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Alternator question

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Jun 25, 2004
Hunter 306 Pasadena MD
Quick question: we have a 3 year-old Yanmar 2GMF. The alternator belt seems to loosen up regularly, so that I have to re-tighten it maybe 3 times a season. Should I try putting one of those "toothed" type lock washers on the alternator bolt to keep it from slipping on the alternator bracket? Is there any downside to this? To answer before the question is asked: yes, I believe I'm tightening the bold adequately, but I haven't used a torque wrench. I've never had a problem with a car alternator before, so I think I have a pretty good idea of how tight to go by feel (not wanting to twist the bolt head off...) Thanks, Jay
Dec 2, 1999
Hunter Vision-36 Rio Vista, CA.
Too tight, too loose

Jay: I would suggest that you purchase a new belt. It is a cheap investment. Save this old one as a spare. If you don't purchase it from Yanmar, be sure to get something like a "Gates brand". Just like everything else, not all products are created equal. I just read that you should be able to twist the belt between your thumb and index finger (90 degrees). If it twists more than 90 it is too loose, less than 90 and it is too tight. Once you adjust the new belt and it has been run for a few hours, I would check/adjust it again. Then mark the bolt/bracket adjustment and see if it is slipping. I agree with you, you really do not want to break it off in the alternator. If the bolt is rusty, order a new one before you do any more adjustments too.



Steve, I realize it probably sounds like a stupid question. I.e., maybe the answer is "sure: try a lockwasher". So part of the reason I asked was really to fish and see if anyone else had experienced this regular loosening up. The other reason is that sometimes there can be unforseen consequences with a high vibration part. For example: a long while ago, I replaced a timing belt on a car, and didn't use locktite on the bolt that holds the pulley on the end of the crankshaft. Of course, the bolt worked itself loose, causing the pulley to wobble and break off the bolt in the end of the crankshaft (on I-95 going 60 miles per hour). After getting towed home, I did manage to get the stub out (with an easy-out), but it wasn't easy. BTW: we also seem to get a lot of black dust (from the belt), but I've been assuming this is just the difference between a car (w/ lots of airflow to blow away the dust) and and enclosed boat engine compartment. Is this perfectly normal? The current belt looks fine, to my eyes. (And landsend, if "What's to loose?" meant to ask "what's too loose", rather than "what's to lose": too loose means just obviously too loose, going by feel, using an ordinary amout of car-repair-common-sense. Loose enough that I suspect I could turn the alternator pulley by hand a bit while the belt stayed put, maybe.) Thanks.
Sep 25, 2008
Alden 50 Sarasota, Florida
belt dust

belt dust is not normally a good sign jay - dust is usually indicative of another problem such as too loose a belt or excessive vibration, worn or warped pulley. Also,if you can turn the pulley while holding the belt, it is obviously slipping (which would also cause wear and dust) and not turn the alternator properly. There should be no movement when the belt is held which is the first test that you have it tensioned properly
Jun 4, 2004
Hunter 28.5 Tolchester, MD
Belt Deflection

My Seloc-Chilton Manual for the 2GM20 indicates 1/2" deflection at the mid point of the longest span of the alternator belt and 1/4" at the mid point of the smaller raw water pump belt. Look for cracks on the inside of the belts, but I'd either get a Yanmar replacement or take the old one with you to get the exact width, depth and size if using another brand.
Aug 9, 2005
Hunter 28.5 Palm Coast, FL
If you're getting a lot of belt dust...

which can wear the belts quickly requiring frequent adjustment. Try cleaning the rust off the inside of the pulleys.
Dec 2, 2003
Hunter 376 Warsash, England --
Black Dust

This either means slippage or misalignment. Suggest laying a metal ruler across the face of the alternator pulley to ensure it lines up with the drive pulley. Also, if someone has upgraded(?) the alternator to a higher amperage then it might be beyond the horsepower of that width of belt to handle.


If the belt slips and squeals then it is to loose,

otherwise is fine. Overtightening it can place undue stress on the alternator bearings. Remember you are not running a 300HP engine with a high output alternator so getting the belt as tight as you get the one in your car is not necessary. Yanmar recommends a 10 MM deflection when a 10 KG force is applied if you want to do it by the book.
Dec 3, 2003
Hunter Legend 37 Portsmouth, RI
Here's what I did...

Although I have not had the problem of my alternator loosening, I took a nut and bolt, along with appropriate washers, (regular flat washer (2) and a lock washer(1)) and threaded it through the slotted bar that is used to hold the adjusted alt taught. I tightened the washer so that it was adjacent to the locking bolt on the alt and keeps the alt from slipping. I did this as a precautionary measure and never had to test it. So now, there are two bolts that control the adjustment of the alt.


Apr 10, 2007
Hunter 25 Elizabeth City NC
Only time I had this problem it was rust

If you have even the tinyest bit of rust on your pulleys it will cause this problem. Only way I fixed it was new pulleys, new belt and lock washer on the adjustment. This should solve the problem for at least 5 or so years.



To address the points raised: 1) I don't see how it could be rust, as this is a fairly new boat (Sept 2004), and I haven't seen any other signs of rust on the engine. I'll check, though. 2) Misalignment: could be, but again, this is a new Yanmar 2GM20F in a new boat. So it's hard to see how a virtually new engine could have become misaligned. I'll check with a ruler. 3) Slippage: I suspect the loosening of the alternator is causing the belt to slip, which is causing the dust. Why the alternator would loosen, I don't know. But I guess a lockwasher or a 2nd bolt in the slot would be a fix. It's hard to believe that belt wear alone (from a misaligned pulley) could cause the belt to _seem_ loose, but I could mark it on the bracket to see if it's slipping. Anyhow, it sounds like this is an uncommon problem. Thanks for all the replies! Jay
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