Serpentine Belt Tension 300E - PeachParts Mercedes-Benz Forum

handyjay · #1 03-26-2012, 12:20 PM

I just replaced my alternator and serpentine belt. I am not sure if I have the tension set correctly. The indicator is at the low end of the ramp. It does not move when I tighten the adjusting nut. The tensioner does not appear to be frozen as it moves when I push or pull on the belt. The belt just doesn't seem as tight as it should be as I see it flutter on idle between the longer distances between pulley's. This is a 1991 300E 4matic. Thanks for any and all help.

JohnM. · #2 03-26-2012, 03:44 PM

If the adjusting rod does not re-tension the belt, I'd say the tensioner is bad. It's a very poor design for longevity. I've done the job 3 times on my 300E in 80,000mi of owning it.

handyjay · #3 03-26-2012, 05:29 PM

Is there any way to tell for sure if the tensioner is bad? They are a bit expensive. I read on another forum that if the indicator is all the way to the high side of the ramp that the tensioner is bad. Does this make sense?

JohnM. · #4 03-26-2012, 05:47 PM

These tensioners are purely mechanical in nature. The adjusting rod simply rotates a collar on the tensioner to tighten the belt. If the adjusting rod is not tightening the belt, it's bad.

Did you loosen the big 19mm holding the tensioner in-place before you loosened the adjusting rod?

handyjay · #5 03-26-2012, 05:54 PM

Will I thought I loosened the right nut but it is a 13mm the same and the adjusting rod. Now I will go back and recheck. Thanks for the info.

engatwork · #6 03-26-2012, 06:02 PM

Yea, look for the large fastener in the center of the pivot point for the tensioner. Follow the torque specs for it when you tighten it up.

I think the 95 E300 has the most well designed tensioner but that is just me

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handyjay · #7 03-26-2012, 06:15 PM

JohnM, I feel like a dunce. I could not see the 19mm until I took off the fan shroud. All is well. Many thanks.

JohnM. · #8 03-26-2012, 06:18 PM

If you didn't undo the big 19mm first, before loosening the belt, you've more than likely trashed the tensioner off the gate. But like I said these M103 tensioners will go bad if you look at them wrong.....so......

handyjay · #9 03-26-2012, 07:28 PM

So I thought I was loosening the tensioner by loosening the rod nut. Obviously that was not the case. I was able to remove and replace the belt by rotating the alternator. So when I tighten the rod nut nothing was happening to change the tension. With the 19mm loosened I was able to increase the tension. So I hope it is OK. Thanks for the help.

JohnM. · #10 03-26-2012, 07:35 PM

Sounds like you got lucky! Glad you figured it out.

handyjay · #11 03-26-2012, 07:51 PM

JohnM, it was with your help. Thanks again.

Ivanerrol · #12 03-26-2012, 09:01 PM

Lucky this didn't happen to your tensioner adjusting rod.

Here's am image of the 19mm bolt location

Unfortunately the tensioners have a habit of failing after being disturbed after long tenure.
Just keep an eye on the tension on your serpentine belt.

Adler · #13 03-26-2012, 10:34 PM

Most likely a bad tensioner . I replaced two in a very short span ,The Lemforder one is holding up fine,the URO/China one did not.
I replaced the tensioner adjusting bolt and shock-absorber as well to be sure all was correct.
As many will vouch the tensioner is a very weak point .
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glenmore · #14 03-27-2012, 02:52 AM

This will tell you if a prospective tensioner is bad. Imagine two parallel lines off the flats of the tensioner bushing, where the tensioner rod attaches (flats shown in pic above). The back of the bolt hole for the belt pulley should be in the middle of the two lines.

hs_300e · #15 03-27-2012, 01:38 PM

If you tightened the tensioner after replacing the belt, you most likely have over tensioned the belt. When that happens, one of the other bearings fail - most likely the alternator bearing. I destroyed two before I realized that tightening a failed tensioner was causing it. Best to just replace the tensioner. Check the service manual under belt tensioner replacement and it gives instructions to check if a tensioner is good or bad.