Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Tensioner "Vibrating" After Alternator Replacement

Hi. I’ve replaced the alternator on my 2014 VW Jetta, and all went smooth with the install, except now the tensioner is vibrating and making a noticeable vibrating sound - it wasn’t doing that before. It’s louder and seems more obvious I filmed of it below. I’ve removed and put the belt back on a couple times, and when running it without the belt nothing looks off kilter or such. Is it possible the alternator pulley is bad? Thanks for any help.

Looks like your belt is on inside out. The grooves don’t match the pulleys etc.


Good catch, I didn’t even notice the ribs of the serpentine belt were facing away from the pulleys.

And now the belt is toast unless it’s supposed to have grooves on both sides. :confused:

Volkswagen uses double sided belts in some applications, the alternator belt on the 2.5 L engine is double sided.

1 Like

So, is this a double sided belt?

It would appear so.


Hi, thanks for the replies. I just double-checked and there doesn’t seem to be an inside out? There’s 6 female grooves on the pulleys and the belt has 6 male grooves on both sides? Unless I’m missing something? Thanks again!

Sorry guys for the delayed reply, I had replied earlier with my above reply but my new account was put on hold while it was reviewed, I couldn’t reply, and it was just now approved.

As Nevada_545 and Tester mentioned, it is a double sided belt.

I noticed the tensioner is getting near the end of travel. The belt may need replaced. But I don’t know if that’s the problem.

Learn something every day about two sided belts. But would an out of round alternator pulley cause the tensioner to jump like that? Or is the tensioner getting weak?

Thanks again for the replies. I inspected the belt during the install and it looked fine, but I could try replacing it.

One note I forgot to mention is the car is 2014 but only has 15k miles on it. I bought it from an older lady about 2 months ago, it was hers and she stopped driving after a couple years of having it and sat in her garage. The battery was dead when I bought it, so I replaced that when I picked it up. The new battery would drain periodically since, so I got the alternator tested and they said it was going bad, and that’s why I just replaced it.

The only reason I was thinking it may be the alternator pulley is the tensioner wasn’t making that noise and jumping like that with the old alternator, just started happening after I put the new one on. I may take this alternator off this weekend and go swap it to rule out that out. Pain in ass messing with the re-install but it wasn’t too bad getting to it, it sits on top.

Just eyeballing the alternator, pulley seems okay, I would go with a new serpentine belt anyway, a 5-6 year old belt is probably becoming brittle. It may not solve your problem, but I consider belt changes as preventative maintenance.

As an idea: when you were removing belt to replace alternator, did you remove the pulley off the tensioner?

I know that for Nissan they forbid doing that in their dealer’s repair manual in BIG LETTERS and instruct to replace tensioner with a new part if it was done. The reason for them is that it detaches some friction producing “thingy” inside tensioner and it starts vibrating similer to what you have on your video.

It is not necessarily applicable to VW, but who knows.

Hi, I finally had more time to work on the car. I’ve replaced the belt and went ahead and replace the tensioner. It’s much quieter than it was before, not clattering how it was before, but the tensioner still does “shake” some, is some tensioner vibration normal?

thegreendrag0n, when I replaced the alternator I didn’t remove the tensioner’s pulley.


This sort of symptom can occur if the pulley’s aren’t in the same plane (i.e. out of alignment). Since the only unknown pulley is the new alternator, that where my first suspect is. Hopefully you have the original alternator so you can compare the dimensions of the pulley, the number of pulley grooves, and the distance from the alternator mounting point on the engine bracket to the center of the pulley. Often there are shims involved with the alternator mounting design that moves the pulley in or out a little, and those can fall off and get lost during an alternator replacement, maybe that’s what happened. I presume you’ve manually turned the alternator and verified it turns smoothly, right? i.e. no hitches.

Thanks George. Ah ok, so there shouldn’t be any movement of the tensioner? With the tensioner moving the amount it is now will it cause other damage?

I unfortunately turned in my old alternator for the core charge. I did turn the alternator when the belt was off, and checked out all the other pulleys and all seem fine, from what I can tell. I’ll double-check the alternator install this weekend, and may just remove it totally and re-install it to be sure it looks correct.

Thank you for your help.

None of my vehicles use tensioners on the drive belts, so no direct experience. But I’ve seen vdo’s of tensioners in action and they do seem to vibrate quite a bit in normal operation. That vibration eventually does them in, but I think they normally last a quite a lot of miles. If the alternator is turning freely the problem — if there is one – is mostly likely the alternator pulley isn’t lining up with the others, or there’s some problem with the pulley itself, incompatible with the belt etc. If you see even slight twisting of the belt between pulleys, that could be an indication of misalignment. You may be able to stand in a way that you can sight them for misalignment. Like if you had 3 poles in the ground and wanted to know if they were in a line you’d stand at one end sight down the line for the middle one sticking out from the other two.

The alternator pulley on my Jeep has a sort of friction slip/one way clutch pulley, supposedly for damping out the pulses from individual cylinders firing. I saw an exaggerated demo video a few years back of what the pulses do to the other equipment driven by the belt if undamped, some component was bouncing all over the place.

Makes sense about checking for the misalignment, I’ll check that out. I think I will go as well to a VW dealer shop here in town to ask their thoughts and if they have a similar Jetta there that I could look at to see how that tensioner is reacting to get an apples to apples comparison.