Mercedes 300SL Serpentine Belt & Tensioner Replacement M104

A flickering alternator light leads to a slipping belt getting replaced.
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[@aloeverga69]

best video on relevant to m104 (as well as m103) tensioner. i greatly appreciated the detail you went into noting the pointer orientation as well as tensioner rod orientation. i have watched many videos on the tensioner system looking for these specific details and yours is the one.

[@FlashDriveFilms]

Glad you liked it! I enjoyed making this particular video.

[@bsclifton]

Missing from the video - but at 7:30 after you route the belt, you need to use a 6mm (I think?( allen insert with a breaker bar turned clockwise to relieve the tension so you can get the belt around all the pulleys 🙂 It might be common sense if you've changed a belt before - but some belt tensioners include a cap over that allen hole and it IS possible (with a lot of force) to get the belt wrapped around the pullies without relieving the tension.

[@williamkennedy5492]

Tempus fugit, at last i am on the road in my SL, it passed the UK gov check with flying colours, I've only done short journeys so far, AND i noticed both cooling fans running, after a short distance, Around 9 months back i thought i had a water pump problem the engine boiled up so bought a new pump, and today i am fitting it, in between rain showers, well it is Cheshire UK , similar to Seattle when i lived there for 6 months. Rain cities i think. I was parked outside my sons school ( I'm and older papa ) and a red VW hatch stopped with a blonde teacher driving it, she looked at me and said what a beautiful car, nice lady ! I replied i bought it for my fiftieth birthday , she laughed and said i did the same with this, so my classic line was, Oh two weeks ago then, i had noticed the year on her registration plate, she laughed and drove off. I guess unexpected things happen when you drive a 1990 SL even on highways i get cars slowing down and pacing me then the thumbs up with big smiles.. 12 years ago i said to my Thai wife i will take my SL back to Thailand, NO WAY she said, as its far too sexy a car and girls will jump into the front seat, WOW she was serious, guess some ladies there have a habit of stealing other ladies husbands but not me i am a happy man. Here's what i did, i put the car in my wife's name, if she owns it for 5 years then she can import it to Thailand and AVOID import duty plus tax at 80% of the cars original value. So sometime in the future the SL may make it to warmer climes . But for now i have to figure out how to loosen that serpentine belt, your video has pointed me in the right direction. Lastly darn fuel gauge stuck at a quarter, i have to get to the sender unit that's going to be a pain but its do-able .Best regard from Cheshire UK and sometimes Sisaket Thailand.

[@FlashDriveFilms]

Well it has been a minute, last I knew you were doing things with the brakes, fuel pump & radiator. I forget if yours is a 6 cylinder or and 8. In any event, I did the water pump on my '90 300SL , back in 2016, and had the devils' own time with it. It's not a simple 4-bolt arrangement, like many cars. On the M104 engines it is tight up under the intake manifold and has a wide assortment of sensors and fasteners to remove. Second hardest car service I've ever done. (the worst being a clutch replacement on an X1/9: mid-engined and done on jack stands in a dirt driveway. Turned into a 7 hour ordeal.) There's an O-ring where the pump body meets the block. Put a dab of axle grease on it to hold it in place when you instal the new pump so it doesn't slip, get pinched, and leak straight away. (ask me how I know.) Anyway, best of luck on your automotive & international adventures!

[@Trippsevv7]

This is by far the best instructional video I've seen to date! Easy to follow, clear and concise, and right to the point without any awkward moments, and you did it in under 10 minutes! Thank you, sir. I am now a subscriber!

[@FlashDriveFilms]

Glad it was helpful!

[@alfacorse]

I´ve just finished this job in my W124 320 E following your video, it´s very clear, detailed and well edited. Enthusiasts like you help other enthusiasts like me to lose the fear of doing a bit of wrenching on our cars. A big thank you.

[@FlashDriveFilms]

Thank you! I love to hear when these cars get kept on the road (and their owners can save a little money while getting to know their cars better!)

[@bobdefalco]

Patience, indeed! Great to have you back!!!

