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Belt advice please – Alternator Belt too long
As a dutiful newbie owner, I tried to replace all the belts on my coach today. When I put the new alternator belt, a Dayco 5120738 on, things just didn’t look and feel right. When I released the tensioner it went all the way to the stop – I could feel the clunk not a soft ending like a tensioner should have when you release it – while succeeding in only just barely tensioning the belt. I said “hmmmmmmm”. Put the old belt back on, snugged up just fine. Took it back off, put the ends of both belts over a long wrench shaft, stepped on it and pulled both belts taught. New belt loops about 3/4″ longer which means it is about 1.5″ OAL longer. The old belt has no visible markings as the back side idler contact has worn them off. All bearings sound and feel fine on the idlers and alternator. None of the pulleys/idlers appears to be of different heritage than the others, but what do I know, this is the third time I have been inside a coach engine bay.
I did later measure the new belt carefully (3 times) in the shop and got an outside circumference of 74 3/4 inches. Spec for the belt shows it should be about 74 3/8″. I don’t think the 3/8″ accounts for the magnitude of the problem as you can see in the photo of me holding the two belts side by side taught. I did not measure the old belt, I left it installed on the coach which is at storage 1 hour away.
Photos below may clarify the scenario.
Any ideas or suggestions would be much appreciated.
Barring any other advice, my planned next step is to get a 5120720 belt and try it. Specs show it is about 1.7″ shorter. My first guess is one of the pulleys/idlers in this drive sequence has been changed to a smaller one from OE at some point.
As a side note when I went to install the three AX74 crank to fan gearbox belts, I found some prior owner or shop had put 3 AX75’s on it. Seemed to have no ill effect.
OLD BELT IN PLACE
NEW BELT IN PLACE – you can see tensioner is at limit of travel
NEW BELT PART NUMBER
BELTS SIDE BY SIDE – new one is the longer one
STYMIED IN MCKINNEY
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21 Replies
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Have you called Prevost for the belt number off of the build sheet? They can give you the manufacture belt number…also there is a update available for the oil line to the alternator from what is on your coach
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One other thing, when the belts are off, and periodically spin the pulleys. They should spin smoothly and make no noise. If you detect any roughness or hear them “growl” get the bearings replaced. It is not often you find a problem, but over my years of ownership I have had to repair two by replacing bearings.
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I think that belt number is 5100510 but confirm with Prevost
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Jeff said,
“there is a update available for the oil line to the alternator from what is on your coach”
I know this involves a larger oil line, but does it really keep your DN50 Alternator from leaking?
1999 XLV X-6872, liberty #440 -
The “COACH FINAL RECORD” from Prevost shows “BELT POLY,A.24V DAYCO 23521000”
A part quote from Prevost parts shows a Prevost part number of 506782
I searched the web and both numbers always seem to cross to a 5120738 belt which is what I tried to put on and was too long.
Going to order the 5120720 and see how it fits.
The 5100510 is the belt from the output of the right angle drive to the fan.
Meanwhile is the alternator oil line fix a “kit” one can buy from Prevost?
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I assume from the pictures, you are referencing the two belts that go around the main and the alternator?
These belts are available at NAPA…. Can you take the old belt to a local store to compare?
They are lawn mower belts.
Pres
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I had the updated alternator oil line installed. Still had issues. Now using air-cooled alternator.
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Tim,
We had the update done last year at Prevost Ft Worth, and no leaks so far. -
The oil line update was a service bulletin and it did not address leaks but did increase flow for cooling and lube.
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Did you do it Jon?
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Sorry David I looked at the wrong number it is Dayco # 23521000 sorry the other is for the fan gear box , my fault
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I did the upgrade for the 50DN alternator oil line in 2015. My records show $135 kit from Detroit. You’ll need the later automatic belt tensioner mounting plate that is drilled and tapped for the new oil line.
Gerald
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Mine had been done when I got the coach. This is just my mind, unfettered with facts throwing out a thought.
When AC units are run through inverters it loads the alternators heavily and as they work to supply power a lot of heat is generated. Without the SB maybe there was insufficient oil flow. I never really load my alternators beyond the loads imposed by OTR so that might be why my previous coach seemed to not have issues as a result.
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I installed Detroit Diesel Upgrade
alternator belt tensioner kit #23527060 last summer. Replacement belt #s are: DD
23521000, Gates K120738 or K120738HD, Goodyear 4120738, Cummins
3098369, ACDelco K120738HD, and Dayco 5120738 -
Dayco # 5120720 = 12 rib 72.0″; Dayco # 5120738 = 12 rib 73.8″
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