Diagnosing Belt Tensioner M103 - PeachParts Mercedes-Benz Forum

OM618 · #1 02-02-2010, 08:52 PM

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 · #2 02-03-2010, 04:00 AM

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 · #3 02-03-2010, 05:34 AM

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 · #4 02-03-2010, 09:35 AM

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 · #5 02-03-2010, 10:04 AM

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 · #6 02-03-2010, 12:13 PM

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 · #7 02-03-2010, 06:54 PM

Quote:

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 · #8 02-03-2010, 08:35 PM

Quote:

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 · #9 02-03-2010, 09:48 PM

Quote:

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.

S-Class Guru · #10 02-04-2010, 12:37 PM

Wow, without the 19mm bolt, the only thing holding the tensioner on is the friction with the little front brace, and the adjustment rod. I would think it would slip around quite a bit.
The turn/click feel you are getting on the adjuster is possibly the end of the rod screwing all the way out of the threads. The rod has lefthand threads, so tightening the bolt CWise drives the rod downward, and eventually right out of the adjusting tube. So, if the rod has screwed out the bottom of the adjustment tube (look down in there and your should be able to see) then there is nothing but the front brace pressing on the tensioner to keep it from just sliding down and out.

Bettter not drive any farther than you want to walk home- when it slips a bit further, the belt will slack, and you will have no coolant pumping.

Oh, just replace the tensioner when you do the fix. In 20 years of owning Old Pearl, the only time she ever let me down was when the rubber tensioner just suddenly snapped in a parking lot 80 miles from home.

DG

OM618 · #11 02-04-2010, 01:48 PM

Thanks for all the replies everyone.

The car is currently sitting in my employers parking lot, awaiting a tensioner change or a tow home for wrenching.

Quick question: S-Class, you said that the threading would slip out the other side - does this mean that I could potentially repair this tensioner and just get the torque bolt installed and be on the road again?

I know I should probably do the whole shebang, but I'm trying to avoid towing it home and in its present location I'm not certain how effectively I can work on it. At least it's under covered parking though, and it's been rainy as all heaven here in San Antonio as of late!

ramonajim · #12 02-04-2010, 01:58 PM

If the big bolt on the front of the tensioner is missing, I sure as heck wouldn't just put the captive nut back on the threaded rod and drive it any further than onto a flat bed.

OM618 · #13 02-04-2010, 06:08 PM

What I'm hoping I can get away with, at least to get thing back to my house for a proper repair without a tow, is to get the tension rod back on, install the 19mm torque bolt, and drive it home.

Does anyone have the specs on that 19mm bolt? Also, there is a washer that needs to go between the bolt and the bracket - is this part a necessity?

I'm trying to avoid paying the dealership $6 for the bolt and $15 for the washer...

If anyone has any links or info where I can get the parts I need for a reasonable price, I would much appreciate it!

S-Class Guru · #14 02-04-2010, 07:21 PM

Sorry, don't have the bolt dimensions.
Dumb question: do you still have the plastic belly pan under the engine?
Any chance the bolt is rolling around down there?

OK, I see your plan: get the bolt and try to get it back together before it all falls out; and get it home. You "may" be able to do that.
Chances are the actual tensioner is still OK, since the lost bolt was the culprit.
1. Check the tension rod and the threaded "tube" it screws into. If you can align them, try to screw the rod back in (turn the tube counterclockwise, it's lefthand threads). Screw it up until it just about pokes out the top end of the threaded tube.
2. Try to align the center hub of the tensioner with the hole in the bracket in front of it. Removing the fan shroud will help. Removing the fan will help even more if you have the proper allen and pulley holder to do that. Might try a screwdriver or something to move the tensioner around. If it won't line up, be sure the tensioner rod is not stopping it (screw the rod up or down to keep it loose. Be sure the belt is not holding the tensioner from aligning with the hole. Be sure the little shock absorber is not binding.
3. If you can get it aligned, slam the bolt in there, turn it nearly tight, mount the belt, and turn the tensioner "tube" CW to tighten the belt.
4. Then BE SURE to tighten the 15mm bolt - Ha.

It's a long shot that you will be able to do this without removing the fan and the water pump pulley, but you can take a stab at it.

Good luck,
DG

whunter · #15 03-17-2013, 04:19 PM

for customer