[@PhillyN]

Thanks for this one! Followed your steps, and back up and running!

[@FlashDriveFilms]

That is The Best! Exactly what the internet is great for. No one is more pleased than I am.

[@davidsalem4526]

Thank you sir. Very helpful during the re-assembly of my 1989 300E (water pump replacement). Nice trick on the finger magnet. Thanks again.

[@FlashDriveFilms]

Thank you! And my hats' off to another survivor of the water pump replacement ordeal! I did that in 2016 on this car and still haven't edited the video yet. Flashbacks every time I open the footage to work with it . (hardest repair I've done since the clutch on a mid-engined car.) Alden

[@4423Scott]

I definitely appreciate this video. You are on point with every step. I did the magnet in the glove and I finally was able to get the nut on there..... that was the best tip!

[@FlashDriveFilms]

Thanks! The magnet-in-glove trick has saved me many times. Glad to be of help! Alden

[@oglinkgog]

Thanks for this video, if only all DIY vids were put together like this! Am replacing the belt, tensioner, alternator and removing the air (smog) pump on my W124 320TE as we speak, and this has been a real help as I'm used to Mercedes Diesels which use a much more straightforward tensioner system. Whoever designed this set-up and where/how it's mounted needs a kick up the a*&$!

[@FlashDriveFilms]

Hi! Glad to help. But if you think the tensioner is bad, just wait for my soon-to-be released video on the water pump for the M104 engine ! (gotta be in the top 5 worst servicing ordeals I have ever done.) Alden

[@oglinkgog]

:) I don't even want to think about that after this job! Only a couple of things I'd add: The leftmost bracket bolt (looking towards the engine) needs sealant on it if you take it out to stop it possibly leaking engine oil,and the manual says the pointer is supposed to move from the right (corner of triangle marking), all the way to the left ('fat' end of triangle) for correct tension. Mine seemed happy at just before there, with good tension and the belt turning 90 degrees with 'reasonable' force. The thing that gave me the most grief was one of the fan allen bolts not wanting to thread back in properly- the very last bolt of course!

[@deltabrogden3350]

I really needed this. Good job on the detail of this review. I am about to tackle this job, and your review was most helpful.

[@FlashDriveFilms]

Glad to hear it! A lot of repair shops don't like to work on cars this age, and even when they do it turns out much more expensive than DIY. Enjoy!

[@robertmurray6990]

Helpful video, especially needed to know what position to start the plastic tensioner "pointer" when replacing it. back to the garage to finish up!

[@FlashDriveFilms]

Thanks! Be extra careful with the pointer. They are remarkably fragile and hard to find replacements.

[@TrainerTakumi]

incredible video with amazing detail subscribed!! thank You so much.

[@FlashDriveFilms]

Thank you!

[@williamkennedy5492]

With each video i watch i keep thinking i have more work to do on my R129, so i say a big thank you for showing me the way with your videos, you are a natural commentator and skilled engineer,

[@FlashDriveFilms]

You are very kind. Don't think you have to do all these tasks at once. Seek a balance between driving and maintenance. I always advocate sticking to the service intervals for things like oil changes and tune-ups and then tackling repairs as they materialize.

[@giantgeoff]

Another excellent video!

[@aphex14]

Great simple and detailed video! I have a 300e m103 that I will be performing this maintenance. Really neat idea for the magnetic glove! Subscribed.

[@FlashDriveFilms]

Thank you! These older Mercedes cars a genuine value, even more so when you can do much of the service items yourself. Glad you enjoyed the video and thanks again for subscribing. Alden

[@MMWA-DAVE]

Thanks for making this video - I am going to tackle this exact job on this exact engine shortly. When i saw a bag labeled water pump I almost threw my beer at the TV screen, water pump removal is a nightmare on this engine, thankfully it is not required ;)

[@FlashDriveFilms]

+MMWA David Xu makes an excellent point below about removing any tension left in the belt before removing the pulleys. On my car there was virtually no tension left and so it was not an issue. Also, be very carful around the front crank position sensor. You don't want to break or disconnect that! Just take your time and it should work out great! Alden

[@edit11]

this exceptionally help me replace my belt tension assembly on a e320 w124 model. Thank you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[@FlashDriveFilms]

Glad it helped! It isn't easy, but it is easier than it looks at first. The E320 is essentially the 4-door twin of the 300SL mechanically.

