So here is the background, I noticed the car overheating a few weeks ago while I was checking out some xmas lights with my ladies. All lights were set to off so we were riding in darkness even from the dash/gauges. Right at the end of the show a red message popped up letting me know car was extremely hot and to turn engine off. I pulled over and popped the hood and got a quick glance before shutting her down. I noticed the fan was not on at all. Let it sit for about 15/20 mins then turned back on seemed okay rest of the way home was highway driving. Did some troubleshooting here at the house with the help of the input from Petar (c230mulac) and Ed (Edgineered) and all fingers were pointing at either thermo, coolant temp sensor, or possibly water pump. No leaks of coolant, no messages whatsoever on the dash, and no CEL's. Took it for a drive around town and the temp stayed within operating temp so I came back home and let it sit until I started to see the temp rise. Took approximately 16 mins from when I parked the car to start creeping. I finally had a chance to take it over to MB to let them diagnose so I would knoe exactly what I needed to replace. They told me that my fan wasn't kicking on when it needed to and that the ECU controlled this. They asked if I had been tuned and I said yes a few months back. They then pretty much blamed my tuner and said they messed something up and that I would need a brand new ECU to remedy my problem, which happens to cost around $3K.


Talked to my tuner and he mentioned that I have a low fan speed (comes on with a/c) and a high speed fan which is supposed to come on to cool the engine down. Both of these have relays and that it sounds like that is my issue.


So has this happened to anyone else? Not necessarily just the need for a new high speed fan relay but being told the ecu controls the fan and ecu alone? They didn't mention anything about a fan relay.


I talked to the service guy at MB and he seems like a straight up guy and I'm not stupid when it comes to cars but its just frustrating that MB would say this when its not even the issue. I will confirm tomorrow when my tuner takes a look at it with me but a new ecu and fan relay are completely opposite sides of the spectrum.


I appreciate any input guys thanks!

 

Check the relays

 

Is there only one in the fuse box above drivers side strut? And does anybody know exactly which relay it is? P,Q......?

 

I forgot to mention that the MB tech was able to turn the fan on with star to 100% duty, but won't turn on by itself when engine is creeping above operating temp without the star commanding it to

 

Fan sensor, sounds like a good place to start, if the fan can be turned on remotely then the control to say come on would be my first choice. Check connectors for corrosion or ?, not sure where you are located ( road salt or sodium chloride liquid). Does the fan start up when the AC is turned on, yes, then go back to the heat sensor for the fan.
?? the MB tech could not diagnose whether or not the sensor is operating correctly ??
I'd try another more qualified shop.

 

Yes the low speed comes on when I turn the a/c on. When you say heat sensor for the fan where is that located?

 

Was initially thinking of the actual temp sensor for the engine, which is where the signal would originate, check connections at this point. Google it, screws into block with connector attachment for wiring. 12 or 13 bucks Rock auto, Just a thought,,,,,,

 

Was going to post what critter said, check the engine temperature sensor.

 

Updates.

Went to peters (c230mulac) this morning and tested out a new relay, fan still didn't turn on when needed. Checked the wires going to the fan (4 total combined a power, grd, high speed, low speed) and they all seem/look to be in working order. We used his voltmeter to check for power and it's there, just not getting the message to the fan. We also tried the same thing on his wires and got the same reading as mine so we ruled that out. If were working downstream from the relays, those wires were next, after that is the fan control module which seems to be attached to the fan itself. I also read that to test if the fan wires/module are out disconnect the coolant temp sensor and turn the car on, the fan should turn on full blast as a fail safe to protect the engine from overheating. I just tried that now and fan didn't come on. Any input?

 

I realize it may not be right but spend 15-20 dollars on a new engine temp sensor and try that.
What do the experts say?

 

Do you mean the coolant temp sensor? I googled and researched the forums and and can't find that we have an engine temp sensor. Besides an ambient air temp sensor. Haven't taken it to an Indy yet since I got it back from the stealership. Like really have a hard time trusting what the dealers/Indy say is wrong with my car. Had a bad experience before and paid way too much money. Would like to figure it out on my own as long as it keeps costs down.

 

Beck Arnley 158-0955 Engine Coolant Temperature Sensor
for your 2006 Mercedes-Benz C55 AMG

 

New MB part number A0009050700

 

^+1^,

Also "000 905 06 00" will work..



ZAYED,,

 

Hey all, since I'm helping diagnose I don't think it's the coolant temp sensor.

Step 1, we tried switching the relay, didn't work.

