Even with the lack of response to my previous post, I got everything ordered correctly and all back together, but I don't think the serpentine tensioner is right. I am following the procedures I have found online, but the tensioner just simply won't put enough tension on the belt. I have the correct belt (2415), and the routing is correct, according to the Pelican diagram, but the tensioner runs out of travel on the adjustment nut before the belt tightens fully. The rod actually unscrews from the nut. The water pump pulley can be turned by hand and the belt on the tensioner pulley is touching the back side of the water pump pulley. New tensioner?

What car and engine?

Sixto
98 E320s sedan and wagon
02 C320 wagon

It would indeed be good to know which car / engine but it sounds to me like the main bolt hasn't been loosened / the area around the pivot of the tensioner needs a good clean

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Sorry, 92 300TE. No, the whole tensioner was off in my hands, and is again, so the bolt was definitely loosened. Issue is that by the time the tensioner puts any tension on the belt, there are no threads left on the rod for the adjuster nut to push. You can put the belt on when the adjuster nut is almost at the end of the rod.

(I hope I'm understanding you correctly)



If it is the same as my M102 (also 1992) this sounds like you're not routing the belt the correct way around the pulleys (assuming the belt is the right part)


For my car:-


With the belt routed correctly - big lower bolt loose (may be a 19mm?) - small upper adjuster (may be 13mm nut?) all the way at the top - adjuster wheel position can be moved freely to tension or not


Winding the top nut in / tightening makes the tension - pulling the tensioner wheel to the right - once the correct tension is shown - looking at the plastic pointer + checking for belt tension on longest stretch then tighten the big lower bolt.

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Belt is routed correctly. Routed according to this diagram on the tensioner replacement page...

https://www.pelicanparts.com/techarticles/Mercedes-W124/30-ENGINE-Drive_Belt_Tensioner_Replacement/30-ENGINE-Drive_Belt_Tensioner_Replacement.htm

Already checked that and made sure I didn't have a long belt. I can swing the adjuster with the belt off. It can swing enough to actually HIT the water pump pulley with the adjuster backed way off. The problem is that when I do get the belt on and try to adjust, the idler pulley is not being pushed far enough away to properly tension the belt. I am thinking the rubber inside the tensioner is deformed. It's probably the original tensioner.

I don't think there's rubber in my tensioner (but it has been such a long time since I had it off!)


If you remove the tensioner (again) any rough running or out of round running should be noticeable (if rough running replace before the bearing dies on you)


One thing that might help is to have the system back fitted as it should be then turn the crank (clockwise from front) for a few revolutions to make sure the belt is settled / positioned on all pulleys correctly - then try and tension again





[General note about Pelican DIYs - they're not always right - checking the FSM is a much better option in my opinion - that's my first port of call anyway (for what it is worth)]

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

The rubber between the inner part of the tensioner that the rod pushes against and the outer part that has the idler and shock on it is what creates a springy tension, instead of a solid adjustment. It has a brand new idler pulley on it. I have replaced both idlers, the alternator, the fan bearing, and the water pump, including removing and re-installing the tensioner to get to the back water pump bolts. New radiator and other coolant hoses and a new expansion tank while the coolant was drained. Guess I'll order a new tensioner and see if that helps. Hell, I have already rebuilt or replaced nearly every mechanical part on this car except the engine itself and the rear end.

Problem solved, it was the old tensioner. When compared side by side with the new one, the flats on the old tensioner that are engaged by the adjuster rod were rotated about 30 degrees, preventing the adjuster having enough travel to tighten it up. I may pull the snap rings and disassemble the old one, just to see what exactly happened.

Good idea - always interested to see carnage (from a purely academic learning perspective of course)

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

The rubber in those tensioners fails with time and mileage. It was a dumb design. Although the principal is similar to a suspension bushing, in the case of the tensioner the rubber is, by design, always twisted. I find they don't last more than 150K.

This one was toast at less than 160,000. I have to say, I am not very impressed with the longevity of the Mercedes compared to my Volvos of the past 10 or 12 years. So far the engine is still hanging in there, but other than the rear end, smog pump, power steering pump and AC compressor, everything else has failed.

I recall reading a post years ago where a member loosened the alternator and tightened the serp. belt that way. I have never tried this.

8 yrs. ago I replaced the belt tensioner on my M103 engine with a German-made Febi. It's holding up OK, but I've never thought the belt tension was as taught as it could be. Domestic and Asian tensioners get the serp. belt TIGHT. Some have said I should have used an OEM tensioner. The car had relatively low mileage when I bought it 21 yrs. ago and still had the OEM tensioner. Belt tension at that time was no better than what I got out of the German-Febi.

I agree. The design of this tensioner was not handled well.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

M-B recognized the fault of the design and modified the tensioner to a spring loaded device, but that was in the well into the production of the M104 (which had the same rubber bush design on the early engines). I think some folks have upgraded to the later tensioner. It may have been quite involved though.