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Posted: Sat Oct 03, 2009 1:36pm

Post Subject: Alternator Problem

Hi Tony, I have a Yanmar 4TNE driving a 40 amp and an 80 amp alternator off one belt. On the 80 amp alternator (Domestic) recently the charge voltage went down to about 13 to 13 .5 volts and the charge current to between 5 and 10 amps even though the battery bank had been discharged by 40 amp hours. My boatyard sent the alternator to a specialist for repair but they said it was working perfectly. On refitting it it did indeed seem fine, charging at 14.4 and with the external regulator reconnected 14.9 v, however the batteries at this time had been fully recharged by the shorepower charger. On spending a night away from the mooring when I came to run the engine for charging the problem returned. I have tried with the external regulator (Sterling) connected and disconnected but there is no difference. The belt doesnât appear to be slipping and the 40 amp engine alternator is working ok. Any ideas please?


Posted: Sat Oct 03, 2009 3:15pm

Post Subject: Alternator Problem

Dear Tim, another brilliant piece of engineering in the marinisation department. It is generally acknowledged that the maximum number of pulleys you can run a V or pollyV belt over without a jockey wheel is 3. Otherwise you are inviting belt slip, especially on the most heavily loaded pulley. Now for reasons best known to themselves E P Barrus seems to have ignored this on a certain range of engines and unfortunately yours would appear to be one of them. Ideal.y you would convert your system so the domestic alternator is drive from it sown belt, leaving the 3 pulley system fro the engine alternator, the water pump and crankshaft pulley, but this is likely to be expensive. Carefully inspect the engine and see if it is possible to moult a "fan belt" or timing belt tensioner off a car. This will allow you to push on the back of a longer belt so the belt wraps over far more of the 800 amp alternator pulley. I suspect that will not be easy so if this was mine I would tighten the belt so tight I could just about twist it through 90 degrees in the centre of the longest run. This will put far more strain on the water pump and alternator bearings which might cause premature failure but I can not see what else you can do. I am almost sure this is all about the belt slipping under load. Within 15 minutes or so of starting the engine alternator will be doing very little so you would be unlikely to notice any slip on that alternator but you would on the domestic one. I also not the combined load on the single V belt is in excess of 120 amps and I know from the belt problems Ford had with 90 amp alternators that your belt is overloaded in my view. This would lead to premature stretching and again slipping. All I can suggest here is that you buy the best quality belts you can. Preferably from an engineering supplier rather than the likes of Halfords or chandlers. Look for the likes of Brammer in your phone book. Also ALWAYS use notched belts because they will drive over a smaller pulley. Tony Brooks

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