[@edvardsz3539]

Thank you very much for tutorial. Helped me a lot!

[@FlashDriveFilms]

Glad it helped! It's great to hear . It helps keep these cars on the road and being enjoyed. Thanks!

[@rakeshsingh9067]

You are really great at what you do and your content is excellent

[@FlashDriveFilms]

Thank you ! I am happy I get to post videos that people enjoy.

[@shixxx8]

Great video and great craftsmanshift! Well thought out. Kind regards from Germany!

[@FlashDriveFilms]

Thank you very much! Glad you liked it!

[@livin4today132]

Great video, very well illustrated,,thank you

[@FlashDriveFilms]

Thank you! Glad you enjoyed it!

[@libran58]

That was a great help thank you.

[@Visionery1]

Very good video! Basically the same tensioner setup as fitted to my M102 230E.

[@FlashDriveFilms]

There was a cutoff date of, I think, 1986, when several things changed. The earlier M102 used multiple V-belts instead. I should really do a video on changing them as well. Alden

[@Visionery1]

@@FlashDriveFilms yes, they changed to a serpentine belt on the W124 M102 from '86 onwards.

[@BigmoRivera]

Great Job 👍🏽 Nicely Done

[@FlashDriveFilms]

Thank you! I hope it helps out.

[@MR-nz5pg]

Excellent work - thanks

[@FlashDriveFilms]

Thank you!

[@billrbca]

Well done video, i have 300sl as well and have been in this are often like how you painted the tensioner marks

[@FlashDriveFilms]

Thank you! Yes, adding a little color to the reference marks really helps make them stand out against the grey aluminum background. Makes things easier every future time you have to adjust or change something. Alden

[@johnsanon9074]

Thank you sir!! great video!

[@FlashDriveFilms]

Glad it was helpful!

[@pierrepotgieter8076]

You save me from insanity......Great video mate Thanks!!!

[@FlashDriveFilms]

Glad I could help!

[@rtsaitw]

Well organized procedure and plan. I hope I know any of MB tech like you in my country so I don't have to DIY any more...

[@FlashDriveFilms]

Thank you! Glad you liked it! Alden

[@striela1]

Great video!! Thanks, well done!!!

[@FlashDriveFilms]

Thank You !

[@sj280z]

thank you sooooo helpful

[@FlashDriveFilms]

Glad to help! I love seeing these R129s still on the road and people enjoying them! Alden

[@TheFrancogt1]

hey thanks for this video , although my car is a 190e 2.3-16v Its similar system and i was able to complete my tensioner replacement , by referencing and comparing this video

[@FlashDriveFilms]

Excellent. Exactly what I make these videos for ! I also have a W201, but it is pre-86, so it uses 3 V-Belts. I'll do an updated belt replacement one of these days. Alden

[@rfbu]

great advice

[@fathallagaara8377]

lot of thanks from egypt.cairo

[@FlashDriveFilms]

Thank you! I hope it helps you keep your Mercedes on the road for many more years! Alden

[@bagerton]

Great Video Thanks

[@FlashDriveFilms]

Glad you enjoyed it

[@giusemancuso]

great video very clear thank you.

[@FlashDriveFilms]

Thank you! I'm glad you liked it and I hope it helps you keep your Mercedes running great! Alden

[@williamillmilable]

Amazing video

[@FlashDriveFilms]

Thank you!

[@pamhannam3669]

Fantastic video, while this job is more challenging than I can do myself at least I know what is involved so I can have an intelligent conversation with a mechanic ( own 1990 300sel w126)

[@FlashDriveFilms]

Thank you ! Excellent. Even if it just levels the playing field with your mechanic, that is a big win.

