The diameter of the original power steering pump pulley wheel on my 2004 Mercedes ML 350 is approximately 110 mm. The diameter of many replacements is 130 mm. Will the original serpentine belt still fit?
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If my calculations are correct and my SWAG (silly wild @$$ guess) of how much your ML350's belt contacts the power steering pump pulley, you will need a belt which is ~15.75mm longer (or 1.5cm) than what you have now. Here is my reasoning:
This picture should represent your serpentine belt configuration:
Pulley #4 is the power steering pump. You can see the belt covers about 1/4 of the circumference. The original pulley, being 110mm in diameter has a circumference of ~345.5mm, while the newer one with a diameter of 130mm would have a circumference of ~408.4mm. The difference is ~62.9mm. 1/4 of that (distance the belt actually rides on the pulley) is ~15.725mm.
The reason why you'll need to get a longer belt in the first place is that the belt tensioner is designed to operate in a given arc. If your belt is too short (as would be your original), the tensioner will be too tight and will wear out sooner. This also puts more stress on all other pulley components which the belt rides on. If the belt is too long, the tensioner will not provide enough tension and will allow the belt to slip across the accessory pulleys. Does this absolutely mean your current belt will not work? By all means no: it probably would work. In your case, you will probably be wearing out expensive components much faster than would be normal.
I'm not sure on your MB, but most tensioner pulleys have an indicator on them which shows belt health. It might have a pointer which, when the belt is appropriately installed, show you if it is in it's proper range. If it does not have one of these (which I'm not seeing in the pictures I've just looked at), you can scribe your own line across the two portions of the tensioner pulley (base where it attaches to the engine and arm which has the pulley on it). When you install the new belt, the two lines should be very close to lining up, if not completely, when everything is put back together. If you are doing this work yourself, it appears you'll need a retaining pin to hold the tensioner pulley in place, as seen in this picture from RockAuto.com: