Alternator for Perkins 4108

[DreamyLady]

I want to replace the alternator on a Perkins 4108 engine. The present unit is a Lucas AC11 with external regulator and warning light control. I want to eliminate the external Lucas devices and fit an alternator with built in regulator, possibly fitting an Adverc charge controller.
There are many different alternator mounting arrangements and the unit I have has 15mm shaft rather than 17mm. Can someone give me the reference of a straight replacement unit which will give at least 45 amp output.

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[TheoSr]

Why not have a look here http://www.coolcatcorp.com/faqs/Lucasalternators.html and keep the old Lucas alternator. This modification eliminates the external relais exept for the regulator. In case you already have a splitting diode you don't even have to modify the alternator with the three diodes mentioned and take field current directly from output I made this modification and it works all right for me

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[Rick]

I just replaced an alternator, going from external regulated unit to an internal.
First thing to check - is you Lucas 11AC negative or positive earth? (These older units came in both).
If negative, fine, if poisitve, then you'll have to change the polarity of anything onboard than using a common earthing system, bonded to the engine and/or any thru hull bondings.
Once all that's sorted, I'd go and find an older local auto electrician, and explain your requirements - young ones probably have never seen an early Lucas unit with it's external parts.
In my case, I went for an internally regulated (but 3 wire) Bosch style unit that was a straight swap on the brackets. I ended up buying a cheap Taiwanese rebuild, however I noticed that the Bosch units fitted to up-market Europeon cars have much better low speed outputs - mine is 70amp @ 6,000, but 17 @ 2,000, whereas the units fitted to some 5 series BMWs are 100amp @ 6,000, and still 60 amp @ 2,000. Similar figures for units fitted to big Saabs, Renaults, Benz etc.


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[silverseal]

With regard to the posting regarding output versus speeds, the important thing to understand is that an alternator can run at high speed, unlike a dynamo, because it has less weight and therefore inertia. A 4108 is likely to be running at 12 -1800 rpm, and the pulleys should be sourced to give optimal output at those speeds of the engine, stepped up to drive the alternator at max efficiency. Dynamo cut in speeds were quite low, and they were self energising, typically 500 rpm. When replaced witha nalternator the same engine required a pulley about 1./3rd the diameter so that the alternator was spinning on tickover of 1800-2000 rpm, running up to 15,000 rpm when the engine was at full chat. The important thing frankly is to visit a few scrapyards and get a variety of pulleys which will fit the alternator shaft, then simply measure the output at say 1500 rpm, which on a 45 amp alternator should be 35-40 amps

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