[@vijaywaingankar8352]

Good.job.sir..❤

[@FlashDriveFilms]

Thank you!

[@user-ok3xl6do5l]

Thanks!

[@FlashDriveFilms]

Glad you liked it !

[@oglaz]

Added to favourites;)

[@FlashDriveFilms]

Thank you!

[@MMWA-DAVE]

7 years later, here I am again watching this, but for entertainment purposes. I'm glad I'm not the only one that specifically dresses like a homeless person when its time to work on the car and get dirty 🤣

[@FlashDriveFilms]

I always figure why mess up some perfectly good clothes when I have a set of perfectly BAD ones to wear? 😄

[@praiselifeworshiplife1ca]

great video. one suggestion more light on the motor when your in the trenches would make it easier to see what your doing. your narrating was awesome and motivational. when you taking out the dampener and a tensioner,was the hex bolt really tight or did it come out fairly easy? thanks 🙋

[@FlashDriveFilms]

Thank You! I agree about the lighting, and have been trying different ways of getting good illumination in tight spots. I'm still learning how to get light, my hands, the tools and the camera all in to a narrow spot, and still be able to show what the action is. I hope to get better each time. The tensioner bolt wasn't really tight, but trying to get a lot of force on it in that awkward spot was difficult. I'm finding more often that wrenches with the most leverage are also the ones that don't get in the tight spots that I need them to. Alden

[@FlashDriveFilms]

[@jerryz1846]

Anything on a 6 cylinder 1991 6 cylinder W126 300 SE water pump replacement ?... I watched some of your videos...you are a genius....I can not find anyone on long island to do this job

[@FlashDriveFilms]

Hi! I think the 300SE uses the M103 motor (Single Overhead Cam) Surprisingly, I am actually working on a water pump replacement video for the M104 motor in the 300SL. I did that repair last October, and it was an epic adventure to say the least. Much reading online and then the experience proved it is not a task to take lightly. Part of what has taken me so long in producing my video is that i have some footage to re-shoot (detail stuff, really) but I had spent so much time trying to get it done, I kind of sped through some segments, not bothering to film. If you do get a shop to do it, expect a labor bill for around 5 hours work. Plus the parts.I suspect the M103 motor is a similar challenge. I will get my video up at some point. (I just want to do a good job of it though.) Alden

[@jerryz1846]

yes mine is the M103 ... thanks for your input...the mechanic wants 850.00 for the whole job...too hard for me to attempt alone

[@FlashDriveFilms]

Honestly, that sounds pretty good. The pump itself is not cheap. (part of the reason why mine went 194K before it failed!) Especially if he is going to provide new hoses and a new thermostat.(and at LI labor rates!!) I would say (*opinion*) if you can swing it, pay the man. I managed mine because I had a bunch of time off already scheduled and a separate car to go get parts with. There are exactly 106 other things I would have wanted to do that weekend. :-) Alden

[@hubertholmes5581]

Great video, out of curiosity, did this fix the flickering battery light? or is that why you later replaced the voltage regulator?

[@FlashDriveFilms]

Hubert Holmes Yes, the alternator light went out with this belt fix. There was no more tension left to take up and I suspect the belt was slipping at low rpm. (and it was really worn out. Over 100K on it at least)The voltage regulator came about from how slow the top & windows seem to be moving this year. And pulling the VR out I saw the brushes down to the nub. I suspect I will also need a new battery before Fall (it is about 5-years old now) (and the top & windows still seem slow, so that may another thing for me to look at ) Alden

[@njoeandjen]

on the tensioner bolt are you sure the smooth side goes towards engine nothced side toward pulley as mine would not adjust thought tensioner was good but purchasing one anyway as looks like center shaft spun a little when i look at new pictures of a replacement as my shock was completely shot but spring bushing seemed ok!

[@FlashDriveFilms]

The tensioner itself needs the flat side towards the engine. It's at 4:39 in the video. The bolt itself is a little strange as it is a reverse thread. Usually "lefty-loosey/righty-tighty" is how adjusters work, but this one applies force in the opposite way expected. Hope that helps.

