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Old 06-02-2014, 12:54 PM   #1
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Alternator belt slip

In search of ideas to solve my belt slip problem. I have a 120/150 amp alternator on both my 1989 Cummins 6BT5.9Ms. This is a recent upgrade from the old 55 amp alts. The problem is the belts are slipping when the alts load up. I have checked the alt, water pump and tensioner for free movement. The belt in the picture is the first belt I put on that slipped. I bought another belt that didnt have the segments in the grooves and it did the same thing. I can see the slipping in the tach response. My research says that the serpentine belt I have should easily handle 120 amp load on the alt. Looking for a little old TF help.
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Old 06-02-2014, 01:28 PM   #2
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Free movement on the tensioner...but is it applying enough tension?

Not that familiar with a failed one but I would look into what specs it should have.
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Old 06-02-2014, 02:01 PM   #3
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Dave,

This is about incorrect belt routing.

I know the book shows to do it this way, but it is wrong.

I will look around for the correct routing which wraps a whole bunch more belt around the alt.

Incidentally if done correctly, it will spin the water pump reverse of what you have been doing, but in actuallity is the 'correct' rotation.

Correct belt routing will solve the belt dust.
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Old 06-02-2014, 05:13 PM   #4
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Dave: you might consider posting your question on BoatDiesel's 6B forum.
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Old 06-02-2014, 05:21 PM   #5
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Old 06-02-2014, 06:59 PM   #6
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That setup is correct for that style engine. There is a different belt routing for that engine when it's set up to have a fan on it. In that case the idle pulley is above the alternator and the belt does wrap around the alternator pulley by about 50%. You should check with Cummins on if and what it would take to change it over.
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Old 06-02-2014, 09:31 PM   #7
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A slightly larger diameter driven pulley would cost a few alternator RPM but would add inches to the contact area the belt sees.
It would also decrease the output a bit at a given engine RPM, but that loss means more power to drive the hull.

That alternator is really built, a beautiful piece of work.
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Old 06-02-2014, 09:45 PM   #8
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It would appear the pulley with the least contact angle is the alternator. Just the reverse of what you want. Yes boatdiesel will provoke some comments, but how did you arrive at the sheave size, belt selection and routing initially?

BTW, did you consider two drive setups, one for the pump and one for the alternator?
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Old 06-03-2014, 08:23 AM   #9
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Quote:
Originally Posted by sunchaser View Post
It would appear the pulley with the least contact angle is the alternator. Just the reverse of what you want. Yes boatdiesel will provoke some comments, but how did you arrive at the sheave size, belt selection and routing initially?

BTW, did you consider two drive setups, one for the pump and one for the alternator?
The sheeve size is the same that came off the original alternator. Two belts would be a significant modification. The routing is the original setup.The belt size is the same as what was on the original setup.
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Old 06-03-2014, 09:55 AM   #10
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Dave, if the belt can't get any tighter, do you have an external smart regulator that allows you to reduce the amperage to a level that the belt can handle? Maybe a belt with a softer, more pulley-grabbing compound and better design?

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Old 06-03-2014, 10:35 AM   #11
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Quote:
Originally Posted by bshanafelt View Post
Dave,

This is about incorrect belt routing.

I know the book shows to do it this way, but it is wrong.

I will look around for the correct routing which wraps a whole bunch more belt around the alt.

Incidentally if done correctly, it will spin the water pump reverse of what you have been doing, but in actuallity is the 'correct' rotation.

Correct belt routing will solve the belt dust.
I think you're right. It looks like a routing issue.

Here's a picture from Cummins plus another. When all else fails, what does the book show?
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Old 06-03-2014, 11:07 AM   #12
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Quote:
Originally Posted by fryedaze View Post
The sheeve size is the same that came off the original alternator. Two belts would be a significant modification. The routing is the original setup.The belt size is the same as what was on the original setup.
Tripling the load will require a lot more belt contact area than the "original setup." Great job, a few more tweaks and you're there!
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Old 06-03-2014, 01:15 PM   #13
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Agree, looks like routing problem. You may be able to alter it; still use the setup you have, and have twice the alternator contact; which is what you need.
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Old 06-03-2014, 01:17 PM   #14
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The manual indicates that the belt should be installed as they have been for the five years I have had the boat.
I think the later model has the wrap that contacts the pulley more. I need to replace the tensioner and add an idler pulley to take adventage of that set up.
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Old 06-03-2014, 02:30 PM   #15
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You can try this. It does help, but if the belt is already glazed, you may have to replace it and then give it a spray.

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Old 06-03-2014, 04:28 PM   #16
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Dave,

lots of ideas.

There is one actual fix.

This is simply a consequence of putting a high amp alternator on your motor.

The manual shows the belt routing you and many with similar year cummins B 5.9 motors utilize.

It is sufficient in the event you keep a stock 55A or mildly increased alternator.

The reverse rotation of the water pump is also apparently irrelevent.


When you and I and others who have added a high amp unit to our motor, we discover the additional force needed to spin the unit is greater than the poor wrap routing will allow, thus the slipping and eventual belt dust.


In my case, I added not only a high amp unit, but also a large frame one. Mine is rated at 200amp. Imagine the squeal, slippage and dust when the alternator went into a bulk mode.

I asked the question on other lists, as well as discussing with the Cummins tech who had supplied the part number for the large frame adaptor. He was dumfounded to discover the manual indicated a belt routing as it does.


Re route the belt. Get a new tensioner while you are at it.

Enjoy the full flex of your new high amp alt's. Just be sure and IR them while under load to make sure they don't get too hot since they are still small framers.

have fun
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Old 06-03-2014, 07:46 PM   #17
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Thanks for all the feedback and ideas. I think I will be updating the tensioner and adding an additional idler pulley. That will give me the wrap in post #14. I dont think using the regulator to derate the alternators is for me. I put in a big house bank and these alternators so I could do a complete recharge cruising between anchorages. I think bshanafelt is right. Just a few more tweaks and I will have her all dialed in.
Thank you
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Old 06-03-2014, 11:41 PM   #18
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The problem with some of the smaller and earlier Cummins engines is that the tensioner is on the tight side of the belt. So as alt load goes up, it tends to lift the tensioner and unload the belt. Then it slips. A properly designed belt loop has the tensioner on the slack side of the belt, i.e., between crank and alt. The later design of the belt loop (shown above in post 14, lower left) tends to keep belt tension much better even with higher alt loads. This should work for the OP. Even better is the qsb loop where the tensioner is in the right place, but strange as it is mounted to the circ pump inlet. But it works.
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Old 06-04-2014, 12:14 AM   #19
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Maybe I am way off base here....it has happened before....
but could you not just get a bigger belt and route it like the photo I modified in red?
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Old 06-04-2014, 08:34 AM   #20
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Maybe I am way off base here....it has happened before....
but could you not just get a bigger belt and route it like the photo I modified in red?
Mike, the tensioner pulls to the right. If it pulled to left I could give your idea a shot.
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