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Alternator Belt Tensioner Mystery
Looking for ideas. Our bus keeps eating alternator belts. It is the Poly V belt to the large 50DN alternator. Lately the belts only last 200 miles before they shred. I really have to get this addressed before we leave to go south.
Ours is the older style tensioner with two pulleys on a slider which gets tightened by a bolt. Prevost says to tighten belts to 350 lbs and once run in check to make sure tension does not go below 250 pounds. Here is the setup with the tensioner pulleys removed.
Here is the tensioner assembly
Here are some examples of the belts. Note the cut in the belt towards the edge. This belt has only 200 miles. The cut is on the back side of the belt.
Pretty quickly the belt will fray and cause all kinds of mischief (unscrewing the oil fill cap, ruining the 3 fan belts, etc)
I took off the tensioning assembly and found a slight bearing noise on the smooth pulley. Could it be that the smooth pulley locks up from time to time and cuts the belt? Putting lots of load on the 50DN (after dry camping or using OTR) seems to accelerate failure.
Unfortunately, the tensioning assembly is one unit, there is no easy way to replace the bearing. I checked with Detroit and they want $950 for the tensioner (23524482). I found it on ebay for $670 but for that price I’d rather upgrade to the latest automatic tensioner. For that the Detroit parts person would need a serial number of an engine in the 1998/1999 range. The change was made right after my engine in late 1997.
So, what to do? I’m not sure the slight bearing noise is the cause of my issues. There may also be a slight alignment issue. The belt is running slightly against the outboard shoulder of the smooth pulley.
Appreciate any insights!
Gerald
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15 Replies
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In my experience with
serpentine belts; a defective bearing causes the belt failure, especially a
repeated failure. Usually the bearing
seems okay when checked by hand; however, applying mechanical leverage, within
reason for the specific application, will show the defective bearing to have
excessive play.I’d replace the bearings in
both pulleys. -
I dont have the details on those pulleys but I know Prevost owners have replaced the bearings.
I replaced the bearings on an idler pulley for my AC belts.
I do know the idler pulleys need to run quiet and smooth and it is part of a check I have always done and so far no belt issues. When I replaced the bearings in the AC idler pulley it apparently was just beginning to have issues and it really was pretty easy removing the old bearing and replacing it. As I recall I got the bearing number right off the bearing which was press fit into the pulley.
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I changed all of my belts late last fall. When I free spun the smoth pulley I could hear a noise that didnt seem exactly right.
My old belt was completely missing one groove. I was very cleanly cut and I didn’t realize it until NAPA couldn’t match it my old belt to anything in their books. I ordered two new ones from Prevost.
The noise in that pulley has been in the back off my mind ever since. This might solidify my observation.
How do you measure the 350/250 ft pound of torque requirement?
When I installed mine, I gave it the good ol aviation mechanic torque, then running it in to allow for stretch, and retorquing it again
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Gerald,
I did replace just the bearings on the pulleys a few years back. I pressed out the old bearings and they were easily obtained at a local bearing supply. Detroit told me they were no longer available and wanted me to upgrade to the auto tension.
I suppose pulley alignment and new bearings would solve your problem.
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I had the auto tensioner on my old coach and never had a problem , 45,000 miles on the belts no problems I would update if I was you
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I have replaced the bearings in both pulleys. Got them from Prevost. They are listed in the on line parts manual.
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Be sure your alternator is mounted securely to its base. All bolts in place and tightened properly.
The bolts have been known to break, come loose and fall out.
JIM
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I have replaced the bearing in the same type tensioner,parts purchased from Prevost.I also found the Alternator mounting bolts had come loose one nearly falling out. I applied locktite and torqued bolts to spec.I now have approx eight thousand miles on the belt with no wear noticable wear. I also put Loctite on the bearing stud when I pressed it back in the mounting bracket.
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Thanks everyone for the quick feedback. Looks like the bearing is the culprit. It would be nice if I could just fix the $10 bearing….
Tom, Gary,
how did you get access to the bearing? There is a dust cap in front that is pressed in pretty far and looks like it would be destroyed upon removal. On the back the pully shaft is pressed into the plate. Did you press the shaft out of the plate and then press the shaft out of the pulley?
Gerald
After having a look, I don’t think there is a way to press out the shaft from the back side of the plate. Just no way to support the plate on the press. So how did you guys get to the bearing?
I’ll have to check the Prevost parts book. Detroit only showed it as a complete assembly.
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John,
To set and check the tension I use a Gates 91132 Belt Tension Tester.
It is also sold as the Cricket belt tension gauge. You put it in the center of the largest span and press until you hear a click and then read off the force required. Napa sells them too. Only about $10-15.
Gerald
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“After having a look, I don’t think there is a way to press out the shaft from the back side of the plate. Just no way to support the plate on the press. So how did you guys get to the bearing?”
To remove the cover/cap over the bearing I drilled a very small hole in the side of the cover and then used a small pick to pry out the cover. Then you can get to the nut that holds the bearing on he stud.
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Ditto what Andre said, I got the covers off by prying. There was a small ridge on mine that you could get a screw driver under to get them off. They’re tight..
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I drilled a hole in the centre of cover , put a screw in the hole then used a slide hammer to pop out the cover , later I welded up hole and put cover back in place..
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You guys made it sound so easy. I did manage to get the dust cover off but it is not reusable. Fortunately, I found both the cover (62mm) and the bearing (double row 30x62x27mm) online. Should be here in a week or so just before we supposed to leave.
Of course, old bearing feels like new now that I have it out of the pulley….
If that does not work I’ll try to upgrade to the autotensioner. The parts book shows 23527060 as “Kit Autotensioner” upgrade. So that is promising but nobody knows what is included. Based on pictures I think one needs a new tensioner assembly, moving the oil fill cover over and install a new cover with the auto tensioner plus gaskets and likely a different belt.
Gerald
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It’s always easy when someone else is doing the work…
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