ECU is pretty common issue since it controls the fan my friends CLS needed a new ECU after his dealer replaced the fan lol and it didn't fix the issue. I think mine is going out too it seems to always kick on even when its not hot at full speed kind of annoying with the windows down.

 

that's great news. Is it possible to replace only the control module or do you have to buy the entire fan assembly?

 

What exactly in the ECU goes bad that the car still start and runs but only the fan is effected? Is the fan controlled by a stand alone chip or something?

 

This is exactly what I am wondering. Something just doesn't add up. It seems with this theory the ECU isn't turning and fan on but EVERYTHING else functions as it should?? There has to be another solution. Maybe if we had the ***** one day to actually take one apart and get a schematic of how its constructed...

 

Not sure what the issue was inside but luckily the ecu was covered under extended emissions warranty so they did it for free.

 

Jake you should go get a good aftermarket warranty that covers the ECU and then get it replaced lol

 

Hi I have the same issue, been told today I need a new fan as the module is broken? Did you have to have it reprogrammed when you fitted yourself? Also how easy was it to fit??

 

The fan swap was pretty easy only took about 30 mins. The hardest part is just wiggling the fan out of the car but once you do that its all cake from there. It is just plug and play no reprogramming but I'm still having a weird electrical issue with mine. The a/c inside the cabin has to be turned on for the fan to actually work. The old fan would only spin at a low speed when I turned the a/c on but now the new one will change speeds to what is needed to keep the car at normal operating temp. Its winter here in MD so its not really an issue but even then when summer comes I always drive with the a/c on anyways. Not sure if I want to pursue this issue anymore or just press the a/c on whenever I need it. Good luck to you though install is easy so you should be good. Post up and let me know how it goes.

 

Thanks for the quick reply, my garage told me today it's a 2 hour job and it's a pig of a job? I'm struggling to source the part in the UK !

 

Its not a 2 hour job and definitely not a pig of a job. I'm assuming that means maybe a PITA job...


But hey they gotta make a profit somewhere right. If you have an hour of free time and a friend you could do it yourself and save the $$$.

 

Hey Guys,
I have the same issue. My C55 radiator cooling fan does turn on when the car is over 80c or even 90c, and does not come on when the a/c is turned on. I did a lot of research on this forum and elsewhere. I replaced the coolant temperature sensor; that didn't fix the problem. Checked the fuse (#51), it was fine.

I then removed the fan assembly and replaced the fan motor and the the fan control unit, cleaning the electrical connections between the engine harness and the the fan assembly, the fan control unit and the fan motor. However, this did not solve the problem. The fan still does not come on when it should; i.e. when the a/c is on or the engine is over 90c.

Any help diagnosing this would really be appreciated. I didn't check the relays; I'd like to do so, but I do not know how. From this forum someone suggested swapping the same color relays systematically (taking notes) to see if any one of them is defective. Any help here would also be appreciated.

But before I can do any of that I have one really big problem. Removing the fan assembly required that I remove the top cross member at the front of the engine bay between driver's and passenger's side. It looked easy, held on by 4 bolts, but really required the removal of the hood latch, two blind bolts attaching it to the rear of the front crash bar and various bolts holding the horn, transmission cooler etc. However, when I re-installed the hood latch (the part on the driver's side behind the light released by a wire connected to the red lever in the cabin on the driver's side under the steering wheel where the driver's left knee would be), I did it incorrectly and cannot now unlatch the hood. Does anyone have any ideas as to how to remedy this *#4%!*! mistake to open the hood, short of cutting through the hood itself? I can reach the fail safe release under the center of the hood, but without releasing the main hood release on the driver's side, the hood will not open. Again, any help would be really appreciated.

Oh, and before I started this project, I had my independent mechanic diagnose the problem, he told me that the cooling fan motor was fried since he could not jump it and his computer diagnosis said that there was an "open circuit" at the fan motor. The car runs fine, gets up to temperature and stays at the right temperature when moving over 35mph, however at long lights, in stop and go traffic, or just idling, it overheats. The new coolant sensor works by correctly and quickly registering those temperature changes.

 

Once you replace the fan, you will need the dealer to perform SCN coding via STAR. If you have a STAR yourself you will be able to do it. I don't know what they charge but I can't imagine it's more than an hour of labor to do it. Just go and tell them you replaced the fan and need them to program it to the car.

As far as the hood, I messed mine up when I did my radiator because the wire running to the release latch didn't seat back properly on the driver's side, but my issue was just that it wouldn't latch on that side. As far as getting the hood back open you'll just have to try fishing around with a flathead or something in the small gap and try to manually turn the hook to unlatch.

 

Thanks Mulac, I'll have the dealer perform SCN coding via STAR to program the fan to the car.