[@matthapaq9618]

Good day mate , you are a Legend 💪🤝 thank you so very much for your time effort to bring shares us your knowledge on W124 ,,, Mate I have problem as I replace water pump after I fill everything bake now time for the belt to install and I found out the yellow tensioner pulley missing 🤦‍♂️ I have not seen it since I start take pump out ,, now engine shaking and misfire,,, any idea how to do it ,without yellow tensioner pulley ??? Thank you again Cheers Matt

[@FlashDriveFilms]

I have honestly never encountered that before. Do you mean the whole pulley is missing or just the yellow plastic tension indicator? I know the indicator also acts as a sort of spacer and that may upset the operation of the pulley. I don't know if that would lead to misfiring though.

[@matthapaq9618]

@@FlashDriveFilms Thank u so very much for your quick reply 💪🫡Yes the yellow plastic tensioner arrow missing Before it was running like a dream no issue at all ,,, everything else could cause any misfire all changed car serviced regularly,, tomorrow I will see my mates if they have anything for me if not I’ll buy it online until then I’ll see in mm the difference and add a washer as a temporary replacement! Many thanks and I’ll get back to you with results 🫡 Cheers Matt

[@FlashDriveFilms]

Just as an idea, look carefully at the connectors on the front of the engine, especially the crank position sensor down low on the front pulley. If you brushed against that it might have bent the bracket or loosened the connector enough to influence the engine firing.

[@matthapaq9618]

@@FlashDriveFilms Mate guess what 😊🤦‍♂️😄 before I start working on E280 I washed her with pressure washing machine with car after I covered what need to be covered even tho with care , so I just remember today 😁😅 and I take all spark plugs each one by one each one I blow it out with my air compressor then clean spark plug with carbby cleaner then blow the inside all cylinders just incase 😅all done 4 cylinders was full of water 😁 now it runs like it was before with no drama, the belt I did it with four steps until I could find the right spot that I undo the pulley bolt then turn the belt tensioner adjustment one half turn the tight the bolt then undo again and repeat it 4 times of course each time I start car until I get there ,,,, thank you again for you kind and care and for thinking for the other possibility kind thinking so I can solve my problem/issue , for me you are a Legend 🤝💪🙏😎🫡 Wish you all the best in the world 🫡 Cheers Matt

[@FlashDriveFilms]

Excellent. Glad to hear it worked out and is running fine again.

[@NCG3]

👌👌👌👌👌👌👌

[@vishnukalainesan]

Hi there I have a w124 230e 4 cylinder and I followed ur tips but I have to tighten all the way to the end of the marker scale because only then it had proper tension. Is there something wrong with the tensioner it self but its a brand new unit along with the correct new belt aswell. Thank you

[@FlashDriveFilms]

Hi! As long as you have correct tension you should be fine. It just means that you will not have any slack to take up as the belt stretches slightly over time. If the belt is an aftermarket brand (not Mercedes itself) may actually have been for a different motor but was "close enough" in length to meet spec. Just a few millimeters difference in length is all it takes.

[@vishnukalainesan]

@@FlashDriveFilms thank u so much for your reply. Yea so I checked and my "correct" belt length is 1980mm with the ac compressor. The tensioner brand is INA so hopefully its a good brand. It's been fine for the last month of usage so I'll see.

[@LoRenzoRozeMusic]

This video really isn’t for my car, I have a 190e 2.3 but I will say this video was waaaay more instructive than the other for my car

[@FlashDriveFilms]

Thank you! I hope it helped in some way. I know the post-1986 190E cars went to a serpentine belt very similar to the M104 six cylinder cars.

[@thaddeauscaldwell7101]

Great video . I was wondering if you could help me . I have a 1995 SL320 . It has two plastic pulleys and I can't seem to find, how to release tension (there isn't any nuts in the center of them) to replace the belt . Can I unscrew the long nut on the tension rod and this reduce enough tension to get the belt off ? Any information will be helpful.

