Diagnosing Belt Tensioner M103

[OM618]

Backstory:

1991 300SE

I broke down and found the belt to be very slack and not charging the alternator. I tightened it up but it happened again. Further investigation reveled that the 19mm torque bolt had fallen out of the tensioner (gone for good), causing it to slacken while driving. Through my own stupidity, I continued driving the vehicle while periodically tightening the belt when necessary. This was simply because it was my only transportation and I didn't have time to work on it (even though it was just replacing a simple bolt, ARGH!) during the work week.

Well, driving with a belt that would become slack killed my alternator, which I replaced along with the belt itself. Again through my own stupidity I forgot to also order the torque bolt, which subjected me to having to drive the rest of this week (I work 10-12 hour days, really no time for auto repair during the week) with periodically needing to re-tension the belt.

Well, I went to tension my belt tonight and every few turns of the socket wrench the tensioner would feel like it was tightening a bit, but then audibly click and loose any tension.

I assume this means the internals of the tensioner are shot, but I wanted to run it by you all and ask if it could possibly be the rod or bolt or....something else. Heh, basically hoping someone may be able to suggest that it's probably just the rod, but also to confirm if I'm definitely looking at replacing the tensioner.

Also, what causes these tensioners to break like that?

Thanks,

Vince

[Hit Man X]

Well that small rod can snap off.

Sounds like a dead tensioner to me.... M103s in the W126 go through accessories pretty quick if you drive on the freeway often, due to the low geared rear diff giving them high RPM.

If your alt has failed, replace it with a large unit from a W140. 143-150A is a great upgrade for today's batteries. I think the W1240 400E came with a 110-120A unit, which is also a nice upgrade. Allows for you to have everything on at idle... rear defroster, A/C on high speed, wipers, headlamps, etc.

[Ivanerrol]

The M103 belt tensioners are a notorious pain. They break down and need to be replaced.

The 19mm bolt must be loosened before the adjuster is turned otherwise it results in failure of the adjuster such as this.



You mention that the 19mm bolt is missing. Hmmmm. The tensioner will not stay in place without this bolt.

Note location of 19mm bolt



There is an indicator that will tell you if the tensioner is at the right place. You can make it out in the above image - it's the "ramp" with a yellow coloured indicator just to the right of the 19mm bolt.

The tensioners have a rubber bearing. The rubber looses it's tension over time. The idler pulley does not sit straight causing the serpentine belt to get loose and squeak.

[mak]

i have been through the routine recently . Seems The tensioner is a very weak point in the 103 engine. They go bad on their own, not due to lack of maintainance. simply replace it and all will be normal ,with my unit a slipping /jerky belt caused the alternator to break the carbon bushes too. replaced the cut out. Normal charging now , no squeals.the telltale sign of a bad tensioner.
mak
300se

[ds190]

I've had to replace two of those idiot things. One actually failed when all I was doing was replacing a worn belt. On some 124 chassis, the FSM recs loosening the alternator to get the belt off and on, to avoid touching the tensioner

Like he said, the internal rubber which holds the tension fails with age and engine heat. Why they designed this boggles my tiny brain, because the 602/603/606 diesel engines driving the same accessory device load use a simple heavy stiff spring and a damper attached to the idler pulley. That design fails very rarely and is easy to fix. Go figure.

NDM, but one can always mailorder parts with overnight delivery.

[junqueyardjim]

That is, would it be possible to install the 603 tensioner on a M103? Those engines seem to be configured so much alike, but if it is better, probably it won't work.

[pawoSD]

Originally Posted by Ivanerrol

The M103 belt tensioners are a notorious pain. They break down and need to be replaced.

The 19mm bolt must be loosened before the adjuster is turned otherwise it results in failure of the adjuster such as this.



You mention that the 19mm bolt is missing. Hmmmm. The tensioner will not stay in place without this bolt.

Note location of 19mm bolt



There is an indicator that will tell you if the tensioner is at the right place. You can make it out in the above image - it's the "ramp" with a yellow coloured indicator just to the right of the 19mm bolt.

The tensioners have a rubber bearing. The rubber looses it's tension over time. The idler pulley does not sit straight causing the serpentine belt to get loose and squeak.

The tensioner on my M103 does not have that indicator or the "ramp" mark on it. I just had to guess when I replaced the belt! Its been fine for 12k so far, so I must have guessed right.....but this summer I am going to replace the whole tensioner out of paranoia.

[Ivanerrol]

Originally Posted by pawoSD

The tensioner on my M103 does not have that indicator or the "ramp" mark on it. I just had to guess when I replaced the belt!

Even the Erzatz tensioners I have seen have the adjustment indicator. Without it adjustment is guesswork.

If the serpentine belt is not adjusted correctly it can effect all manner of functions - not least is that the pulley on the fan clutch will slip when its required for cooling and not do it's job.

[pawoSD]

Originally Posted by Ivanerrol

Even the Erzatz tensioners I have seen have the adjustment indicator. Without it adjustment is guesswork.

If the serpentine belt is not adjusted correctly it can effect all manner of functions - not least is that the pulley on the fan clutch will slip when its required for cooling and not do it's job.

Guess I just have that magic touch , I just pressed on the belt and tightened it little by little until I found where I thought would be "ideal" tension. Fan works great, no squeaking....no issues in 12k of driving. The new tensioner I install this summer should have the proper markings on it.