Alternators

[davidk65]

Talking to a fellow boater, he told me that he had just had to replace his alternator. He said "It just blew up". Not really sure what he meant by that , other than it obviously stopped working. He took the old unit to an auto electrical place and they confirmed it was US.

I asked if they had given him any indication of a possible cause, but they did not offer any explanation. I thought they may have suggested what to check before fitting the new unit, but again nothing.

Which leads me to ask what are the most common, or perhaps the most uncommon, cause of failure.

 

i) I know that diodes fail occasionally.

ii) That running an alternator without an electrical load on them cause failure.

iii) Getting the battery cables the wrong way round cause failure.

iv) excessive brush and slip ring wear is not good for them.

 

What else can lead to failure? What about about arc welding.

 

DavidK

[MoominPapa]

Which leads me to ask what are the most common, or perhaps the most uncommon, cause of failure.

In boats with "proper" engine rooms, where the alternator is sharing the space with people, the cooling air channels can get blocked up with fluff, dust and hair, leading to overheating and failure.

 

 

MP.

[chris w]

The 3 most common causes, apart from the man-made hair problem listed above are probably:

 

1. Blown diode = new rectifier = £15 on Ebay

 

2. Faulty regulator = new regulator = £10 on Ebay

 

3. Worn brushes after long service = might as well replace the whole regulator (which usually includes the brush assembly = £10 on Ebay

 

At the end of the day, a whole new alternator can be had on Ebay for around £40 - £70 depending on which day you look.

 

Chris

[Sir Nibble]

Having spent far too much of my life inside alternators, I am afraid there is no simple answer. Some types of alternator rubbish their diodes on a regular basis, some suffer from rapid brush wear and some have brushes so hard they burrow into the slip ring and write it off. Some are very reliable but fail in certain applications (like the A127-65 fitted to rovers so as to draw their cooling air off of the exhaust manifold!) How long is a piece of string? Give me a type and I can probably tell you what it tends to die of.

 

Should have read the OP more carefully before posting shouldn't I. Yes, arc welding can wreck an alternator, so can hurried and inexpert jump starting or boost charging. It is a standard precaution to isolate batteries before welding.

Edited June 1, 2009 by Sir Nibble

[Theo]

ii) That running an alternator without an electrical load on them cause failure.

 

DavidK

 

Running them without a load is not a problem. Switch off the master switch or disconnecting the battery while they are running is certainly a problem.

 

Nick