I recently tried (emphasize "tried") having my fan clutch replaced. My tech initially installed a new ACM brand clutch I purchased online. After installation, apparently the clutch remained locked up regardless of temperature because I could hear the fan roaring all the time. He ordered a Fichtel-Sachs clutch (costing almost 2x as much). He installed that and it also remained locked up even though the radiator and area around the clutch was cool (confirmed with infrared temp sensor and when engine is cold). We tested this many times. Unbelievably, he ordered another Fichtel-Sachs and upon installation had the same situation. He couldn't believe this and thought maybe there was something weird going on in my car. We installed a clutch from another vehicle that we knew worked and it worked fine on my car, with lockup and disengagement at appropriate temps. Even installed the new clutch on the other car and.....it remained locked up. Anyone have experience with this? Fan clutches are this flaky? So first, is my mechanic and I missing something? Secondly, I'm wondering if I should get a replacement ACM clutch as the Fichtel-Sachs apparently is not having a great track record, either. Is there a qualitative and quantitative difference between the two? One quantitative difference, of course, is that the ACM is almost half the price. My tech is willing to not charge me additional labor for the installation of the next clutch. Obviously, he orders new clutches, he'll keep changing without charging labor. I order, it doesn't work, I'm looking at additional labor charges. With this luck, I'm not going to Vegas anytime soon. Thanks.

Not sure where you're ordering from. If from a dealer, the dealer parts dept. employees have technical people at Mercedes (part distribution network) they can get involved in researching this, even to the point where they can have someone in the warehouse check the fan clutches they have on the shelf. Might not be a bad idea to have someone at a dealership (sevice dept) check for DTBs about this as well. Also the parts dept gets DTBs of their own concerning replacement parts problems.
Gilly

__________________
Click here to see the items I have up for auction at EBay

Click here to see a photo album of my '62 Sprite Project Moneypit (Now Sold)

The problems you are experiencing may be due to how the viscous couplings have been stored.
They should only be stored in an upright position,it's acceptable to lay the coupling flat for a short period during installation but never with front face down.
Don't ask me why this is,probably to do with the fluid distrbution in the coupling and the lockup situation you experienced may have resolved with more extended use,that is if the fan blades didn't part company first.
Geo.

Thanks for the responses. I read about the upright position in a link from one of the posts here, as well. I mentioned it to my mechanic and he said that may be true, but in real life, in his 25 years of experience (most of it at the MB of Beverly Hills dealership), they've never had that be an issue. In addition, I checked out the box the Fichtel-Sachs clutch came in, and it shows that the only requirements be that the temp sensitive metal clutches be on top. Horizontal positioning of the clutch being apparently o.k. and vertical position not requested.

I asked my mechanic about break in period as well and he said no. Initial lockup (5 secs or so) and then it should free up and work right away.

I'll check with the dealer about DTB (what does that stand for?, I can surmise it refers to some trouble report) for this part. I got my part online arrives overnight even though it's UPS Ground because it's local (from the UPS tracking info), my mechanic gets it from some distributor locally, arrives in about 2 hours.