the terminals are...

B+ the positive output terminal, goes to Battery +ve.

B- the negative output terminal, goes to the negative bus or engine block as applicable

W connected internally to one stator winding before the diode pack and provides an AC output of about 7VAC. This can be used to pulse (drive) a tachometer as the frequency of the AC is proportional to the speed of rotor thus proportional to the engine speed. W stands for 'waveform'.


D+ goes to the warning light. Stands for ‘Dynamo Positive’ on old style generators (AKA dynamo). It is through this terminal that the alternator is initially externally excited. Before an output is established, the current flow through the rotor winding is from the battery +ve, via the key switch (ignition switch), through the warning light, to the D+ terminal and then through the rotor winding via the brushes and finally to B- (ground). At this point the warning light has current flowing through and thus is illuminated. However once an output is established, the output voltage is supplied (internally) by the regulator to the D+ terminal. This causes the the current to cease flowing through the warning light and so it is extinguished. The alternator is now self exciting i.e. it provides it's own rotor voltage/current internally.
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Old 24-05-2021, 16:23   #4
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Re: Need help with Perkins 4108 alternator wiring know how

I'm planning to replace it for sure. Does the way the 2 red wires are joined and attached to the battery and the green wire not being attached to anything make sense?
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Old 24-05-2021, 16:26   #5
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Re: Need help with Perkins 4108 alternator wiring know how

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Originally Posted by Wotname View Post
The function of the terminals are...

B+ the positive output terminal, goes to Battery +ve.
Thanks! That is very informative.

What does "+ve" mean?
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Old 24-05-2021, 17:29   #6
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Re: Need help with Perkins 4108 alternator wiring know how

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Originally Posted by lambrettaman View Post
Thanks! That is very informative.

What does "+ve" mean?
It means "positive"

And -ve means "negative"
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Old 24-05-2021, 18:36   #7
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Re: Need help with Perkins 4108 alternator wiring know how

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Originally Posted by Wotname View Post
It means "positive"

And -ve means "negative"
I love it when it is that obvious.
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Old 24-05-2021, 18:41   #8
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Re: Need help with Perkins 4108 alternator wiring know how

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Originally Posted by lambrettaman View Post
I love it when it is that obvious.
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Old 25-05-2021, 09:01   #9
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Re: Need help with Perkins 4108 alternator wiring know how

I have a Perkins 4107 with an alternator I took off the previous Peugeot Indenor XD2 engine.

Its one of these Lucas A127 it comes with Left and right hand versions. Ill soon need a new one as the other has been soaked in seawater from a leak https://www.ebay.co.uk/itm/324193342...UAAOSwuDte4AeB

Connections are as mentioned earlier +ve -ve D and W. I have a spare rectoifi3er which I will try first though, thats one reason I chose it. https://wiringall.com/image/lucas-ac...-diagram-7.gif
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Old 25-05-2021, 09:09   #10
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Re: Need help with Perkins 4108 alternator wiring know how

That alternator will probably burn up with an external regulator. It should be replaced. Also be aware that your tachometer is probably driven by the alternator and will need re-calibration when the alternator is replaced.
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Old 25-05-2021, 12:40   #11
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Re: Need help with Perkins 4108 alternator wiring know how

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Originally Posted by stormalong View Post
That alternator will probably burn up with an external regulator. It should be replaced. Also be aware that your tachometer is probably driven by the alternator and will need re-calibration when the alternator is replaced.
Got it!
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