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Old 01-18-2011, 06:28 PM
Benz Mondi
Join Date: Nov 2006
Location: Sacramento area
Posts: 308
Belt Tensioner Replacement- Simplified w/ tips and tricks 300E

I did this twice in succession for my 1990 and 1992 300E's...

Belt Tensioner Replacement- Simplified w/ tips and tricks (1992 300E)

Items needed: 2 hours (it should only take 45 mins to 1 hr), new tensioner , new adjustment rod and shock (might as well), blue Locktite, spray lube, breaker bar, extensions, mm sockets (19, 13, 12, 11), two 13mm box wrenches, 13mm open end wrench, allen wrenches, thick gloves to save your knuckles when the tools slip, a large tray below the engine to catch the crud that will fall, as well as, the falling parts and tools, good light, , marker pen, two rubber bands, magnetic parts finder wand, and an extra pair of strong hands for about 10 mins.
  • Remove the engine back splash tray below and the radiator fan cowling (no need to remove the fan itself) from engine compartment
  • Make a note of the routing of the serpentine belt (make a drawing)
  • Move the belt away from the right side of the engine by first loosening the tension on the serpentine belt with the turning the adjustment knob (12mm socket, spray lube the threaded shaft first) so that it moves down the shaft until the top of the knob is even with the rod in the middle of the knob (OEM rods are actually counter clockwise, while aftermarket rods are the conventional “righty tighty, lefty loosy”), which will release the tension. I started by removing the belt from the water pump pulley, then the power steering pulley, and then the A/C compressor.
  • Remove power steering pulley by using two 13mm box wrenches opposing one another. Hold the one on the right down while you push down (to loosen) on the one on the left. Repeat for the second bolt. For the third you’ll do the same (actually re-tighten the one on the right while breaking loose the one on the left). That’s OK because once the bolts have been broken loose, you can actually hold the pulley with your right hand while you re-loosen the last bolt. Put a piece of masking tape on the pulley to mark it versus the water pump pulley (they’re actually the same part just facing different directions). Set the pulley and bolts aside.
  • Remove the water pump pulley in the same manner by first locating a bolt and moving it to the 12 o’clock position. You will see just a few threads sticking out of the hub at the rear. Now take a marker pen and draw a line toward you from the bolt threads across the hub edge and the rear of the pulley. This will help you quickly align the pulley on the hub when you are re-attaching the pulley. Remove three 13mm bolts, but make sure that you have a good fit on the bolt head with the socket. The concave shape of the pulley makes it not possible to see the bolt head so there’s a danger of stripping the bolt head. Get an extra pair of hands so that both persons can be pushing down on the socket head while pushing down on the wrenches. You’ll thank me later. For the last bolt, move the bolt to the 8’oclock position and insert an allen wrench, that is as thick as possible, through the top hole. The allen will now wedge itself against the body of the water pump and will hold the pulley so that you can push down on the last bolt to break it loose. If the last bolt is so tight that the allen starts to bend, you’ll have to re-install a second bolt, and screw it in so that you can use the opposing wrenches technique to break loose the last bolt. The other bolt will come out easily with the allen wrench inserted. Set the pulley and bolts aside, separate from the power steering set.
  • Remove the tensioner shock by first locating the lower 11mm bolt and removing it. Then remove the 13mm bolt at the top. Set the whole set aside.
  • Remove the belt tensioner by first removing the two 13mm bolts at the right side of the bracket facing you. It’s in the shape of a “Y” laying on it’s side with the two legs at the right, next to the power steering pump. Start with the bottom bolt. It goes all the way through the housing and is held in place by a nut at the back. Follow the alignment of the bolt rearwards to see the nut. As you loosen the bolt, the 13mm nut will usually just slide downwards and stay on the little shelf that it’s on. I used a magnet on a wand held in place along the shaft of a 13mm open ended wrench by two small rubber bands to make sure that I didn’t lose the nut. Now remove the upper bolt (it’s shorter than the lower bolt) and just screws into the housing. Now using a 19mm socket, remove the tensioner bolt. If it’s been on there a while, this is likely going to take at least an 18 inch breaker bar to break it loose. Remove and set aside the bolt and washer. Now with some effort move the bracket upwards, so that it’s just even with the top of the tensioner housing that the bolt just came from. Make sure that the left side of that bracket doesn’t pinch anything. You just want to slide the tensioner down and out from behind the bracket. Once I got the bracket to budge enough, I slid the top bolt back into the top hole of the bracket so that I could have something easy to hold while I slid it up. You should now be able to slide the old tensioner and adjustment rod down and out of the hole they’re in. When you have them in your hands, note how the adjustment rod is mated to the rear “eye” of the tensioner. You want to copy the exact alignment. If the adjustment rod becomes separated from the tensioner, no worries. I’ll give you the alignment below. Also, note the yellow indicator arrow that is on a ring fasten to the front of the tensioner. Carefully remove that ring. You’ll be re-using it on the new tensioner.
  • Install the new tensioner and adjustment rod, by first making sure that the adjustment knob is wound down so that the top of the knob is even with the top of the threaded rod. Now place the tensioner on a flat surface with the wheel the 9 o’clock position. Attach the tensioner rod at the rear so that the bottom of the rod is closest to you (versus flipped over so that where the rod attaches to the ring is toward the engine and away from you) and is also at the 9 o’clock position. This is the proper alignment of the adjustment rod to the tensioner. When you install it, the wheel will actually be closer to the 6 o’clock position. With this in mind, place the whole set up in place by first working the adjustment rod into the hole in the bracket that it fits into and then the tensioner. You’ll need to slightly twist the whole set up. I twisted the both parts in there at the same time. With both in place now install the yellow plastic indicator arrow collar from the old tensioner so that the arrow point is at the right-most end of the scale (thin part of the sideways triangle). The arrow moves from right to left as you increase the tension on the tensioner. Now take the shorter of the two 13mm bolts and place some anti-cease paste on the threads. Now install the two bolts that hold the bracket in place by sliding the bracket down until the eyes are aligned with the holes in the housing. You’ll likely have to push the bracket away from you to get the top bolt (the shorter one) to slide in place. For the longer bottom bolt, place the nut that it threads into at the rear into the 13mm open-ended wrench with the magnetic wand placed as a back stop at the jaw by the rubber bands. In this way all you have to do is turn the bolt while leaning the nut against the end of the bolt and it will thread itself enough to get it started. Make sure to finger tighten any bolt before using a socket to avoid stripping the threads. Do not screw them all the way in yet. First slide the tensioner slightly upwards so that it’s lined up with the eye in the bracket that the big bolt slides into. Now take the 19mm bolt and washer, put some anti-cease paste on the threads and screw it into place. If the bolt doesn’t line up, slightly lift up the tensioner with you left hand while feeling for the hole with the bolt in your right hand. It shouldn’t take much movement. Make sure that you keep the yellow indicator arrow properly aligned before you bolt the tensioner down. I used the same 18 inch breaker bar to do the final “snug fit” tightening.
  • Install the new shock by first screwing in the 11mm bolt at the bottom. You might have to persuade the top eye of the shock to line up with the bracket eye. I levered a screw driver until the shock eye was at the correct level.
  • Re-install the water pump (line up the marks) and power steering pulleys (insert an allen in one of the holes to help line up the PS pulley holes) and bolts (use a little BLUE-ONLY locktite) on the threads.
  • Re-thread the serpentine belt. I followed the reverse order that I used to remove the belt. For the last part of the belt to slip over the power steering pulley, I worked a long screw driver between the tensioner pulley and the A/C pulley and moved the tensioner pulley slightly to the left to get enough slack. Once you have made sure that the belt is on properly (look from underneath the engine) re-tension the belt by turning the adjustment knob so that it moves up the shaft. It becomes harder to turn the knob and the yellow arrow point moves to the left. Keep turning the knob until the arrow is at least half way to the left. (After a couple of days driving, try to move the arrow all the way to left, thick line mark, but make sure that you are not deforming the bracket that knob threads through.)
  • Re-install the fan cowling and you’re done and with a lot more green in your pocket.