The problem I have with the hood is that there is no room between the top of the drivers' side headlight and the front lip of the hood to get a screw driver in there. When I reconnected the wire to the hood release latch I must have done it so it doesn't pull the spring that releases the hood and, like an **$% I slammed the hood shut without testing the release first. Now the hood is tightly shut --

Can the dealer reprogram without opening the hood?? Any other ideas as to how to get in there -- use coat hanger? or other really thin, reasonably stiff, but flexible strip of metal (if anyone knows what such an animal is called, let me know).

 

You've got me lost on the hood thing..You can't remove the bumper or headlight without getting to the bolts that require opening the hood. I hate to say this but you may have to break something to get in there, which in this area the cheapest thing would be the grille. The grille has nubs on it on either side that sit in holes at that little piece of bumper that extends up between grille and headlight. You may be able to twist/shift the grille enough to get it popped out and maybe move it to make some room. Maybe ask the dealer what they think?

They only need to plug into the OBDII with STAR to do the programming so they won't need to open the hood for that.

 

Unfortunately I learned the hard way too....always check the cable routing before the hood gets closed lol


Tracing the ends of the radiator bridge with a pencil is a great idea too

I tried for a few days to get it open but ended up just taking the fast way out. Bought a long sawzall blade and sawed off the loop. I did cut away some underhood insulation so I could visually get the blade on the loop for sure, but my insulation is a little tired anyway. Replacement part was pretty cheap online too. I won't say I didn't study lots of under hood pictures though!

 

Well guys thanks for your help. This is what happened. I removed the CLK Star Grill installed on my car without breaking anything. That grill clips (with nubs) into two small holes on the left and right like Mulac says, and the tab attached to the cross member with screws is similarly clipped onto the back of the grill with two plastic nubs. So it can just be wiggled out without opening the hood or breaking it. Then, after trying to release the hood with various screw drivers and a long flat, narrow bar I did not even come close to opening the hood, so I took the easy way out and went to the dealer (we have a decent relationship and the mechanics like my car).

After waiting a couple hours (the service guys drove me to a business meeting and I got a ride back to the dealership), the team leader who knows my car and had been at that dealership since my car was new, arrived and put my car on a lift, removed the belly pan and from underneath the engine with a three foot screwdriver type tool with two prongs on the end rather than a flat head, managed to release the hood in about 30 min. He explained that these hood release latches are designed to be tamper proof and unmovable without the hood release cable because of the thick plastic cover around the base of the mechanism. It turns out that I had installed the wire correctly but failed to notice that the plastic stop on the end of that cable had partially broken and could no longer hold in the latch mechanism. Rather than replace the hood wire (which they told me was a three-four hour job) the mechanic zip tied the cable in place and everything seems to work just fine. This is what they wrote on the final invoice:But more importantly, when I asked them to SCN code my new fan, they said that they never heard of such a thing, never did it and that replacing the cooling fan should be plug and play. So they hooked up my car to STAR found that an open circuit prevented the fan from turning on and traced it to a broken wire in the wiring harness between the front SAM and the radiator fan connector. They repaired that broken wire and I'm good to go. Fan works great. This is what they wrote on the final invoice: Now all I have to do is return the fan motor I didn't need to buy in the first place. Also, I was in the shop and watched them perform these repairs. By the clock I was overcharged 2 hours, but by book time I suppose they billed what MB specified.

 

I was gonna say that it sounded like a broken wire problem. Similar to the pre facelift models .
Good to hear that you got it sorted out.

 

It just so happens that I bought this car and I found out that it overheats when the a/c is off. I live in Florida so its not an issue, but I would like to get it fixed so it is perfect. It looks like JakeMS3 did a lot of testing on this, but do you think it has anything to do with the tune and I may need a new ECU? It is going into the shop on Monday but now that I have all this info it may be more helpful. There are a bunch of used ECU's for sale but I am hoping it isn't that because I like the way it drives.
Any thoughts?

 

Check the fan thermostat, when AC is on the fans run continuous, two different circuits.

 

Go back a few posts and read the second half of Tump43's post and start from there.....

 

So I just got back from the MB indy and spent a few hours trying to isolate the issue with no answer. Apparently, the tune had some sort of effect on the fan operation, it's not telling the fan to turn on when the ac is off. My indy is very familiar with the C55 and has worked with a lot of tuned mercedes and basically said just keep the ac on. He could reprogram everything but they said it may cause other issues on an otherwise perfect car.

Outcome was that I just keep the ac running and I have a flawless car. Didn't feel like fixing a small issue and possibly having other issues pop up.

He did say after a nice overlook of the car that it's a beautiful example of a C55. Luckily it's Florida and the ac is always on so I don't really mind. I'm going to run it hard with the ac off and see if it heats up unless I'm in traffic.

I feel like a weight was lifted off my chest, he could have said that I have some serious issues going on but as long as the ac is on it's perfect. I was hoping it would be a sensor and it would be a perfect car but I like the tune so I'll live with it.