Hi Guys,

I have to change the alternator on my W124 MY'91 300TE-24V. I have the fan and shroud off, I have loosened the tensioner adjuster but I cannot see where the tensioner is locked. Can I please have a hint as to what to undo please?

Also any handy hints as to how to remove the alternator please?

Thanks chaps!

Daz

Daz, it's important to loosen the 19mm center bolt on the tensioner 1/2 turn or so BEFORE turning the adjuster CCW to loosen the drivebelt. The horizontal center bolt locks the tensioner once the drivebelt is properly tensioned. If you failed to do that, chances are you ruined the tensioner ... order a new one and chalk it up to inexperience. Instead of replacing the alternator you may only need to replace the voltage regulator which bolts to the back of the alternator. When the voltage regulator brushes wear down too far the alternator fails to recharge the battery properly.

__________________
Fred Hoelzle

Fred, I always like to learn new stuff (but then I forget it, Ha), but I just can't grasp how the tensioner elastic could be harmed by backing off the adjuster rod bolt first. (If this tensioner is similar to my '91).
The adjuster rod is tied to the inner shaft, as is the big center bolt.
So, when you back off the adjuster rod, nothing happens, as the large center bolt holds the inner shaft in torsion against the rubber doughnut.
Then, as you loosen the 19mm center bolt, the center shaft just relaxes the torsion on the rubber doughnut, and all is good.
Now, in re-installation, if you tightened the center bolt, and then tried to tighten the adjuster rod, you would strip the threads, as the adjuster could not turn the center shaft. Is this logical?

tks,
DG

Thanks chaps!

So, assuming I haven't buggered the tensioner, I will have a look later to try to get the belt off. I was lining up to undo an Allen headed bolt earlier so I don't know which one that was! Does anyone have a pic of the bolt so I know I am aiming in the right direction please?

Cheers

Okay, I have located the bolt, it is the allen headed one, the belt is now loose so I can progress. I just need to wait for daylight so I can remove the alternator.

Thanks again

Daz

Daz, I don't have a photo, but you might do a search here.
The bolt is a 19mm std 6-point head, and it sticks right out the front of the tensioner. Can you see the tensioner pointer and adjustment marks? The bolt is directly under the pointer. Kinda hard to see under there.

DG

Hi DG, strange but it is definitely an Allen headed bolt on ours, hey ho, it is loose now. As you say it is hard to see in there, and mine is covered in a thick layer of 184,000 miles worth of dirt too!

Now to tackle the alternator, I guess that is straight forward?

Cheers

Oh well, sorry for wrong advice, but you found it.
While you have the belt loose, you might push/pull on the tensioner pulley and be sure it moves smoothly and has a lot of resistance in both directions to movement by hand (may need to tighten the allen bolt again). All it is, is a big rubber doughnut that flexes to maintain tension. But when they fail, you walk home. Actually, if you have that many miles, you maybe should just replace it on principal - the only time in 18 years that my MB failed me was when the tensioner failed last summer 90 miles from home - no warning.

Also, the guys are right about the voltage regulator. They seem to be the most common fail mode for the alternator. They cost less than $30, and can be replaced in about one minute with two screws. They include new brushes.
So, if your brushes look badly worn, but the alternator bearings and commutator still seem okay, you might just pop a new regulator in and see.

DG

Thanks for your advice DG and no worries about the wrong info, it stil led me in the right direction

I have done lots of DIY on cars in the past but not much on this E300 and MB do things differently to the likes of Saab and MG.

I now have the belt off, it was well and truly done in, with missing chunks of rib . I did buy the replacement a while back but have never got round to changing it, if I have realised it is not a hard job I would have done it agens ago. I have checked the tensioner and it appears to be fine, but I will follow your advice and procure a replacement. I have loosened the alternator now so as long as the replacement arrives tomorrow then I am in business.

As far as replacing the brushes and regulator.... I think I may well keep this unit as a spare and get replacements for it in due course.

In the mean time..... thanks for your advice and take care.

Daz

If the commutator is in good condition then brush replacement is possiblity. If commutator is grooved (from brush wear) and lots of mileage, then complete alternator replacement warranted. Regards, Abe G

Well I have just finished the 'fight' with the car to get the alternator out. I have a steel undershield fitted which I have to drop so I could remove the alternator from underneath. It was a pig of a job. Still, in recompense the pulley came off with no real hassle. I received the replacement alternator yesterday but the connections differ, I have 2 screw terminals but the replacement has a spade type connector. I will sort a replacement tomorrow now. I have found the belt sorely needed replacing so at least my efforts in removing the old one have not gone to waste.

I looked at the brushes and they are only protruding by about 3mm so I guess that is the problem.

We have had the car for 5 years now and this is the first real work I have had to do on it (apart from servicing) so I am not complaining at all.

The fan shroud etc. came off really easily but thanks for the words of advice Hristo.

Thanks again all of you and with a bit of luck with the weather and suppliers I will be able to leave a post tomorrow telling of my success!

Daz

Well I managed to well and truly cock that up! I am now in the market for a new tensioner as I managed to successfully strip the thread on the adjuster!!

Bum!

The adjuster is available separately.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Music to my ears, off to the local MB dealer I am then!

Thanks Steve

Daz, in a earlier post I mentioned that the center bolt on the drivebelt tensioner was 19mm (it's actually a 10mm allen head bolt) and it needs to be loosened before turning the adjuster.

__________________
Fred Hoelzle