Unusual alternator behaviour

[lexicon]

Hi all

 

Any advice on this greatfully received.

 

While running my engine to charge batteries last night I noticed when I went to switch off engine that the ignition light was illuminated.

When first started the engine the belt screeched a bit and then stopped.

The ammeter showed charge going in even when light was on. And batteries had charged from 40% to 70% on smartgauge.

 

Alternator is a year old and has 112 hours use following engine rebuild.

[Tony Brooks]

You say "last night". was it dusk or dark when this happened because blown field or Aux diodes can cause that but it's usually more of a glow than full illumination. That fault is also likely to cause a higher than normal charging voltage.

 

To be sure we need the make & model of the alternator in case its an "odd" one. Some alternators use an internal circuit to turn the warning lamp off.

Edited May 11, 2016 by Tony Brooks

[magnetman]

Rectifier breaking down maybe? As TB says this would result in a glow less bright than the normal lamp which comes on before the engine is started. I don't know how much less bright but I remember a failed rectifier causing a glow I would say was about half the intensity of the standard lamp.

 

I'd be inclined to take the alternator to an auto electrics workshop where it can be tested and repaired if necesssary.

[lexicon]

Lamp was regular brightness and reduced charge volts 13.5

[Tony Brooks]

That sounds like a duff alternator so take it for testing.

[Wanderer Vagabond]

Hi all

 

Any advice on this greatfully received.

 

While running my engine to charge batteries last night I noticed when I went to switch off engine that the ignition light was illuminated.

When first started the engine the belt screeched a bit and then stopped.

The ammeter showed charge going in even when light was on. And batteries had charged from 40% to 70% on smartgauge.

 

Alternator is a year old and has 112 hours use following engine rebuild.

Most new alternators come with a year's warranty, is yours now out of warranty?

 

Having had an alternator problem whilst in Braunston I don't think I'd bother with the 'having it checked and repaired', for £78 I got a new alternator (with a years warranty). When I offered the old alternator back to the company concerned he replied that they'd only throw it away since they don't consider it worthwhile servicing them when they are less than £80 to replace.

[ditchcrawler]

Most new alternators come with a year's warranty, is yours now out of warranty?

 

Having had an alternator problem whilst in Braunston I don't think I'd bother with the 'having it checked and repaired', for £78 I got a new alternator (with a years warranty). When I offered the old alternator back to the company concerned he replied that they'd only throw it away since they don't consider it worthwhile servicing them when they are less than £80 to replace.

Most repairers will test them for less than that. Its only 5 minutes to spin it up and check it.

[lexicon]

It's always the simple things that make you look stupid with boats.

Got home today. First chance I had to lift engine boards and look at things. After a bit of faffing about, discovered the connection on the back spade connector had loosened. Tightened up all running well and back up to 14.4 and no ignition light. Sigh of relief.

[ditchcrawler]

It's always the simple things that make you look stupid with boats.

Got home today. First chance I had to lift engine boards and look at things. After a bit of faffing about, discovered the connection on the back spade connector had loosened. Tightened up all running well and back up to 14.4 and no ignition light. Sigh of relief.

Good job you didn't just say I'll have a new one please, throw this one in the bin".

[Wanderer Vagabond]

Most repairers will test them for less than that. Its only 5 minutes to spin it up and check it.

 

 

Good job you didn't just say I'll have a new one please, throw this one in the bin".

In my defence I had a particularly annoying experience of alternator 'servicing'. At Debdale Marina they took my alternator off and sent it away to one of these companies specialising in servicing alternators. Came back all allegedly fixed and away I went rejoicing. Six months later the d*mn thing failed completely, now I cannot be bothered with trying to 'fix' or 'service' them, if they don't work get a new one. Of course it is always worth checking the tightness of the terminals first

[Loafer]

It's always worth looking for the 'old geezer', with really dirty hands, who runs a side-street garage in most towns. My suggestion is to buy a new alternator right away, then get the old one serviced by said old guy. My last one cost me £40 to repair, although it has yet to be tested in operation! Having a spare gives me great comfort.

[Tony Brooks]

It's always the simple things that make you look stupid with boats.

Got home today. First chance I had to lift engine boards and look at things. After a bit of faffing about, discovered the connection on the back spade connector had loosened. Tightened up all running well and back up to 14.4 and no ignition light. Sigh of relief.

 

Now that really is odd behaviour unless there was some solar or mains charger input. 13.5 implies a degree of charging voltage yet the warning lamp on brightly says no charging.

 

It all depends exactly what spade terminal was lose but if it was the warning lamp one the alternator should no have energised so you would only have battery voltage or if it did energise with an intermittent connection it should have stayed charging with the lamp off even if you pulled the spade terminal right off.

 

It could be that the alternator was charging earlier and the 13.5 was just surface charge but as I said unless you slowed the engine right down the charge should have continued.

 

Its one of life's little mysteries, I just hope that really was the cause. I am pleased you fixed it.

[ditchcrawler]

 

 

It all depends exactly what spade terminal was lose but if it was the warning lamp one the alternator should no have energised so you would only have battery voltage or if it did energise with an intermittent connection it should have stayed charging with the lamp off even if you pulled the spade terminal right off.

 

Maybe there is a split charge relay or something connected to it giving a circuit to keep the light on and the alternator was self exciting

[dor]

When first started the engine the belt screeched a bit and then stopped.

 

 

A screech when first starting up usually indicates a slack fan belt. The demand on the alternator is highest just after starting, so if the belt is a bit slack it will slip - hence the screech. As the demand on the alternator drops it doesn't make as much resistance so stops slipping.

 

And the high demand on the alternator drops very quickly - a 90A alternator might give near 90 A when first starting, but will drop very rapidly.