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Old 07-01-2020, 07:37 AM   #1
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ISL 400 HP replacing serpentine belt

As a preventive measure it's time to replace the serpentine belt. It looks like I need to remove the AC compressor belt first. When reinstalling the AC belt how do I determine the correct pressure on the belt. I think the serpentine belt is self adjusting.
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Old 07-01-2020, 07:42 AM   #2
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ISL 400 HP replacing serpentine belt

Usually want about 1/2 deflection in belt once tightened.
Do not get it too tight this will cause premature wear.
Too loose and you will hear it squealing when the compressor engages.
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Old 07-01-2020, 09:07 AM   #3
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We used a belt tension gauge in the shop for various belts and sizes

Usually a good push and eyeball the deflection as Spk64 noted works. Only down side is some belts tend to stretch more than others so installing a new one I tend to go a bit tighter
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Old 07-01-2020, 10:22 PM   #4
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If you don't carry a spare belt. I find that rotating my old belt into rolling stock for emergency usage, each time I put a new one on - is good insurance. (Enough to get you do a place to buy a new belt, if something were to go wrong!)

$.02!

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Old 07-02-2020, 08:42 AM   #5
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I went to the coach yesterday to get a heads up and I have a couple of questions.
Picture 1 below; I tried moving the tensioner and it did not budge. I will need to get a extension tube to fit on my socket wrench but is the arrow depicting the correct direction to move the tensioner?
The AC compressor appears to have a tension adjustment bolt. Correct? Is this shown in picture 2.
Which bolts on the compressor do I need to loosen, Bolt 1,2, or 3, or all of them.
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Old 07-02-2020, 08:43 AM   #6
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Bolt 3
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Old 07-02-2020, 08:50 AM   #7
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ISL 400 HP replacing serpentine belt

I think you need to loosen all three bolts on compressor. You will know after loosening the bolt on the slotted adjustment.
Also you may want to consider replacement of the serpentine belt tensioner. Cummins recommends changing when belt is changed. Yes, your arrow is correct.
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Old 07-02-2020, 09:39 AM   #8
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Your going the right direction on the tensioner, may need a longer rachet or a pull handle. The tensioner maintains 100 lbs. of belt tension on these size engines.

Best practices are to replace belt and tensioner as a set. Theres a spring dampener and bushing that wears out that causes the biggest issue.

See below video... about the 1 minute mark.

https://youtu.be/h8pQT3YZAwo
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Old 07-02-2020, 09:58 AM   #9
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The bolt you've labeled as " Bolt 2 " is the mount for the base of the adjustment slide , and should not have to be loosened .
However , there are 8 bolt " ears " on your compressor , two; one front and one rear will be pivot points , and depending on your mount , two will possibly be attached to adjuster slides .
It's important that all the slide lock bolts and pivot bolts, be loosened , or the adjuster tension bolt won't move the compressor . Yellow arrows .
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Old 07-02-2020, 12:58 PM   #10
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Quote:
Originally Posted by mackwrench View Post
Your going the right direction on the tensioner, may need a longer rachet or a pull handle. The tensioner maintains 100 lbs. of belt tension on these size engines.

Best practices are to replace belt and tensioner as a set. Theres a spring dampener and bushing that wears out that causes the biggest issue.

See below video... about the 1 minute mark.

https://youtu.be/h8pQT3YZAwo
Agreed, definitely change the tensioner
I use a two foot belt tool and can move the tensioner so yours definitely needs replaced
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Old 07-02-2020, 01:38 PM   #11
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Thanks guys, I will replace the tensioner. Cummins part # 3976831 is superseded by Part # 5259022 for the tensioner $130 from Cummins. Dayco part # 89481 cost from various internet sources 50-100 bucks. Hard for me to believe a genuine Dayco tensioner costs 50 bucks but it's hard to tell if one is a knockoff and another is not. For that reason I'll stick with genuine Cummins. Pay a little more maybe a lot more but not worried where the item is made and if it will preform.


Knockoff LED lights, step motor, awning motor, etc OK but not Cummins engine parts.
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Old 07-02-2020, 03:27 PM   #12
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Either Dayco or Gates builds the part for Cummins...
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Old 07-02-2020, 03:34 PM   #13
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Quote:
Originally Posted by mackwrench View Post
Either Dayco or Gates builds the part for Cummins...
Probably Dayco.

I recently replaced mine with a Gates, and it looked and felt like a higher quality part than the original.
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Old 07-02-2020, 03:51 PM   #14
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Quote:
Originally Posted by mackwrench View Post
Your going the right direction on the tensioner, may need a longer rachet or a pull handle. The tensioner maintains 100 lbs. of belt tension on these size engines.

Best practices are to replace belt and tensioner as a set. Theres a spring dampener and bushing that wears out that causes the biggest issue.

See below video... about the 1 minute mark.

https://youtu.be/h8pQT3YZAwo
Well, well. I learn something new every day. Thank you.
I've not heard of this before.

So what is the recommended time and or mileage window to change the belt and parts?
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