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So my well-maintained clean 1999 C280 Sport w/ 166k miles started making noises under the hood a few thousand miles ago. First part recommended changing was the expensive tensioner pulley (tho installed for $175 was a deal I'm told). That took care of some "whining" and grinding type noises for awhile. Then 1k miles later a similar sound came back, this time worse with a weird high-pitched whine sound from the cab that sounds like it might be an electric car. From outside it sounds like grinding. Mechanic now said its a Idler Pulley- but thankfully it's only $60 with a $50 install. However I failed to replace it right away, and tonight while I was driving all of the sudden I hear a loud smack against the undercarrige as if I just ran over something metal that flipped up and banged around. Kinda scared me but the car continued to drive fine for a few miles but then suddenly the Battery light came on and the power steering went out simultaneously. I stopped and checked under the hood- one of the drive belts was just hanging there, some black grease was splattered around and a noticeable pin that should have a pulley on it near the top was just sticking out. I figure thats the idler pulley now a gonner.
Funny thing is the car kept driving fine despite the lack of power steering. I have noticed the thermometer is running a little higher but well out of the danger zone. Electrical seems fine. I'm guessing that belt runs the water pump, power steering pump, and alternator.
Am I right about all this? Kudos to MBZ for still being able to run after this happened!!
 

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Yes.

Stop driving it or you are likely to be stranded or worse.

Had you posted prior, we could have told you how to DIY for $20 and 15 minutes, and that's if you are slow. ;)

Have it towed in, it will have to be inspected to see if there is any collateral damage. Good luck.
 

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I will have it looked at- thanks. Hoping I'm good and just need that Idler Pulley replaced. Can you still reply with those DIY instructions? Thanks for any and all help.
 

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If you go over to the garage/knowledge base at benzobsession.com you will find a DIY by Gregs210. It's for a W210 but it is the same engine (larger displacement but still the M112), part and process.

I would probably fit a new belt as well. Look VERY carefully for anything that may have been damaged by the belt flapping around. The part is cheap online, call NAPA to see if they have one, it would still be a good quality and cheaper than the dealer.

Check your manual for the proper routing of the belt.

You'll need a 1/2" drive ratchet and (I think) a 17mm socket to compress the tensioner. And an E12 (IIRC) torx socket or wrench to pull the bolt. You can buy a basic set anywhere for about $20.

Good luck.
 
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