Serpentine Belt Tensioner Heads-Up

This past Monday, the wife and I were on a leisurely drive (50 mph) from Nashville, TN to Natchez, MS on the Natchez Trace Parkway "leaf-peeping". The scenery was beautiful, the front windows were open and we were driving South into a bright sun, so it was a little difficult for me to see the dash. I usually am obsessive about checking the Scangauge readouts and the dash instruments, but I eventually looked down and noticed the battery light was on. It took me a few seconds of wondering why before I looked at the dash temperature gauge and saw the needle at the 250F mark ready to go into the red. Then, I knew I had a serious problem and pulled over immediately and shut'er down. The 1st picture below shows what I saw when I opened the hood. The tensioner pulley/arm was broken away and was lying in the bottom of the fan shroud. Of course, that meant the water pump was not being driven and the engine had been running for an unknown amount of time with no/minimal coolant circulating. There was no smoke or steam or loss of coolant from the radiator cap vent plus only a little boiling heard inside the engine, so maybe not too long w/o circulation. The Trace is somewhat remote from services, but I was able to get a platform wrecker to tow to a nearby campground (you can stay 14 days for free anywhere on the Trace - no reservation required), get on the phone to Europarts and have a new one overnighted to the nearby park district office and got back on the road by 3 PM the next day. The van seems to be OK having driven from there to Houston, TX w/o further incident.

When I was on the phone to Europarts, I was alerted to an apparent connection between a bad alternator decoupler pulley and tensioner problems, but, by the hand test, mine seems to be OK (Bosch 150 amp replaced in June of '07 at ~106k miles due to decoupler pulley slippage). Presently, it locks in one direction and slips in the other.

At this point I'm thinking there was some tensioner internal issue that caused the arm pivot to freeze and that overloaded the arm causing it to fail. Supporting that hypothesis, I noticed when recently replacing the water pump after short service (less than a year and ~18k miles) that the tensioner didn't pivot smoothly - like it had some corrosion or grit in it. Plus, the low water pump life may have been a result of tensioner problems. And, I've noticed recently on a couple of occasions that the belt squealed a little right after startup. Also, the second picture seems to show that the broken tensioner's pivoting part is now stuck somewhere in the middle of its travel and not resting against the provided stop.

When we get back home, I plan to use my old bar-type torque wrench to compare the torques required to rotate the tensioner pivot for the broken one and the OEM unit I replaced it with ~25k miles ago (should have carried that one with) and report back.

But, given the possible serious consequences of this type failure, I recommend that the tensioner pivot be exercised regularly (every oil change?) to assess its condition.

Maybe it's just the photo, but it looks like the left side of that break has been there for quite a while.

Possibly, but it's only been in service a little over a year.

mendonsy said:

Maybe it's just the photo, but it looks like the left side of that break has been there for quite a while.

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Because it's darker? I think that's just a lighting issue.

Thanks for this heads-up, TH

glasseye said:

Because it's darker? I think that's just a lighting issue.

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Yeah, It's hard to tell from the photo whether it is darker or just different lighting.

talkinghorse43 said:

I noticed when recently replacing the water pump after short service (less than a year and ~18k miles) that the tensioner didn't pivot smoothly - like it had some corrosion or grit in it.

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When a belt tensioner does not pivot smoothly, in spite of heavy spring tension, it MUST be replaced.

Easy to evaluate with a long flex head wrench w/17mm, 12 point socket, from under the vehicle.

Beware of aftermarket tensioners.

Doktor A

abittenbinder said:

Beware of aftermarket tensioners.

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This was labeled INA from Europarts - one to beware of?

abittenbinder said:

Beware of aftermarket tensioners.

Doktor A

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From what I can glean from the web, all replacement parts, no matter where sourced, are "aftermarket". Again, is INA (tensioner is labeled "made in Germany" http://www.schaeffler.com/content.schaeffler.de/en/index.jsp) a good brand to use?

Autopsy of the failed tensioner shown in the first post of this thread answered my question of the previous post. I will show in this & following posts the results of that autopsy and argue that the design of this tensioner dooms it to a short service life and is likely the cause of its catastrophic failure. So, this brand tensioner (I bought it, and another like it (now installed), from Europarts - others vend it as well) is NOT a good brand to use.

