Both brand new? If so I rekon you bought cheapies? I always buy Gates V-belts, which I learned are well worth the extra $. Not just because they last longer, but they grip better too. I suggest you stay away from others, especially store brands. I would not make it tighter in order to compensate, like to keep it from slipping because tension is hard on the alternators front bearing and the water pump which I assume is on them too. Belts are cheap and easy to replace, alts and pumps not so much. The main advantage to two belts (two matching ones that is) is you can run less tension so it's less stress on those bearings.
The mfg may have decided the tension of one belt was too much on the water pump, maybe due to the design of the pump or maybe even the mount for your alt is weaker, who knows. I think more likely reason is your alt takes more torque than normal to drive it. Diesels run at lower rpm so I imagine the alt is wound to match which means more torque. My truck for example has a generic gas powered car alt, which is wound for typical car rpms. I rev the engine much higher than normal which I bet took the poor alt past 20k, so I finally put a much smaller crank pulley on it to slow everything down. Point being is at idle the alt is not putting out enough juice because it's not wound for low rpm. If wound for lower rpm it would take more torque to make the same power. I imagine to add to that your car draws a lot of power too, so for one or a several reasons the mfg said it needs two belts. Odds are two belts are overkill which means you can run lower tension to spare the bearings.
On cars with V belts I've always ran them as loose as possible for that reason. So much so that I have to adjust much more often, but again, easier than buying/replacing a pump or alt. I also replace the belts more often because they lose grip with time and I'd rather not tighten to compensate, which is how I discovered cheap belts suck. They stretched more, and sooner, and usually have less surface area for traction so you had to run them tighter. The smaller pulley in my truck is aluminum and despite my low tension it shows wear after low miles. Steel pulleys can wear too and the tighter the belt is the more wear you get, and the more wear you get the less the belt grips it and it wears the belt faster. Wear also makes the belt sit deeper in the pulley so if one groove in the pulley set were worn more than the other then you could end up with the problem you have. Say for example the last owner was too cheap to buy both belts so he had to tighten it to keep it from slipping which wore that groove deeper. So another point for looser belts. All these problems are basically solved with serpentine belts which have way more surface area for traction yet less friction, plus the automatic tensioner is awesome.
TMI I'm sure, but maybe it'll help you or someone.
Lost it? Like you left it somewhere on your daily errands or it fell out of your pocket? Good thing you didn't lose the rest of the motor...
I'd say a junkyard if you really want a factory replacement, which I have never done because I hate oem fans. I typically replace with an electric fan, or if that doesn't cool enough I get an aftermarket like a Flexalite, eg; jegs.com/i/Flex-A-Lite/400/2017/10002/-1
You just have to be sure the diameter is correct to fit in your shroud as best as possible because air tries to get in the gap from behind rather than pull it thru the radiator which obviously decreases air flow thru the rad. If too close the it may hit the shroud when the engine moves, which is a common way to tell if a motor mount is broke. So measure the shroud opening and get the closest size smaller. It's likely meant for a metric fan so this may or may not work to your advantage. For example if it's 18" then you have to get a 17" fan. If it's say 18.5" then an 18" fan is probably ideal but no doubt too close and will hit if the engine moves too much. It all depends on how much it moves and where the fan will sit in the shroud. So lets say your hole is 18.5" so you buy the 18" fan, which leaves 1/4" all round. Then you floor it and the eng rises 1/2" and, well, ever hear a wood chipper eating a big branch? Plus odds are your water pump won't be perfectly centered in the hole anyway so that fan may already hit.
Also be sure the rotation is correct because there are a few reverse rotation fans. The one in the Jegs pix is normal rotation which when viewed from the front of the car (and the Jegs pix) turns clockwise.
You may also have to buy a fan spacer, maybe, depending on how many parts you lost. You want the fan to sit at least half way into the edge of the shroud, again because air will try to get around it otherwise. So looking down at the installed fan the blades basically take up about 2" of thickness front to back, so you want it spaced so it's half way to all the way in but no more. Like the diameter it's not the end of the world if you can't get it perfect, it just costs you cooling and the further from ideal the more it costs you, probably squared. So say 1" too far back will cost you 20% cooling flow. 2" too far back probably 80%. 3" back it's basically useless.
My guess is oem is sounding better and better, so Ebay is another source for old oem parts, as is Craigslist and I hear Facebook has a sellers section. What arrives may not be what you want since you don't have the old one to compare, so in that case junkyards are much better because they will greatly increase the odds it'll fit.
been looking for a junk yard no luck yet also e bay its not so easy to find stuff on eBay i have started looking at any selling old Mercedes parts and then go to their store you get to see a lot more stuff
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