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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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Old 06-12-2010
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Can't keep alternator belt tight.

I have a Yanmar GM20. I tighten the alternator belt using a wrench or flat bar, to pry the alternator off the motor (and tension the belt), and cinch it with the bottom adjustment bolt. The access to this bolt is tough (of course!, but I manage to get it very tight. Then I tighten the pivot bolt on top. It seems like the belt has loosened (more than the recommended 10 Millimeters) after I run the engine. I've tried pushing on the alternator to loosen it, in case my adjustments were not tight enough, and every thing remains tight until I run the motor. Nothing is stripped. I'm not talking sloppy loose here, and the Alternator still functions. However, it is definitely looser than when I adjusted it. Any tips?
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Old 06-12-2010
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There is a gizmo called an alternator tensioning device (or other names) that looks something like a turnbuckle with a curved "horn" on each end. You place the tensioner in between the two pulleys (alternator and engine) and the "horns" are expanded out until they push against both pulleys. Then you keep turning the turnbuckle until you haver enough tension on the belt. Snug up the alternator--which can't slip--and finally remove the turnbuckle (the tensioner). If it doesn't hold after that, you need to look at bent brackets or lock nuts.

The tensioners never were very popular, grease monekys always figured a bigger crowbar was just as good. But any old-time auto supply store, or one that carries a wide variety of tools, should be able to find one for you, probably all of ten or fifteen dollars.

If not...it often helps to have someone else physically hold tension onthe alternator while you tighten the bolts down, less chance of slippage that way.
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Old 06-12-2010
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lock washers or nylatch nuts...?? The vibration could be loosening things ..

when I get things where I want them, I usually push the alternator, while watching the mounts, towards "loose" just to make sure everything is tight and doesn't slip with my pressure on things.
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Old 06-17-2010
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If bolts are still tight, i would replace belt, might be stretching.
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Old 06-17-2010
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Use only QUALITY belts such as GATES (blue) or Dayco 'Topcog'.

Make sure that the inner surfaces of the sheeves are not pitted from rust. Especially the pressed steel sheeves on the Yanmar fresh water pumps are VERY subject to surface pitting - replace; ditto on the alternator sheeves.

A 1/2" v-belt is at maximum service limit at about 90 amps. If all the sheeves are not pitted, etc. then if you have an external regulator consider to de-rate the alternator's output ... lessens V belt wear. An external 'smart' regulator will allow for 'slower alternator ramp-ups' ... also promoting less belt wear.

Use a Belt Tensioner (from NAPA, etc.), these are inexpensive and prevent OVERTENSIONING which stretches the belts, promotes rapid belt wear, and promotes rapid waterpump and alternator bearing failure.
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Old 06-19-2010
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Check the alignment of the pulleys too. People seem to add washers where they should not and not replace them when they should . The slight misalignment causes belt wear quickly and it then seems loose. Donīt tighten the bottom bolt so much ,its just a pivot point . Make sure the top adjustment arm bolt is tight at both ends . Sometimes the holes wear and slop around.

Too tight a belt puts force on the water pump pulley and that goes next. Sometimes as that wears it seems the belt is loose .Check the engine water pump pulley to see if it wobbles

Last edited by tytower; 06-19-2010 at 05:11 PM.
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Old 06-21-2010
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Quote:
Originally Posted by tytower View Post
Check the alignment of the pulleys too. People seem to add washers where they should not and not replace them when they should . The slight misalignment causes belt wear quickly and it then seems loose. Donīt tighten the bottom bolt so much ,its just a pivot point . Make sure the top adjustment arm bolt is tight at both ends . Sometimes the holes wear and slop around.
To make sure we are talking about the same thing (think we are), here is a picture. My pivot is on top (correct ?) and the adjustment bolt is in a slotted arm below (Not in photo). BTW, thanks for all the help and thoughtful responses
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Can't keep alternator belt tight.-2gm20-altrenator.jpg  
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Old 06-22-2010
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In this case dont try to tighten the top through bolt excessively 50 lbs will do . The adjustment bolt and bracket in your case is below and this is the common place for end wear where it is bolted to the engine
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Old 06-22-2010
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While I had the alternator hinge on the bottom on my old Yanmar, I did have some slippage as well which I solved by adding a bolt and nut through the slot and tightening that after tightening the original. The combination of the two worked well enough.
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Old 06-22-2010
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Also, you should order a new Yanmar tensioner bolt that meets factory specs. Somebody replaced the tensioner bolt on my water pump with one that was 1/4" too long, and it wouldn't hold the tension on the belt.
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