New Alternator problem

[Andrew_B]

I have just replaced the Hitachi 35 amp alternator on my Yanmar 3GMF with a Prestolite 70 amp.
Other than the positive and negative wires the hitachi had 2 wires - charge warning lamp and another?
There is only one connector on the Prestolite marked IND which I am told is for the charge warning lamp.The other wire I am told is now redundant.
When I turn the ignition on the warning light comes on as normal.When the engine is running it stays on and the Alternator doesnt charge.
I have returned the alternator to the place I bought it 3 times this week for them to test and they tell me it is working fine.
I have tested the voltage in the warning lamp wire before the engine is started and it is normal.When the enging is running however the voltage drops to 2.5v.
If I feed a different 12v supply to the terminal the alternator starts to charge but I can only detect 17amps at 1700rpm.
What am I doing wrong?
Please help before I throw the thing over the side!

Many thanks

Andrew

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[colvic]

Your alternator though designed to charge, theoretically, at upto 70amps, I have one, the output is regulated by what the battery requires at any particular time.

I did throw one away because it wasn' charging, even flat out, and I spent weeks trying to sort the problem, reire etc.etc. only to discover the fan belt was slac, shown by the alternator starting to charge as the engine slowed down as it was switched off. Make sure there is no more than 12mm deflection in the fan/drive belt on its longest span. Also, the increase in load placed upon the engine by such a larger alternator is quite surprising. Our old Thorny T90 slowed when it was pumping out in the region of max amps. Was told it took at least 2hp to drive the alternator.


Phil

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[Heckler]

if i remember right, the 4th one is for a supply from a switched 12v source,so you are doing the right thing, i think that is the one that supplys the regulator. you are doing the right thing by connecting a temp from the battery to it. the reason it is switched is to stop battery drain? the light one works by when not charging volts flow thru the bulb and light it when it starts charging it flows the other way and nullifys the effect and the light goes out? the wattage is i believe critical so that the voltage balances and light goes out.
my nigel calder is on the boat so cant look it up,
on the other subject of hp, 750 watts = 1 hp <> 12 v times 70 amps = 840 watts, just over 1 hp + innefficiency perhaps 2 hp?
v belts drive on their sides, if they are worn and bottoming in the pulleys they wont produce,
the regulator decides the charge amps and so the biggest alternator in the world is npo good unless the reg says "more rope", that is why sterling smart regulators are a better bet than a bigger alternator, it can make a 35 amper put out 35 amps when it is needed and a 70 amper the same.

stu


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[smb]

With regard to output, not only is 'what the battery needs' a relevant consideration, but so is the alternator RPM. I fitted a 90amp model earlier in the year, but before I get anywhere near full output the alternator has to be doing something like 9000 rpm which with a 7'' crank pulley and 3'' alternator pulley requires the best part of 4000 rpm at the crankshaft - not something I see too often, but I do get upwards of 60A at cruising revs if the battery needs it. I think there is a graph on the Prestolite or Leece Neville website that shows the rpm to output curve.

And yes, every time the Adverc cycles to extract maximum output the engine note on my 62hp motor changes enough to make me think the rope cutter is fighting another hawser.

Steve



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[WayneS]

Are you sure that it is not a pulley diameter problem. I had a similar situation with a new alternator. ie. Charge light would not go out. Turn's out that the new pulley was fractionally bigger with the result that it would not spin up fast enough to kick it into charge mode.

Replace with the orig pulley and all was fine.

Wayne


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