[@FlashDriveFilms]

Hi! Yes,The SL320 uses a very similar version of the M104 engine of the earlier cars like mine. The plastic pulley do have a center bolt holding them, it is under a cover/plug that you have to pry off. They are flush with the center, use a good light and you will see where to pry them out.The best way to release the belt tension though, is to loosen the bolt at the center of the tensioner and then back off the 13mm bolt/rod combination that locks it. You only have to slacken it a little, so that the pointer moves down some, and then you should be able to flex the belt enough to remove it off the pulleys. Hope that helps. Alden

[@thaddeauscaldwell7101]

@@FlashDriveFilms , thank you for your reply. My car has the 10mm allen bolt. This is the main tensioner bolt , right ? And it is torqued to specs after the rod bolt is adjusted , right ? If so , then I understand your instructions. Thank you again.

[@FlashDriveFilms]

Yes, that is correct. Loosen the hex bolt at the center of the tensioner before backing off the adjuster. The rod is thin enough it will snap if the tensioner is still torqued. When you put the belt back on and adjust the tension, then torque the center bolt to spec, 75nm. And good luck! Alden

[@thaddeauscaldwell7101]

@@FlashDriveFilms , Great , thank you for pointing out, to release the tensioner allen-bolt before the rod-bolt . Again ,thank you so much Alden. Best Regards

[@RobertSmith-di5ll]

What more do you need - Spot on

[@FlashDriveFilms]

I try to make the videos that I needed to watch before I do the thing that I end up making the video about. Nothing truly bad ever happens to my cars, it just generates new material! Alden

[@BerlinettaFerrari]

Why is your belt so loose compared to mine when I am stillllll trying to get this thing started....

[@Biffamild]

Hey man. I have a w140 s280 that has the same tensioner setup. I don’t know how but I keep snapping the rods and the belt doesn’t seem to tighten at all :( any idea ?

[@FlashDriveFilms]

Truthfully, I've never worked on a W140. The best I can guess is to make sure the center bolt that clamps the tensioner is loose enough to allow it to move. I actually snapped a rod years ago because I was trying to tighten it against a tensioner that wasn't free to rotate. I loosened that center bolt, and the tensioner worked as it should.

[@thaddeauscaldwell7101]

know this question may be off topic. Is there any sealant that needs to be used when changing the water pump on the M104 engine. I have a 1997 SL320 R129 with the M104 engine and the water pump kit has the rubber O- ring . Is the O-ring sufficient to seal the water pump to the block or is sealant needed . Any information is greatly appreciated.

[@FlashDriveFilms]

No, they do not need a sealant. BUT, I dab a little axle grease on the O-ring to keep it in place in its' groove. The first time I did the water pump, the O-ring slipped and got pinched and wow, was that a lot of work to repeat. Second hardest fix I've ever done. 2nd only to replacing the clutch on a mid-engined car. Good luck to you!

[@thaddeauscaldwell7101]

@@FlashDriveFilms great . Thank you so much for the information. I haven't changed a clutch on a mid engine car before. I'm sure it could get frustrating.

[@FlashDriveFilms]

It was actually the first clutch I ever changed. I had no idea what I was doing and I had no lift. So jack stands in the driveway. With the water pump I was forewarned and had many more tools. It was just so many fasteners and sensors and hoses to remove from tight spaces and keep track of. Hours and hours of engaging fun.

[@lordperezident]

Where did you buy the parts from?? You never mentioned it in the video

[@FlashDriveFilms]

I bought the parts at a local, independent auto parts place in my town. If you shop online (and these are NOT endorsements, only *suggestions*) you can get these parts at websites like Rock Auto, Pelican Parts or eEuroparts. But definitely shop around for the best price and service. Alden

[@rioraton]

where can we get the shop manual you used?

