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Hi

My son went out on our canal boat and ran the engine for about an hour with the ignition switch in the off position 

Now the battery charge light won't go off

Got a new Alternator but still have the same fault

Any Ideas please 

Battery voltage present on alternator.

Exitation voltage is 4.5 volts from the charge light .

Winding throttle up makes no difference 

 

Any ideas ????

Neil 

 

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18 minutes ago, glider pilot said:

 

Excitation voltage is 4.5 volts from the charge light .

Winding throttle up makes no difference 

 

Any ideas ????

Neil 

 

The voltage on the D+ excitation tag with the engine stopped, ignition on should be zero, bulb lit.

With engine running fast enough to charge, bulb not lit it will be 12v plus. With the engine running, fast, the bulb should have 12v+ plus relative to battery negative on both sides.

Your 4.5v is wrong for some reason.

 

Is the replacement alternator a good one? Is it correctly wired? Are the connections all in the correct place?

Are you confusing the D+ terminal and the W tacho terminal?

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If you have W and B only it is a self exciting alternator. A rare item. What does the wire from the warning light connect to if this is the case? B is the battery + connection. W is for the tacho only, nothing else.

 

Send a photo of the back of the alternator.

 

Just now, Tracy D'arth said:

If you have W and B only it is a self exciting alternator. A rare item. What does the wire from the warning light connect to if this is the case? B is the battery + connection. W is for the tacho only, nothing else, and is an AC voltage.

 

Send a photo of the back of the alternator.

 

 

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Photo of back of alternator please. Are there ant blades down a roughly rectangular hole.

 

Running with the ignition off should do no damage to the alternator its running with the master switch off that might.

 

I disagree with @Tracy D'arth. With the engine stationary and ignition on the D+ (warning lamp terminal) will usually be in the centre of a voltage divider formed by the warning lamp and the alternator rotor so although O have no idea what voltage would be there it would be less than 12V.

 

If it is a self exiting alternator then it should charge when running, ignition  on or off. There is something we are not being told.

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That is a standard A127 alternator. What connections do you have, rings or spades?

 

The 3 spade connections in the odd shaped hole are 2 X Battery, B on the large spades and D+ excitation on the small one.  The small tag in the little hole below the nut terminal on the left could be a W tacho connection

 

One of the big nut terminals is also B+ for battery live, the other will be ground, B-for battery negative...... If you do not have a negative connection it will not work. The original alternator may of had the insulator removed from the B- one the allow it to ground via the alternator mounting bolts.  Have a look at the original one.

 

Can you post a photo of the wiring on the back of the alternator as connected please specifically as to which terminals are connected?

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4 connections

large brown to B+ ( ring)

black to case stud ( ring)

brown and yellow to terminal ( at top of row of 3 )small spade

Black and Blue to W terminal (ring)

 

can i ask does the small spade produce a voltage or is it for excitation ?

 

just left boat so cant send picture

 

 

 

 

 

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21 hours ago, glider pilot said:

Hi

My son went out on our canal boat and ran the engine for about an hour with the ignition switch in the off position 

Now the battery charge light won't go off

Got a new Alternator but still have the same fault

Any Ideas please 

Battery voltage present on alternator.

Exitation voltage is 4.5 volts from the charge light .

Winding throttle up makes no difference 

 

Any ideas ????

Neil 

 


Does sound a bit odd! Firstly running the engine with the ignition switched off won’t damage the alternator. Running the engine with the battery isolator switched off, or worse switching the battery isolator off with that engine running, will do.

 

What should happen with ignition on is that the warning light is connected to battery positive and the other side of it goes to the alternator D+ terminal, and then internally via the regulator, slip rings and rotor, to negative. The small current flowing through the warning light and the rotor, provides a bit of magnetism to get things going once the engine starts, so that ac power is generated in the stator windings. Some of this power is rectified by the field diodes which are connected to D+. This brings the voltage on D+ up to battery positive voltage and thus there is no voltage across the warning light so it goes out.
 

The voltage on the D+ terminal with the ignition on and not running should be a few volts, your 4.5 is perhaps a bit higher than expected but it depends on the wattage of the warning light. So it should all be fine.
 

But it isn’t! Therefore something else must be afoot!

 

When a boat has a single alternator, often a split charge relay is used to connect engine and domestic batteries together when the engine is running. The relay coil can be powered by the warning light circuit. It is placed between D+ and negative. When the ignition is switched on, the few volts provided via the warning light isn’t enough to operate the relay but once the engine is running and D+ goes up to battery positive, the relay operates.

 

Hypothesis: the warning light is connected to the relay but the connection to D+ is broken. This would explain all the symptoms including the slightly higher than expected voltage on D+, well that is if you are not actually measuring the voltage on the D+ spade, but somewhere else that you think is connected to D+ (but isn’t).

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@nicknorman's reply above is exactly why I asked for a photo of the back of the alternator, little did I expect the OP to take the thing off the engine. I suspect a wiring fault but can't tell until we see what's connected where.

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