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Old 28-08-2008, 14:52   #1
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alternator

Hello all,

My alternator making a screeching sound when revving up form 1000 and 1500 rpm at the initial starting of the engine and then it goes away after 3 to 5 minutes of engine running. Is the alternator going bad?
Nelson
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Old 28-08-2008, 14:59   #2
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It sounds like it may be the belt slipping . check the belt tension .
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Old 28-08-2008, 15:01   #3
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It's probably the belt getting old or it is not tight enough. The screeching sound is happening when the belt is under load which makes sense when you first start an engine and the alternator is putting out more current working to top up the battery.

I would first inspect the belt for wear. If it is an old belt then replace it. If the new belt screeches then tighten it. It is still screeches get some belt dressing. Try that before assuming you have a bearing failure.
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Old 28-08-2008, 15:45   #4
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wow!Ok I will check it this weekend.Thanks everyone.Nelson
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Old 28-08-2008, 16:15   #5
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see other forum!
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Old 28-08-2008, 17:39   #6
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Nelson - When you look at the belt look for:

- tension - should deflect about 3/4 inch in the middle with light thumb pressure
- Belt Dust - Look around the engine area and on the engine covers. If you see splattered black "dust" this it the results of the belt slipping and shedding
- Belt width - compare the width of the belt with the width of the pulley. As the belt wears it gets thinner and we be noticably thinner than the pulley
- Glaze - the slipping will heat up the belt and the "sides" of the belt will look extremely smooth (glazed) if you belt is at this point change it. It won't grip properly on the pulley any longer under load.
- Cracks - If the inside diameter of the belt has any cracks replace it

If it never slipped before and has started slipping now it is probably time to replace.
If it has been slipping for a few weeks it is probably shot

Before you go to the boat, buy one. Then you can change it if it needs changing and be done. If not, you have a spare.

Changing it is usually a one wrench, one "pry" bar operation. Very easy to do.

If you do change it. Check it a couple of weeks later and tighten after initial stretch.
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Old 28-08-2008, 17:54   #7
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While the most likely cause is the belts I would suggest checking the bearings first as it is very easy to do and the costs of having a bearing weld itself to the shaft spells quite an expensive repair, especially if it is a large framed alternator.

If you choose to do so then with the drive belt off rotate the alternator by hand and check for anything other than a dead smooth rotation. There should also be no axial (sideways movement) possible. The usual bearing to fail is the one at the pulley end and another useful check is after the alternator has been running a short time feel for any significant temperature difference of the alternator casing between the drive end and the rear end (watch your fingers).

If the belts seem tight enough already before you make any further adjustment to them then I would be inclined to suspect a bearing (or maybe other internal problem), not run the engine and take the alternator for a service check as overtightening the belts will precipitate failure of any already sick bearing.

For the sake of good order will repeat that I am not claiming it is the bearings, just that it is so simple to check those and their failure usually expensive that I am just suggesting that it would seem silly not to do so prior to anything else .

If following Dan's advice you find you should replace the belt(s) then I would suggest that you go for a good brand, specifically Gates, in order to minimise further belt troubles and black dust throwing.
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Old 28-08-2008, 18:01   #8
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MLO - Great point. However as you point out the bearings can't be checked without the belt off. If the belt comes off, definitely check the end and side play of the rotor.

Also you make a great point about belt tension. If the belt condition is bad, no amount of tightening will fix it. Changing the belt (~$15) is really cheap insurance although I didn't say it previously, if it were mone, I'd just change the belt.

The belt slips because it is loose or worn. The power draw is significant across the pulley when the batteries are low and could be over 3hp.
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Old 28-08-2008, 20:06   #9
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Do not use belt deflection as a measurement of tension.

This was told to me by my Late mentor in this business.

With over 35 years experience, he said that deflection is OK on Cars because they have several belts applying pressure in different directions...alternator, airconditioning, water pump, power steering.

To get the correct tension....tighten the belt/adjust alternator....until you cannot turn the pulley with your hand....usually grasping the fins with your fingers.....
too loose....belt flops....to tight premature bearing wear.

If you have a small diesel, you may actually turn the engine over.
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Old 28-08-2008, 20:38   #10
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I'll follow up after this weekend. I may have a spare on board and give it a go.
once again many thanks.
nelson
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Old 28-08-2008, 20:53   #11
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Quote:
Originally Posted by Chief Engineer View Post

With over 35 years experience, he said that deflection is OK on Cars because they have several belts applying pressure in different directions...alternator, airconditioning, water pump, power steering.
Disclaimer - Do what the engine manual for your engine says.

Having said that the above paragraph doesn't make any real sense. The direction of deflection of the shaft will not by affected by the number of pulleys the belt has gone around - unless specifically the alternator is double belted in opposite directions.

The deflection will be in the middle of the angle formed by the belt entering and exiting the pulley.

3/4 is a rule of thumb.

Another great rule of thumb pointed out by Chief is the tighten it until the pulley can't slip. I actually use a socket on the pulley shaft and use my "calibrated elbow" to set the tension.

The difinitive answer is still what is contained in the engine manual.
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Old 28-08-2008, 21:29   #12
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I think what he was telling me was the side pressure on pulleys and the relatively longer "runs" on the belts.....On a yanmar you might have 12 or so inches from center to center of pulleys.

And I have seen some floppy/dusty yanmar belts.
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Old 29-08-2008, 11:34   #13
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While it screeches spray the pull/belt with a little wd40 If it stops the belt is likely bad or loose if it continues you should have the bearings checked.
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Old 30-08-2008, 09:25   #14
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All good points. bearing noise usely will not go away.but if the alternator has a lot of hour on it ,bearings now would be alot cheaper then after the rotor eats in to a stator.Back to the belt no need to even get out a wrench out just start engine let squeal for 1 min. turn off engine (TURN OFF ENGINE) then go feel the pully ,careful it could be very hot if it is good and hot belt is slipping.
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Old 30-08-2008, 09:29   #15
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O keep old belt an old belt is better than no belt
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