I used the indicating torque wrench shown in the first pic to get an idea of the difficulty of pivoting the failed tensioner compared to the OEM tensioner (Litens brand w/ MB star logo - next two pix) it replaced ~25k miles before the failure. (Since the failure was obviously the result of overloading, the relative torque required to pivot the tensioner is important.)

The OEM tensioner pictured required 15 ft-lbs to begin to pivot from the fully-extended stop and required 30 ft-lbs to reach the fully-retracted position (the new Litens brand tensioner I recently bought from my Freightliner dealer is the same). The broken tensioner required 25 ft-lbs to begin to pivot from it's partially-extended position and 45 ft-lbs to reach its fully-retracted position. After pivoting the broken tensioner multiple times through its range (stop-to-stop) to loosen it up, it required 20 ft-lbs to begin to pivot it and 35 ft-lbs to reach its fully-retracted position. Although it's not easy to get an accurate reading on the INA tensioner now installed on my '02, it's essentially the same as the broken tensioner after loosening it up. (When testing the installed INA tensioner (~4k miles since installation), I noticed that it makes a creaking noise/feel and that it takes slightly more torque to get it to begin to pivot than to keep it pivoting.)

So, the failed INA tensioner required significantly more torque to pivot than either the OEM Litens tensioner or the newer INA tensioner, suggesting internal condition issues.

More to follow....

Continuing...

The first pic shows the back of the failed INA tensioner and the point of attack I used to disassemble this unit. I determined that the plate shown pivoted with the top of the tensioner and that told me there was a central shaft. Therefore, I drilled out the the back end of that central shaft deep enough to release the back plate. The second pic shows all the parts of the failed tensioner arranged in their respective positions. Proceeding from the left of that exploded view, the third pic shows the inside worn surface of the back plate and a central seal (white O-ring which on probing with my fingernail appears to be a very soft PTFE-type foam material). The 4th pic shows the back surface of the body of the tensioner and the heavily-worn surface of a plastic insert that had been in contact with the back plate. This plastic insert appears to be a hard brittle thermoplastic similar in character to Bakelite. Also seen here is one of the two sleeve bearings through which the central shaft passed. These bearings seem to be coated with a white plastic material reminding me of a PTFE-coated frying pan. This sleeve bearing's coating is partially worn away.

More to come...

Continuing...

The ends of the spring shown in the exploded view in the post above fit into recesses in the tensioner body and the inside of the top of the tensioner to provide torsional resistance to pivoting. The spring is also compressed on assembly and provides clamping force to hold the back plate in firm contact with the plastic insert. This effectively makes the back plate/plastic insert a brake or shock absorber, apparently meant to dampen oscillations. I haven't disassembled the OEM Litens brand tensioner because I plan to use it as a spare part in the future, but experimentation with the torque wrench shows me there is no such damper in the Litens brand tensioner. The 1st pic below shows the top of the tensioner body and the cavity for the central spring. Also, this shows the top sleeve bearing for the central shaft. Again, this bearing appears to be coated with PTFE, which is partially worn away.

The last pic shows the central shaft attached to the top of the tensioner. The surface of the shaft in contact with the two sleeve bearings is corroded, which accounts for the increased pivoting torque and the overload failure of the roller mounting arm. Note also where the shaft meets the top of the tensioner. It can't be clearly seen, but an O-ring like the seal at the bottom is also fitted here. This is where I believe the water responsible for the corrosion of the central shaft entered. Since the design of the tensioner depends on the friction between the back plate/plastic insert to provide damping, that friction pair will lose thickness due to wear which will remove the top O-ring from contact with the sealing surface (top of the central tube of the tensioner body) allowing the seal to be lost.

Therefore, the very design of this tensioner dooms it to early failure.

talkinghorse43 said:

So, this brand tensioner (I bought it, and another like it (now installed), from Europarts - others vend it as well) is NOT a good brand to use.

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A rather comprehensive postmortem. Beware of aftermarket tensioners.

