Alternator charging problems

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catalina1

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I have a Bukh 20 fitted with the standerd Marchal altenator fitted.
Its rated at 38amps and had it checked by local Alternator man who says it works buts it old and not as new ( to be ex[pected)
I decided to upgrade to new alternator and bought a new 90 amp.
They advised me that the connections are the same but use slightly different teminogly.
I fitted the alternator and I have connected it as per instructions.
I cannot seem to get a reading on the batteries with the engine is running. ie only showing 12.3 v which is about what the batteries read with out the engine running.
I have also checked the output on the back of new alternator B+ terminal and earth and can get no reading

I would have expected a reading of at least 14 v. the old alternator produced a reading of 13.6 v
What am I doing wrong
Do you think these alternators are incompatible?
Some one said one may be battery switched and the other maybe internally switched?
Can I change this?
Advice please
Or can any one recommend a boat electric firm / handy guy on the Hamble river.
 
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Not enough information I'm afraid - what is the make and model of the new unit, please?

Some alternators require an excitation voltage to be provided - a connection to the "ignition" switch, for others a connection through an ignition warning lamp is enough.

Your initial diagnosis looks as if this is the cause.

To save others looking up the details of the make / model, you could describe the terminals on the back of the unit and the letters beside them - that might help.
 
You will have four terminals on the back of the alternator. You have B+ and B- which are the + and - outputs of the alternator and should be connected to the battery bank. You have W which goes to the tacho and you have D+ which goes to the igntion.

Check the voltage at D+ when you are running. If there is no voltage there then the alternator will not produce anything. You are looking for an output reading between B+ and B- of about 14.2v
 
Hi
If that is the case.
Have meausured the volts between B+ and B- at only 3.5 or so
Do I need another wire from ignition light spade connecton to where?
I have indentified all of these terminals.
 
You need to excite the field windings -
wire from 12v to ignition switch through little bulb on to F connector on base of alternator.
 
So to clarify jneale's post.
Take seperate wire from 12v positive terminal of battery to the bulb terminal on ignition switch then to the spade terminal ( ignition light) on back of alternator?
 
When I replaced the alternator from our bukh DV20, the old alternator used the connection from the alternator case to the engine as the -ve ground.

My new alternator didn't have this connection so it wouldn't charge.

I wired a suitably thick (80amp) wire from the gnd (I think) on the alternator to the mounting bolt. Ta dah it worked!

Pic here
 
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No.

If you do what I did, and take the wires off my Paris alternator and attach those wires to your new alternator, it won't work. As all you have is the +ve wire and another wire going to (the amp light) on the electric panel and no thick black ground/-ve wire

My old alternator was using its internal connection (from its casing, to the -ve part of the alternator) to be the GND.

The new alternator you have will probably have (at least) 3 connections.

The +ve
The other wire (for simplicity)
and a GND/-ve connection

I took a wire from the other side of the bolt securing the alternator (so bolt it up as usual, then add the wire with a crimped terminal, then add another bolt) to the GND bolt on the back of the alternator and everything worked swimmingly.

I should be back on the boat this week so I'll be able to tell you the colours of the wires if you need to know
 
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catalina1 said:
Snooks That look like the very same alternator ( white)
So I must earth the casing of the alternator to the engine?
Click to expand...

No because you dont know whether the alternator has an insulated return or not but in any case the casing will be earthed by its fixings top and bottom. . Just make sure that the B- terminal ( it will be marked) is connected to the negative system ( usually a bolts into the crankcase) by a wire thich enough to carry the 90 amps.

Ditto with the B+ terminal which should be connected to the positive side of the boats system by an equally thick wire. You can usually do this by connecting to the +ve feed to the starter motor - the thick wire.

That leaves you just needing to excite the alternator by applying voltage from the starter system to the D+ terminal. You dont need an extra wire. Just a connection to the switched side of the existing starter system.

Assuming you have all this and still it doesnt work then check the wire which will run from the built in regulator to the B terminal. I struggled with a new regulator / brushes on a bosch type alternator last week only to find that this terminal had been crimped on badly and was making only intermittent contact. It gave just the symptoms you describe.
 
Fixed Thankyou

Just to say I finally got the allternator sorted today.
It was indeed the need for a separate earth from the alternator to the engine.
Now charging at 14v ish.
Let me just say how helpfull you all have been with your advice and comments.
This forum is brillant for its members knowledge and experience
 
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