Orange Posted November 8, 2009 Share Posted November 8, 2009 I've just replaced the alternator on my 300 Defender... What came off was a Magnetti Marelli 65 Amp jobbie and I've replaced it with a 100 Amp version of the same make. (Wiring is also dealt with...) the 65 Amp one has a larger pulley with an external fan, but the 100 Amp one has a smaller pulley with an internal fan. The pulleys are not interchangable (I tried) as when you tighten the larger pulley onto the 100 Amp alt. it seizes solid. I have just put the 100 Amp unit on with it's smaller pulley and replaced the auxillary drive belt with what appears to be no ill-effect... I am nervous, though! Even though there is plenty of movement left in the belt tensioner, is it a problem to run a smaller pulley on the alternator?? My guess is that the 100 Amp unit needs a higher RPM to produce it's output than the 65 Amp unit would have done, hence the smaller pulley, but can an adult confirm this...?? Cheers Adrian Quote Link to comment Share on other sites More sharing options...
western Posted November 8, 2009 Share Posted November 8, 2009 only 1 pulley shown in my parts book & microcat for a 300tdi alternator & no part numbers stated. this thread might help http://forums.lr4x4.com/index.php?showtopic=7336 IIRC the info in the link came from my older EPC CD's. Quote Link to comment Share on other sites More sharing options...
Mark Posted November 8, 2009 Share Posted November 8, 2009 Adrian are there arny spacers on either of them such that you can put the right pulley on the new alt. without if siezing solid? Quote Link to comment Share on other sites More sharing options...
white90 Posted November 8, 2009 Share Posted November 8, 2009 Adrian you have done exactly what I did no ill effect after 2years or so larger pulley means it turns less. Quote Link to comment Share on other sites More sharing options...
Aragorn Posted November 8, 2009 Share Posted November 8, 2009 Smaller pulley on the new alt means it spins faster for any given engine RPM. Given that a TDI only revs to 4k i very much doubt it will cause any issue, and will mean you have more power available at idle. I would guess its come from an engine with a different size of crank pulley, so the alternator pulley is different too. Quote Link to comment Share on other sites More sharing options...
jericho Posted November 8, 2009 Share Posted November 8, 2009 There are two pulley sizes for the 300 tdi,and two different belt lengths listed in the Gates catalogue to match. You want the shorter belt,but IIRC there is only about 25mm difference in the two belt lengths,so you may well get away with running the longer one. Quote Link to comment Share on other sites More sharing options...
Orange Posted November 9, 2009 Author Share Posted November 9, 2009 Ahhh. Didn't know there were 2 'genuine' belts available. The parts diagrams I have only list 1... I had it pointed out to me that I could just go to Halfords (other motor factors are available) and just buy a similar belt that's a bit shorter... The problem is that I'm not really going to be able to tell what's going on until it's all under load, i.e. belt slippage, or just lack of 'drive', so I might buy another belt and keep it in the spares box just in case... Thanks all... Quote Link to comment Share on other sites More sharing options...
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