Doktor A

Unfortunately, it's not in MB's interest (since they depend on parts sales & service as an important part of their business) to issue specs and approve parts to those specs like they do for oils. Sure would be nice for us DIYers to know which parts are adequately designed and constructed. The failure of this tensioner could easily have cost me the engine of my '02 and the parts supplier (in this case, Europarts) would have had no liability other than the 1 yr warranty on the part. I've learned my lesson and will from now on only buy critical parts (probably difficult to tell which part is a critical part) sourced through MB; or maybe I should buy all parts only in the MB box?

talkinghorse43 said:

I've learned my lesson and will from now on only buy critical parts (probably difficult to tell which part is a critical part) sourced through MB; or maybe I should buy all parts only in the MB box?

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It does help to know which OEM MBenz parts are available directly and more affordably from the source supplier. Starters, alternators are a good example of identical aftermarket from Bosch, the source supplier.

Water pumps, belt tensioners, EGR valves are examples of parts NOT available aftermarket from the OEM source manufacturer.

Doktor A

abittenbinder said:

Water pumps, belt tensioners, EGR valves are examples of parts NOT available aftermarket from the OEM source manufacturer.

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Speaking of water pumps, the OEM for mine went 216k (shaft seal failure) which I replaced with one from Europarts. The replacement lasted just a little less than a year (replaced this past summer - shaft seal again). Replaced with another from Europarts and the shaft seal is leaking now too. Might be due in part to the tensioner issue, but I just bought one in the MB box for the third replacement - not a job I like to do.

This all has me wondering about other possible effects of bad tensioners - mainly harmonic balancer, key & crankshaft bolt failures. Probably those failures are due to metal fatigue and metal fatigue is due to load, yield strength and # of cycles - lots of cycles at 2500 rpm. We do know that balancers with the dual belt provision are more susceptible; probably due to the additional load imposed by the second belt. Hopefully, the roller arm on the INA tensioner that failed was also deficient in strength and was the weak link protecting the harmonic balancer & crankshaft bolt. Right now, I don't see any web cracking or feel or see any wobbling of the balancer. Time will tell.

Just looked at the Europarts online catalog and saw that their offering for the tensioner is no longer from INA, but from Febi-Bilstein.

http://store.europarts-sd.com/serpentinebelttensionerassembly.aspx

Picture looks just like the OEM and MB parts supplier Litens brand tensioner. Anyone know if the Febi-Bilstein is equal to the Litens?

Also, text there attributes the failure I had to a seized alternator decoupler pulley. I've tested mine (Bosch 150 amp purchased from Europarts in '07) by hand many times since the failure and it is NOT seized. But, I can only test it with the belt off; don't know if load from the tensioned belt would cause it to seize.

Same thing happened to me....after market belt tensioner pulley.....lasted only a couple thousand miles......broke in a different way than talkinghorse.....here is pics for the record.

TH,
Just found this post.

Here is my story: My original tensioner lasted more than 300k and eventually the spring locked up. Replaced it with an aftermarket one (not from europarts) which I had with me as a spare and I thought it would take a while until I would need to buy a spare. Well, about 38k later, driving in KY I see the battery light on, pull over right away and the tensioner pulley locked up. No spare with me, so next day someone brought me a new one from the dealer. I don't have a spare now, so I was going to order one from europarts, thinking they were OEM. But I will steer clear of them.

Now, my water pump story: The original water pump started leaking I think around 250k or so, so I ordered one from europarts. Installed it and 18k later the seal started leaking again (it only took me less than 3 months to drive 18k). Europarts replaced it under warranty, so in goes the new one. Well, guess what? 12k later it starts leaking again. I bought one from the dealer, not trusting the pumps Europarts sells any more. They said that they never had problems with those, but just didn't want to risk it again. So they refunded me the price I paid for the pump. I think the pump was Meyle, but I might be wrong.

FWIW.

mawsea said:

Same thing happened to me....after market belt tensioner pulley.....lasted only a couple thousand miles......broke in a different way than talkinghorse.....here is pics for the record.

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What brand?

Talkinghorse,

Excellent analysis.

I checked the stock tensioner in my 3500 Sprinter. It has a force range of 15 - 25 ft-lbs.

My spare INA tensioner has a force range of 20 - 35 ft-lbs. I have concerns using this part now. It could fail like yours did. In addition, is that too much tension for the belt? Will bearings wear prematurely?

I'm waiting for an email reply from europarts-sd.

Cal