[@FlashDriveFilms]

The manual was actually a CD that came with one of my other cars when I bought it. (it's really for the W124-bodied cars, but many systems on those cars are the same as on the R129) I just printed the page from the disc to use in the explanation. I'm going to guess similar CDs are available on places like eBay or Craigslist. Alden

[@sauldean6430]

How do you release the tension if you only changing the alternator

[@FlashDriveFilms]

You can loosen the belt by prying the tensioner pulley with a large screwdriver/prybar. ( the tensioner is springy) That should slacken the belt enough to slip it off the alternator. Or loosen the center bolt on the tensioner and run the 13mm adjuster rod back (remember it is reverse threads) to achieve the needed slack to pop the belt off the alternator pulley.

[@sauldean6430]

@@FlashDriveFilms so clockwise to loosen it and anti clockwise to tighten it, mine does not have the screw in the middle of the tensioner, it has that 10mm hex

[@FlashDriveFilms]

Correct. Some came with a 10mm hex bolt. Most were an allen-head cap screw . Don't remove it, just loosen it a little to get the tensioner to pivot when you slacken the adjuster rod.

[@sauldean6430]

@@FlashDriveFilms ok thanks a lot, I really appreciate your help

[@SuperBranFlakez]

Where can I get the pointer online for tensioner?

[@FlashDriveFilms]

I don't know of any source for new ones. You might be able to get one on eBay. I don't think the dealerships even list them as a separate part.

[@hussy67]

Is this the m103 or m104. Thank you.

[@FlashDriveFilms]

This is the M104. Twin cams, 24 valves. No waiting. The M103 is single cam and has a different setup for the serpentine belt.

[@hussy67]

@@FlashDriveFilms Thanks I have a M104 E320, trying to do the tensioner and fan pulley.

[@carsxarexhard4504]

My tensioner bolt doesn’t tighten the belt at all and just slips thru the bracket 😢

[@FlashDriveFilms]

I'm assuming you mean the left-threaded tensioner nut on the bolt bolt at top behind the belt/pulley? Don't be embarrassed to use a washer under the nut. You might have to try a few to get the right diameter, but it ought to keep the nut from sliding through the bracket.

[@carsxarexhard4504]

@@FlashDriveFilms yeah the 13mm adjusting nut , it slips thru the bracket and won’t tighten anything . I’ll try the washer technique and see if that helps. Is there a email I can send pics too

[@FlashDriveFilms]

No need for an email, just post the pics to your "Community" page. I can see them there and there may be an even better fix I haven't thought of that your community might know.

[@BerlinettaFerrari]

I am having issues...Why cant I get the belt tight? Did I install the tensioner wrong? I put the flat side to the engine, yet......

[@FlashDriveFilms]

It's hard to see it from here, but make sure the large center bolt in the tensioner is loose enough for the assembly to pivot, too tight and you'll just snap the adjuster rod. Also the adjuster rod is reverse-threaded, so lefty-tighty/righty-loosey, that faked me out the very first time.And if the belt seems loose with the pointer in the middle, tighten the adjuster more and don't be afraid to move the pointer all the way to the line at the end of the scale, if that's what it takes.Make sure to tighten the center pivot bolt when the tension is set.

[@BerlinettaFerrari]

FlashDriveFilms Great ideas! Trying them now!! I couldn’t have done this without you, thank you.

[@BerlinettaFerrari]

FlashDriveFilms What is loose for the adjuster rod, when it is threaded “down” all the way with the rod showing out the top??

[@FlashDriveFilms]

@@BerlinettaFerrari Is there any chance at all that the ring part of the adjuster on the back of the tensioner is 180 degrees out? Like it is flipped and the rod is pulling it int he opposite direction?

[@wesleyp47]

Where can I get one of those shop manuals?

[@FlashDriveFilms]

Mine is actually a series of pdf files on two CD discs that came with one of the other Mercedes cars I had. (either the 260E or the 300TE, I forget which) It is actually for the W124-bodied cars, but the R129 shares so many parts with those that it often does translate. You might try someplace like eBay or Craigslist for manuals. Alden

[@wesleyp47]

Thank you...

[@RobertSmith-di5ll]

you sure will

[@BerlinettaFerrari]

What about the water pump while you were in there?

