Serpentine tensioner

[Ron Davis]

So I suspect the issue here is my serpentine tensioner but I'm not sure so wanted to confirm with anybody else who might know...

Got back from our trip to Memphis....  Noticed the serpentine belt was off by one track of the lines but only on the AC  one line away from the engine.. and not perfectly centered on the tensioner either...    I loosened the tensioner but the belt on the pully correctly , the tensioner seems to pull it tight went and started the engine let it run for maybe a minute notice it basically slipped itself back into its same off position again Shut down the engine repeated the steps and it happened again.
 One thing to note is after replacing the serpentine belt a month ago,, seeing there was a vibration with the serpentine tensioner that I hadn't noticed before and I feel like I would have noticed..like a lil bobbing to it when at Idle, had son watch it when revving and the vib bobbing stopped.
   As of now I'm thinking to call up cat or freightliner and get a tensioner....but if there is a different idea I am all ears.
Thanks in advance
Ron

[Frank Bergamo]

Ron,

Sounds like an alignment issue with the AC compressor. Check the mounting brackets and any bolts that may be loose. Also check pulley on AC compressor, making sure nut is tight. Also, the serpentine tensioner pulley has bearings in it that might be failing. Hope this helps.

[Ron Davis]

Serp Tensioner I can push down by hand with some effort but not a lot....but according to various sources the belt is within spec of tension twist no more that 90 deg and up or down push at midpoint is less than 1/2 inch.  Everything seems to spin without noise and smoothly.  Maybe some friction in the spin but these are heavy components...so spin is slow but smooth.... I didn't remove the AC from the main front bracket that mounts with like 10 bolts direct to the engine.  Nor did I remove the tensioner. So not sure how its getting out of alignment...unless the AC pully or tensioner is slightly angled due to bearing wear or something odd ... freightliner wants like 260 bucks for tensioner..  any other sources

[Frank Bergamo]

Ron,

If all pulley bearings seem ok, then I would try to figure out what component is out of line. Has the AC compressor been replaced recently? If the AC compressor is the component that is misaligned then maybe the AC pulley needs a spacer for proper alignment. As far as the tension pulley, I would take it off and compare it to a new one, looking for any spacing differences between the two before buying a new one. Hope this helps.

[Fred Cook]

My tensioner was also vibrating so…. As a preventative maintenance measure, I had a CAT dealer replace it and also two belt pulleys that appeared rusted and a bit stiff to turn by hand. Maybe a bit much but it’s always good to have peace of mind driving down the road.

[Eric Maclean]

Ron
As the spring in the tensioner gets weak a vibration is one of the signs that your tensioner is not doing the job under heavy load as in acceleration with A/C or neavey alternator load the belt will bounce or chater which can lead to the Grove displacement and running out of line.
The tensioner spring should be tight enough that it requires a good force on a breaker bar or pry bar to get the belt on if it doesn't chances are the tensioner spring is weak.
Also check the range of movement on the tensioner as they sit in one position most of the time I have seen several that where partially seized and would not move through their full range of travel meaning that they couldn't compensate for belt stretch under acceleration.

Hope this helps
Eric

[Ron Davis]

So narrowed my issue down to what I believe was the wrong Belt itself.   After reinvestigation... the belt i put on a month ago is a Napa 080700.  8PK1775 ... from what I am reading others are using the 080710 .. which is about 1 whole inch longer .. that doesn't make sense to my setup unless something in my pulleys is way different ...  either way its made me review this belt.  Discovered the depth of this belt from NAPA is .5 mm shorter than my old belt (unknown brand) is 5mm depth and the new one is 4.5 mm .. to my way of thinking thinner belt makes it not as strong or heavy.... thus it flops more as well as won't last as long as a proper thickness.
 One might. that being said also the original belt I took off has ridges or the Vs themselves were pointed more and this newer one has flattened tops.   That may well make the .5 diff alone....
My sense has been to get a shorter belt, as the tensioner seems pretty relaxed.....both the old and new tensioner same issue exists....so the tensioner replacement solved nothing for me.   ERIC or others with a 90s beaver and 3126 side rad have any more concrete info for me?

Also my chassis manual says to use dayco 20-2693  which seems a lot shorter....lol

Thanks

[Eric Maclean]

Ron
I'm running a NAPA#25-080700 belt on my unit as we speak it's never given me any problem and it's go about 10,000 miles on it now.
Are you sure everything is in alignment ?
Also check the pulleys themselves for built up dirt rubber etc in the groves or a possible bent pulley.

Eric

[Ron Davis]

Hmm..  Ok.  Thanks