Current through Alternator Sense Wire

[aluijten]

I was wondering if anyone knows how much current flows though the Alternator Sense Wire. On my boat I've got a VP2040 engine with a Volvo Penta charge splitter leading into the starter- and domestic battery.
The past winter I spend some time reorganizing the cables to and from the batteries and installed a fusebox to avoid lots of cables going to the battery-posts.
So I also routed the sense wire from the standard Volvo Penta alternator (yellow wire) via the fusebox and put a 3 amp fuse in line.
To my big surprise this turned out not big enough and I had to install a 5 amp fuse.
Is this because of some initial current? Or is the sense wire required to conduct more then 3 amps when the engine is running.

Thanks in advance,

Arno

 

[Upnorth]

I'm surprised. As this is just a sense wire I would have expected virtually no current - as all it's doing is reporting the voltage at the battery to the regulator in the alternator.
This is just a theoretical answer, no practical experience.

 

[aluijten]

Thanks Upnorth, at least now I know my own thinking had some logic into it

A.

 

[VicMallows]

In any case the difference between a 3amp fuse and a 5amp fuse in practise is almost insignificant (and other fuse characteristics will be more relevant).

Chances are you either inadvertently caused a short and blew the first fuse; or had a dud one to start with.

 

[Billjratt]

I'm very unhappy at the thought of a sense device drawing more than three amps from the battery it's reporting on!
Why don't you measure it with a multimeter? You may have another fault or have some self-inflicted problem.

 

[nigelmercier]

Chances are you either inadvertently caused a short and blew the first fuse; or had a dud one to start with.

I agree, I'm pretty sure I've got a 1A fuse in mine.

 

[earlybird]

I'm not familiar with Volvo alternators, but, just as a suggestion, has the wire described as the sense wire been connected to the field terminal?
An alternator rotor winding connected to the battery might typically take around 3 amps. plus, and hence blow the fuse originally fitted.
Don't know the effect on the alternator, I suppose it might be driven at maximum output at a guess.
My external regulator, an X-Alt, has a 5 amp fuse.

 

[pvb]

To my big surprise this turned out not big enough and I had to install a 5 amp fuse.
Is this because of some initial current? Or is the sense wire required to conduct more then 3 amps when the engine is running.

The "sense wire" surely provides the field current, doesn't it? So a 3A fuse probably wouldn't be enough.

 

[Billjratt]

No, the sense wire senses. (In a battery-sensed setup)
The field current is provided by the third set of diodes in a 9-diode alt, or from the B+ indirectly, in a 6-diode alt.
More common is machine sensed, where the regulator just looks at the voltage at the alternator end of the wire, and assumes it's thick enough.
Are we sure the yellow wire is a sense wire and not for the rev counter or a relay. ( in which case it will be connected to the raw AC in the altenator and will blow a fuse if misconnected.)

 

[aluijten]

Thanks all for your replies.

Some more light on the topic. The yellow wire is most definitely the sense wire as it's described in some detail in the workshop manual. The manual describes how to connect it in combination with a charge splitter as well. The manual also mentions that the wire should be 1.5 mm2 or 16AWG. That leads me believe that a current between 3 and 5 amps could be expected. When back at the boat I'll try to measure the current.