Alternator works/fails

[kingfisher]

We're finally back from our annual cruise, this time Belgium and Holland. Going from Nieuwpoort to Dunkerque, with the engine running, there was no charging current. In Ostend, a repair guy stopped by. We measured all the connections, and he left with my alternator. He came back two hours later, saying that they had rigged the alternator to a test bench and that it worked fine, even under load. We reattached it and it worked fine.
Next day, we leave Ostend, and no more charging current. Three days later, she charges again. So about one in ten the alternator decides to provide current. What gives?
Volvo Penta 2001
Alternator to split diode to 2 batteries, sense wire connected to batt A, and working fine.

 

[ccscott49]

Bad connection somewhere or a diode or something similar breaking down?
The worst possible fault to find, an intemittent one!

 

[savageseadog]

On some alternators some of the case fixing screws also double up as connections. It's worth whipping the back off and cleaning everything up put some silicon grease on and tightening up

 

[Salty John]

If the alternator is OK and it worked after reinstallation (at which time I assume all connections were checked), it might be an intermittent fault between the engine block and ground.

 

[Danny]

Alternators need current through the field winding before they can generate electricity. When the alternator is spinning it effectively feeds itself (via the regulator). To get it to generate in the first place requires a seperate current path to the field winding. This is normally accomplished by the "ignition" switch and warning light. If either of these are open circuit the alternator may not start up. Sometimes it will start to generate due to residual magnetism. Once it's going it'll keep going.

I'd check the switch and lamp and associated wiring.

 

[ZEBEDEE]

I would check the brushes this is one of the most common causes of an intermittant fault and the cost is minimal

 

[steve_cronin]

\"Brushes\" was my initial reaction too..............................nm

. .

 

[kingfisher]

ah, so the fact that that the instrument panel lights don't work anymore might be related to the alternator issue?

 

[maxi]

ah, so the fact that that the instrument panel lights don't work anymore might be related to the alternator issue?

Correct, with no initial load to excite the alternator (eg a light) it will not start generating.

I always fit 2 warning lights, so if one dies, the alternator will still work.

 

[kingfisher]

I was told by the mechanic that the field was excited by the yellow send wire, which I connected to the plus of batt 1, and reads a nice 12.4V.

I always thought that the send wire serves to give the alternator an idea of the voltage drop of the diode.

So, time to check the connection between the D+ and the warning light.

 

[Danny]

[ QUOTE ]
I was told by the mechanic that the field was excited by the yellow send wire, which I connected to the plus of batt 1, and reads a nice 12.4V.

I always thought that the send wire serves to give the alternator an idea of the voltage drop of the diode.

So, time to check the connection between the D+ and the warning light.

[/ QUOTE ]This wire you're talking about may be a remote sense wire. If you use a diode splitter there's a forward voltage drop inherent in the diodes so using remote sense (rather than internal sense) allows the regulator to compensate for this. I don't think it has anything to do with your problem.

Certainly if the instrument panel lights don't work then this is likely to be the cause of the problem. If it's all the lights then look for a bad connection. If it's just the "ignition" bulb blown then replace the bulb - it'll fix your alternator problem immediately. The bulb just provides a suitable current limit resiostance for the initial excitation of the field coil. When the output of the alternator rises the voltage acrosss the bulb falls and it goes out.

 

[Danny]

[ QUOTE ]
Correct, with no initial load to excite the alternator (eg a light) it will not start generating.

[/ QUOTE ]The purpose of the bulb is not to act as a load on the output of the alternator but to initially feed current to the field coil until the alternator output rises. (apologies if that's what you meant...)

I like the idea of two bulbs - in parallel, of course.

 

[kingfisher]

It's all the lights.

 

[Danny]

Reckon it's the + lead to the instrument panel then.

Thinking about it your engineer may be right in that the remote sense wire could also be used as the + lead to the panel as well as providing the reference voltage for the regulator.