The serpentine belt on new autos together with the tensioner that it uses appears to have been designed with one of the goals being to leave a motorist stranded on the side of the road when the tensioner fails catastrophically.

One the other hand maybe I was negligent in failing to replace my tensioner when my vehicle had 98K miles on it. I’m not really that familiar with tensioners and serpentine belts as I have only one other car with one; it’s a 92 300E and I did have that tensioner replaced at 88K mile because I could tell the drive belt was not that tight when I pulled on it.

About a week ago the tensioner on my ML failed catastrophically. It caused a belt with only 50K miles on it to shred. This is the first time in 34 years of driving I’ve had to have a vehicle towed because I could not get it quickly fixed on the side of the road sufficiently to get it home.

In the past if a drive belt would break I could just replace it, tension it manually and be on my way. Actually the only vehicles I’ve had that have broken belts are old Porsches and VWs. I did have an 83 240D that burned up a belt when the A/C compressor froze up. Not good, I had to drive without A/C but I could still drive it.

So bottom line should the tensioner be replaced based on mileage or age as part of the general maintenance on a vehicle?

Just replace it sometime before it fails and you'll be fine........ :p
If the compressor seized on the 240D, the fact the belt burned up is kinda secondary to the problem.
The newer style tensioners seem to be much longer lasting in general, and the belts seem to be lasting longer, but I know that's a small consolation after one of them failed on you.
I think we're all alot better off with serpentine belts rather than seperate V-belts, IMHO.

Gilly

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It is very uncommon for tensioners to "just die", they usually give warning signs such as noise. It's common to change parts during the course of a service that we can see as having a iminent failure but when questioned as to "why haven't I noticed" by the customer or "can I see the BROKEN parts" they find it hard to believe that even though it seems good now that it will fail shortly.

Most new tensioners will last a long time, but then again, so do the belts relative to the older v belt style.

Your tensioner may have been showing signs for a while, you might not have noticed them.

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Alex.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32

My tensioner never made any noise whatsoever before it failed without warning.

I'll usually notice when something sounds different or out of the ordinary. I noticed the sound of a very slight exhaust leak, on start up which was coming from my driver's side Cat. Once the vehicle warmed up the noise was gone as the metal expanded enough for the leak to be essentially sealed. It turned out to be a hairline crack around the perimeter of the weld on the Cat. My point is I think I would have been able to figure out the tensioner was about to fail if it gave any warning. But then maybe not. Thank goodness my favorite car has the old fashioned separate v-belts.

So is there a recommended mileage or time when a tensioner should be replaced? I really don't want this to happen again.

Dealer replaced my tensioner and the belt along with the ole Harmonic Balancer just before the warranty ran out. It seemed okay to me. Ron, do you still have your original H/B?

No, I had them put a new one on right before it went out of warranty along with a new fuel pump with the sending unit.

Those older style V-belts were recommended to be changed every 5 years or 50,000 miles whichever occured first...

Seems like the newer serpintine style belt would be about the same...

The belt tensioner mechanism should easily be reliable for 100,000 miles or better...

But taking a wild guess I would say to change the belt tensioner mechanism before the 200,000 mile threshold... better safe than sorry... no one wants to break down in a bad area after hours...

Whenever a new engine fan belt is installed the old one [in usable condition] can be placed in a tool box in the trunk along with a flashlight, any spare relays on hand, and the small satchel of tire changing tools...

Hope this info is helpful to someone somewhere...

Elusive 190e

I've seen in the Asdit /Performance Products catalog a snap-together adjustable length plastic emergency belt selling for about $35. Has anyone ever used it? It seems like a worthwhile item to have on-hand in case of an emergency, since you don't have to remove any other parts to install it.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)