03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio

Last edited by Benz Mondi; 01-19-2011 at 04:31 PM. Reason: spelling
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Old 01-19-2011, 12:17 AM
sptt's Avatar
Registered User
Join Date: Sep 2009
Location: Orange County, California
Posts: 691
Very nice job! Thank you.
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Old 01-19-2011, 03:51 PM
Mike Murrell's Avatar
Registered User
Join Date: Feb 2000
Posts: 2,574
Yes indeed - a very nice job.

I replaced one of these in December on my M103 motor.

Two things that I would like to add.

- Field of vision is greatly improved with the 2 large radiator hoses removed. This of course requires you to drain the radiator first and reconnect and refill on reassembly.

- MB documentation(forget the procedure number - available here in PDF format) mentions loosening a bunch of hard-to-get-to power steering bolts so that the PS unit may be swiveleing away. In the end I found this to be totally unnecessary. What improved access to the tensioner/rod/shock assembly was what I mentioned above -having the radiator hoses out of the way.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 01-19-2011, 04:24 PM
Benz Mondi
Join Date: Nov 2006
Location: Sacramento area
Posts: 308
Your Welcome!

The ideal timing for this proceedure would be when the coolant needs refreshing, because then I would not only drain the coolant, but take the time to remove the radiator for some additional work space and rad inspection.

My next post will be on changing the aux fans. I also did the same job on both my cars... busy weekend! My son was taking the 90E back to San Diego State so I wanted to make sure that all the work was done already. While at it, I changed the fluids and the belt.

03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio
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Old 01-17-2013, 03:09 PM
wielder of thor's hammer
Join Date: Jul 2003
Location: Long Island, NY
Posts: 321
I recently tackled this job and unfortunately snapped the threaded tensioner adjustment rod while tightening it. I was curious if this mite have happened because I needed to slightly loosen the main 19 mm bolt for the tensioner, or if this bolt should remain tight while the rod is being adjusted.


p.s. replacement rod looked terribly inferior to the original, so I'm thinking it mite just be a result of cheap replacement parts.
headrivenoise... listen.
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Old 01-18-2013, 04:04 AM
Registered User
Join Date: Jan 2008
Location: Melbourne Australia
Posts: 834
You need to loosen the 19mm bolt first.

When loosening the bolts on the water and steering pump pulleys, leave the serpentine belt on. It helps to give you a non slip grip on the pulley when slackening the bolts - use a pear of non slip gloves to grip the pulley and belt at the same time.

Depending on the market and AC unit installed you will need to remove the fan and Y bracket to get at the pump bolts. Some pumps are held on with allen headed bolts not regular bolts that a socket can be used on.

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