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10.5V at d+ on aux alternator


mattcyp

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Hi all,

The title is the short version  - My auxillary alternator warning light is staying on despite the alternator pushing out it's usual current (I think - it's at least pretty much the same) and a quick investigation with a mulitmeter found 10.5v at the d+ stud. 

From my understanding of how warning lights work, one side is supplied with +12 (or  more likely 13 - 14.7 when cruising)  and before the alternator is excited the other is at 0v, causing the bulb to light.  When running normally the d+ terminal would see roughly the same as the b+ terminal and so with no potential difference across it, the bulb goes out.  In my case, I think the 2 or 3 volts across the bulb is keeping it on. 

This isn't such an immediate problem at this time of year as when sailing the alternator just pushes out max currenty all the time, but once the batteries get near to absorption charge (14.7v for me) there is minimal drop in current and the alterntor will just keep charging until the batteries reach a high voltage (it got to 15.5v ish before I noticed). 

Probably foolishly, I replaced the regulator (it was only £10 on ebay) but this has made no change.  Does anyone have a suggestion for what to try next? 

 

Here's a pictuer of the alternator, though I can go back for more.  It's the 90a auxillary fitted on a canlaine 38. 

20171016_181119.jpg

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Sounds like a field diode has blown. This is making the regulator see the 10.5v and selecting max field current in order to try to get the voltage up. But best to wait for an alternator expert as I’ve been known to be wrong! And not even sure if it’s a normal 9 diode model.

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Not as expert as some here but I agree - field diode.

I suspect that for the price a new/exchange alternator may be the easiest way to go because the slip rings do wear and once you get into new brushes, slipr ings and diode pack it only leaves the bearings.

 

 

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Thanks for the input all.  A blown field diode sounds plausible. 

Bulding on Pearley's suggstion of fitting a new diode assembly sounds worth a try.  Does anyone know where I can source them from?  There are plenty of units on ebay (e.g. link), but plenty of results for Lucas a127 that don't look the same as my alternator (e.g. the picture below), so I'm concerned it won't fit (and trying not to dismantle the unit as it's getting a fair bit of use at this time of year). 

 

Thanks again. 

 

Ps Tony - I think it's only done 300 or 400 hours so I'm keen to keep it going.  An exchange alternator is on my mind though. 

 

image.png.1461745472956431f741fb2835f7db5b.png

 

 

Edited to add (and tidy up), a bit more googling suggests it's the alternator below, which I haven't had much luck looking for parts for (ebay link). 

Prestolite 66021590M:

image.png.7fb87bc2db859f2804e58e9bd9b011ef.png

 

 

 

Edited by mattcyp
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Your alternator is not the same as the Prestolite. It is a Lucas, now Magnetti Marelli. 

Have look here

http://www.jcrsupplies.co.uk/products/ALTERNATOR_REBUILD_PARTS/LUCAS_ALTERNATOR_RECTIFIER_DIODE_PACKS

From you picture you need a diode pack with a stud output terminal rather than 3 pin Lucas. You don't say where you are but you should be able to find a local specialist who can identify the correct model of alternator, assuming there no visible markings, and sell you the correct part. 

Edited by pearley
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Look at the mounting bracket on the Electroquest photo, nothing like yours as far as I can see in your first photo.

Rebuilt A127s come with all sorts of labels on them, one being Prestolite. Actually I think the  Electroquest one is a different version of an A127, the regulator shape gives it away.

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2 hours ago, mattcyp said:

Thanks for the reply. 

I'm surprised that it's not a prestolite - the one I picuted is identical in every external way to the one on my boat. 

I'm near Skipton on the L&L so if you do know any local specialists, that would be great. 

Call in and see Ian at Pennine Cruisers, he'll put you on to someone local.

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As an aside, the only part of this unit common with the A127 is the regulator and I have not been able to find any other parts. Diodes I would replace individually but I think the "triode" of three field diodes can be surgically transplanted from an A127 rectifier. Not really a diy job.

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On 23/10/2017 at 18:40, pearley said:

It's a Lucas A127.

 

21 hours ago, mattcyp said:

There are plenty of units on ebay (e.g. link), but plenty of results for Lucas a127 that don't look the same as my alternator

 

2 hours ago, Sir Nibble said:

this is decidedly not an A127.

It appears that Pearley’s post confused matters somewhat. 

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5 hours ago, WotEver said:

 

 

It appears that Pearley’s post confused matters somewhat. 

Yes, it probably did. I should have given closer attention to the pic in the original post. Instead I looked at the second set of photos which clearly shows 2 different alternators. 

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Thanks for the inputs.  I might try and catch Ian to see what he has to say.  It's a shame the spare parts aren't available in the same way that a127s are though.  The Lucas Industries wikipedia page did turn up this though: " With a monopoly in place, Lucas proceeded to supply electrical equipment that was commonly cited as the best reason not to buy a British car ". 

 

Sir Nibble, thanks.  I've sent you a PM

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