[@FlashDriveFilms]

The water pump is a whole different task. ("ordeal" actually springs to mind) The serpentine belt isn't like the timing belt on most modern cars, the cams are driven by a chain, so it is not as imperative to do the water pump at the same time as the belt. That said, I will eventually put up my video on the water pump replacement. The pump alone is $100-$200 depending on if it is rebuilt. Then there is a raft of small stuff you have to remove to get at it, and some blind bolts to remove essentially by feel. For me it was the second most difficult task I have done on a car, 2nd to replacing the clutch on a mid-engined car. yeah...it was...fun? Alden

[@BerlinettaFerrari]

I am halfway there, well maybe.....

[@BerlinettaFerrari]

FlashDriveFilms Did you happen to video a tutorial for the water pump?

[@FlashDriveFilms]

@@BerlinettaFerrari I did shoot a bunch of footage, but never edited it into a how-to segment. Maybe this is the nudge I need to consolidate that into something useful.

[@FlashDriveFilms]

@@BerlinettaFerrari Just a couple quick tips: use small bags and cards to keep track of all the fasteners you take off. There are a lot of them and they can get lost easily. I assume this is on the M104 6-cyliender motor, so there is an O-ring that mates the pump to the block, use a dab of grease to hold it in place when you get ready to attach the pump so that it doesn't slip, get crushed and leak. (ask me how I know) Take your time, some fasteners will make you apply force in awkward angles, fingers and hands will tire out, take breaks. I found myself using the 1/4 inch socket set a lot, just to get clearance for the tools. And Good luck! Alden

[@thelogician1934]

I don't think you should remove one pulley when the belt is still tensioned. The belt should be loosened first.

[@FlashDriveFilms]

David Xu Well definitely use your best judgement. In this case, I assure you there was essentially no tension left in the belt. The tensioner was cranked beyond the end of the scale and the belt still slipped under load. But, yes, if there is any tension left in the mechanism, best to slacken the adjuster rod nut first. I do NOT recommend just cutting the belt, even though I have seen that done (by others!) in the past. Alden

[@BerlinettaFerrari]

So/......why isn’t my belt long enough all of a sudden?

[@FlashDriveFilms]

Make sure you have routed the belt correctly over all the pulleys. There is a black plastic idler just above the alternator on the exhaust manifold side, I skipped that once and the belt length seemed all wrong. Double check the belt path diagram in your workshop manual, or sometimes the cardboard sleeve the belt arrives in has a diagram. Also if you have a 27mm socket, you might want to rotate the engine around a few times by hand using the crank pulley nut (only turn it clockwise!) just to be sure the belt is properly seated throughout before you crank the engine over at starter/running speed.

[@BerlinettaFerrari]

FlashDriveFilms So I got it back on, but wow it is close to the water pump pulley. Got it all done and.....still leaks from somewhere I cant see. It is dripping down the starter now, I give up!

[@FlashDriveFilms]

@@BerlinettaFerrari Exactly what happened to me. It was heartbreaking. So I walked away for a while, cleaned up my tools and organized them, took a break, got a coffee, did other stuff. Came back to it and using mirrors, feel, and a flashlight discovered where the leak was originating. For me it was that the O-ring had slipped and gotten crushed when I tightened the bolts down. So, I took it mostly apart again, easier this time because of the experience and did not have to remove everything the 2nd time. Took the O-ring to the hardware store and matched it up (50 cents! I bought two) Came back, did the grease trick carefully this time, really made sure it was seated right and tightened the bolts in a criss-cross-pattern. Filled it, hoping....... that was 24k miles ago with nary a weep. So yeah, I've been there, my brother. You will prevail.

[@Tigerfire75]

Get junkier cars and you can make more videos quicker or often.

[@FlashDriveFilms]

You nailed it. Just as "Calm Seas Never Trained Good Sailors", reliable cars never make good KZbin repair videos. (And you'd think my 50-year-old Fiat would be an endless wellspring of slapstick comedy breakdowns, it has actually been as reliable as a dry stone wall.)