<img height="1" width="1" style="display:none" src="https://www.facebook.com/tr?id=918799731584652&amp;ev=PageView&amp;noscript=1">
FCP Euro Service Kits

FCP Euro Kits

This article will cover how to avoid an alternator bracket and tensioner failure and how to
replace the alternator upper bracket and alternator on a Mercedes W123 240D.

Just what does goes boom in the night? An alternator bracket that snaps apart! While it might
not sound very exciting, any loud sound of “explosion” from under the hood is or should be
alarming.

On my way home last week in my 1981 Mercedes 240D, I was quite taken aback by an
exceptional thud that sent a mild shock wave through the car. While in the middle lane of a 6
lane highway doing 70mph (yes, that was the legal speed limit), I thought it wise to at least drift
to the far right lane in case the shoulder was needed. Thankfully it was not, and I proceed to get
home with no “known" problem.

After supposedly inspecting every square inch of the engine and undercarriage the next day,
along with a friend who is a true mechanic, we discovered “nothing” missing, and I was to chalk
it up to a random object from the road that entered the engine bay, and was later ejected.

As the photo shows, this “random” object did some damage and passed through a space 10 in.
by 7 in. and then exited. I did indeed hear an object fly out from under the car while driving but
again chalked it up to road debris. It is not uncommon in the Scottsdale, AZ metro area to
encounter a large number of stones and rocks on the highway, but this certainly would have
been a biggie.


Photo (L) shows air intake hose split and photo (R) shows 2.75 in. long damage to hood from
underside out

The following day while driving the car since my mind was not settled that something wasn’t
wrong, I noticed an intermittent metallic vibration and scraping sound. Once again I looked at
everything and again, to no avail.

Longer story short, my latest trip led me to the problem. Indeed it was not a random object off
the road that caused the damage and the noise, but the upper alternator tensioning bracket had
snapped—sheered off and launched through the engine bay. Along with the chunk of bracket
were the tensioning bolt and nut, and all being violently propelled by striking the fan blades of
the alternator.

It wasn’t until my last trip that half of the alternator fan blade gave way and once again shot
through the engine bay, once again, while I was in the middle of a drive. This time I knew for
sure no debris was to blame and so pulling over and raising the hood for the tenth time, I finally
saw the wreckage at the alternator.


Note the top portion of the fan is missing in the photo above.


Photo above shows where the tensioner and support bracket snapped off.

Thankfully the two water lines just inches from the site were unaffected, the belt which also
serves the water pump remained intact and tight and so being very close to home I decide to
proceed, carefully watching the temp gauge and ultimately arriving without further
complications.

While this story was not planned, it hopefully serves as a benefit to some degree and so we
launch now directly into just how to properly change out an alternator and bracket on a 1981
Mercedes 240D and similar models.

How to Avoid Early Model Mercedes 240D Series Alternator Tensioner Failure

First, inspect your alternator tensioner carefully. In no way should the 90º tensioner bolt be bent.
The bracket should also look only like the photo shows.


Photo above shows exactly how each component should be aligned when installed.

Should your set up look something like this, (photo below) you are in for a likely problem.

Mercedes designed this bracket with a “safety catch” should the tensioner come loose as I now
know mine did. Should you discover your bracket or tensioner is not exactly as is shown in the
prior photo, you should replace or repair all necessary parts.

How to Replace a Mercedes 240D Alternator, Bracket and Tensioner Bolt

Should you need to replace the alternator, there is a specific sequence that must be adhered to
to avoid damage and unnecessary expenses.

Remanufactured Bosch alternators are still plentiful as are new 90º tensioner bolts and new
upper brackets. The tensioner bolts are not cheep however and neither are the brackets.

The first step in removing the alternator is to disconnect your battery, remove the air cleaner
housing and detach the air hose, unplug alternator, and loosen the 3 major bolts retaining the
alternator. NOTE: What might appear to be a nut on the upper most section of the mounting
bracket is NOT as the photo below indicates. DO NOT attempt to loosen the alternator
tensioning “adjusting nut” until the alternator is loose. Should you attempt to loosen the
tensioner “nut” first, you will likely ruin the 90º tensioner bolt and/or tensioner nut.

Once the alternator is sufficiently loose, remove the tensioner “nut” and proceed to remove the
two mounting bolts on the lower portion of the alternator. I found it easiest to simply remove the
bolts and lower the alternator to the ground through the engine bay.

Now disassemble the bracket from the alternator and replace the tensioner bolt, nuts and
washers if needed. You will also see that there is a metal spacer that must be returned when
reassembling.

Install the new bracket, spacer, 90º tensioner bolt to the top of your new alternator. DO NOT
tighten and do not fit the tensioner bolt adjusting nut at this point.


Photo shows under side of bracket with 90º bolt and spacer that needs to be fitted to alternator.

Again, I found it easiest to simply slide the new alternator with bracket under the car and lift it
into position from above.

With the alternator in place, slip the lower most retaining bolt in to position and begin to thread
but do not tighten. Now insert the up retaining bolt and loosely thread.

Now hold alternator close to engine block and slip belt over pulley.

The two retaining bolts may now be tightened up just enough to just keep the alternator able to
pivot but not so loose that it will take you much to lock it in to position.

Now attach the 90º tensioner bolt adjusting nut. It will require a 13mm deep socket to adjust. As
the adjusting nut is turned clockwise, it will tighten the belt. DO NOT OVER TIGHTEN. When
the belt is sufficiently tight (set tension according to Mercedes spec), now it is time to tighten all
three bolts well. Remember, if you are driving a diesel you are subjecting all bolts to an extreme
amount of vibration. If theses and all nuts are not torqued to spec, you are asking for further
problems as things rattle loose.

At this point, plug in your alternator, reconnect your battery and start the car. Always listen for
odd noises or unusual movement of any component.


Photo shows what completed installation should look like on early W123 Mercedes Diesel

Now you have a properly installed alternator and bracket for many years of excellent service.
Also be sure to return your old alternator for your core charge refund.

HELPFUL TOOLS:

  • 13mm Deep socket
  • 17mm Wrench for retaining bolts.
  • Belt tensioner “scale” tool

HELPFUL PARTS:

Shop Mercedes Benz Parts Online Catalog Lifetime Warranty


author image
Written by :
West Kenyon

West Kenyon is a Multi-Media Design Consultant and automotive enthusiast. Cars in his collection include a 1956 Austin FX3-D, better known as a London Taxi, a 1965 Rolls-Royce Phantom V, and a 1975 450SL and 1981 240D by none other than Mercedes-Benz. West said, “owning one or a number of vintage cars is often an expensive proposition", which is why he does the majority of mechanical and interior work/restoration himself. West added, “not only does it save a significant amount of money to do the work yourself, it also provides full opportunity to know the job is being done with excellence and allows for an unprecedented appreciation and knowledge of the car.” For FCPEuro, West will focus on providing in-depth information on both the Mercedes-Benz R107, 450SL, D-Jetronic version and the W123, 240D. Glücklich Autofahren! (Happy Motoring)


More Related Articles