Cruisers Forum

  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
Thread Tools Search this Thread Rate Thread Display Modes
Old 15-03-2017, 12:46   #16
Registered User

Join Date: Oct 2014
Location: Caribbean - currently cruising Virgin Islands
Boat: Lagoon 39
Posts: 129
Re: Yanmar 3YM30 Alternator Belt Replacement and Adjustment


I'm just "your average bear in the woods" in that I am a beginner at maintaining these Yanmar engines.

The OEM belt part number is 128990-77580 also imprinted on the belt is 13 120. Hope this helps. The V groove is significantly deeper than the belt I replaced it with.

I put a V belt on that is 1" longer but is already starting to slop after 12 hrs on the engine. On my prior post you will not I had to leave the tension bar off due to the holes no longer aligned with the mount. So a put another new belt on just to bet me where we had to go today figuring I had 10 hrs before this one too started slipping.

In regard to the OEM belts I still have the original belt on and its working well and not showing signs of wear. Hours on that engine are almost right at 1,400. The three pulleys each have five metal edges that the each V groove sits on. Not sure if yours or similar or this may explain why the belt is lasting.

One interesting note, I checked with a reputable, knowledgeable Yanamar dealer when I bought the boat, planning to purchase two spare belts. When I informed him what type I had, he said "your wasting your money because those belts will out live you. No need for spares." Well either I'm on the verge of dying and simple too stupid to realize it, or the dealer is wrong. I like the second option.

My engines are going through raw water pumps at a pretty fast clip (3 raw water pumps within 1,100 hrs) and I am starting to believe the two issues are related. Here's why. The same Yanmar dealer shared that the reason why the raw water pumps are going bad so quickly is that owners/mechanics are over tensioning their Yanmar 3ym30 alternator belts. And furthermore he said, this piece of equipment on the engine was redesigned from its predecessor because of a faulty design. Yet it too, is failing frequently on these engines.

Given the amount of tension that is required to even get the belt onto the pull, it leaves no room to tension and already extremely tight. I do have to say my other engine though is the same equipment and working fine so it may be my lack of experience.

I now carry spare alternator belts aboard.
faa50 is offline   Reply With Quote
Old 15-03-2017, 13:22   #17
Registered User
macman4190's Avatar

Join Date: Jan 2017
Location: Waukegan - Lake Michigan
Boat: 1979 Islander 36
Posts: 91
Re: Yanmar 3YM30 Alternator Belt Replacement and Adjustment

Originally Posted by jacktheflyer View Post

Looking up the p/n for your belt it would appear to be 998mm circumference. When I bought my boat one engine had 995 mm and the 1000mm belts. Of course I bought replacements the same as what was on them and found the 995 mm belt difficult to fit and the 1000mm to be perfect.


Last year I replaced both belts on my 3YM30. It is critical to get the correct impeller pump belt. There are two OEM options, and it depends on your engine serial number. One belt is slightly larger than the other. It was a pain to install it, but following the manual was enough and no alterations were necessary. However, if the parts guy had been slightly less knowledgeable and thorough, and sold me the smaller belt ...
macman4190 is offline   Reply With Quote
Old 15-03-2017, 14:39   #18
Registered User

Join Date: Apr 2013
Location: Manly, Qld, Aust
Boat: Fusion 40
Posts: 174
Re: Yanmar 3YM30 Alternator Belt Replacement and Adjustment

Sounds like you have serpentine belts rather than V belts. This is indeed an answer to my problem which involves just the standard V type fan alternator. The conversion is expensive.

When you install a new belt, readjust after 15 min might give you a better result.

If you are having trouble with your raw water pumps, research 'Philips' pumps. Parts are much cheaper than OEM parts and you can get plates, impellors and stuff but the housing is a special build for Yanmar. my pumps have done 2000hrs and the housing is still intact but ahve replace impellors, end plates seals and bearings recently. I use ASAP in the UK for parts.
jacktheflyer is offline   Reply With Quote
Old 15-03-2017, 15:38   #19
Moderator and Certifiable Refitter
Wotname's Avatar

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 18,313
Re: Yanmar 3YM30 Alternator Belt Replacement and Adjustment

Be aware that Yanmar use a variety of water pumps even on the same model engine. These are often country specific and certainly serial number specific. Thus even raw water hoses and belts vary. Some Yanmar dealers know this and some don't!
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 31-08-2019, 12:01   #20
Registered User

Join Date: Dec 2017
Location: Maine
Boat: Toni Hansen Trimere 33
Posts: 66
Re: Yanmar 3YM30 Alternator Belt Replacement and Adjustment

The top bolt that holds the altenator and shroud. It is not a standard hex head. It just has two flat sides. That is what needs to be loosened.

Ps. Leave the shroud off to help cool the altenator.
Supers0nic is offline   Reply With Quote

3ym30, alternator, men, yanmar

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Alternator Belt issue 3YM30 jacktheflyer Engines and Propulsion Systems 42 10-03-2022 10:45
Yanmar 3YM30 burned out alternator and starter motor pugwash01 Electrical: Batteries, Generators & Solar 11 04-10-2016 01:56
Adjusting V Belt tension and removing alternator Yanmar 1GM Overlord34 Engines and Propulsion Systems 10 03-04-2014 18:36
Yanmar 3YM30 Raw Water Pump Belt Change Muhsin Cifter Engines and Propulsion Systems 1 17-06-2013 05:20
high output alternator on 3YM30 Yanmar? c.spots Engines and Propulsion Systems 13 01-04-2012 14:53

Advertise Here

All times are GMT -7. The time now is 13:50.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2023, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2023, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.