Step 2, we unplugged temp sensor and turned car on. The temp gauge just stayed down at the bottom. When car is turned on cold, it heats up to operating temperature normally. I would think the sensor is okay as it seems to be reading temperature. If it were bad I would think the gauge would be acting funny. Too much of a coincidence for it to only be heating up during idle and not while driving to be the sensor.

Engine temp sensor = coolant temp sensor, they are the same item

Step 3, with the voltmeter set to voltage, red wire is hot, brown wire is ground, the other two I'm not sure what they are, must be low and high speed control. Readings taken with key in position 2 with engine off (Jake am I remembering that correctly?).

Red to brown - ~12V
White to brown - ~12V
Green to brown - ~0-0.003V

We took the same voltage readings on my harness and got the same values, so we know that the wire harness running to the fan is receiving power and none of the wires are broken upstream of that.

You can't officially buy the fan module separate of the fan assembly, however I have been able to find an aftermarket module but not sure how reliable it is. That is, if in fact it is the fan module.

However, like the others said, it may not be a bad idea to just swap the sensor and see if that fixes it. It is important to note when my water pump was going bad I had the exact same symptoms, the only difference is that I got the "coolant visit workshop" message on the dash each time it started to overheat; and sometimes it would overheat when at lower speeds around 10-20 mph, not just idle.

 

I may be off track but the sensor is in fact a thermistor that takes temperature and converts it to a voltage or resistant value to be used by downstream devices to activate other components, the gauge and the fan.
If you can turn on the fan remotely then I would think it could be the source of the signal, ie the sensor.
Just sayin,experts where are you.

 

As far as the water pump issue, this is only a pulley attached to a shaft which in turn spins and impeller inside of the water jacket. Failure is 99.9% caused by bearing failure brought on by coolant bypassing the seal. This will cause a loss of coolant as the system is under 16 or so pounds of pressure during operation and will diminish as the car sits and cools. So in effect a system could dribble coolant for days or weeks before the light comes on. Best to look under the car periodically or check you parking place for any unwanted drips, could save you money in the long run. Overheating is caused by low coolant not a leaking or noisy/loose water pump although it is the initial aggravate.
Just sayin

 

Yes, you are right, the sensor measures resistance and converts it into a temp reading. I actually swapped mine out when I was having issues and took the old one and measured resistance at the terminals. Then I held it under hot running water for a while and measured resistance and the value had changed.

As far as the water pump that could be the case, but I actually don't recall whether or not my pump was leaking...maybe it's that I caught it early enough and the pump had not entirely failed. In any case, it's worth mentioning I did see some sort of fluid below everything on top of the belly pan when Jake came over but we were thinking it's because it had been raining. But we didn't actually check to see if the fluid was water or coolant

 

In any case, Jake bought a water pump, pulley, and thermo so maybe we should just replace them all and see if that solves it. If not, well at least you've got a brand new pump and thermo!

 

Another small tidbit, the sensor will not measure air temp or steam it must be in liquid to operate properly. So in effect if the level of coolant is below the sensor then an improper reading will be given on the gauge, and fan. Also another is the heater blowing cold/cool air but the temp is okay, low level!!
Is the reservoir empty?

 

Let's see where we are:
The fan turns when triggered via STAR system. That means the fan motor is working, the relays are working since I would guess STAR communicates with the fan via relays, the temp censor is working since the "overheating" message comes up. But the system doesn't work together under normal conditions. Is that a good summary?

 

I have checked coolant levels all seems to check out. My car is parked same spot for days at a time and no sign of any leaks. No squeaks/squeals of any sort from the pulley either. I do have a replacement for it just in case, but at this point it doesn't seem to be the issue. The car only overheats while it's been sitting at idle for 15+ mins. Exactly when the fan should kick on and start to cool the engine down itself. I'll try and switch that sensor out on this weekend to rule that out as an issue. I'm hoping it's not control module because new fan and module is at least $600.

 

Before going crazy on parts, run the car with the reservoir open, when the car reaches operating temp check the temp in the reservoir, you may be experiencing a faulty thermostat. Or when you say overheating is that according to the gauge or is there steam present, turn on the heater to see if you are blowing hot air.

 

Okay so I just got the car back from the Indy who determined I have a bad fan control module. I'm relieved to finally uncover the issue but dam these fan are expensive. I found these so far http://www.ecstuning.com/Mercedes_Be...Fan/ES1728538/


http://www.rmeuropean.com/Part-Numbe...MTfu92pWA.aspx


I'm worried about buying the cheaper versions off ebay so should I stick with an OEM replacement for this part? Has anyone had to replace their cooling fan before?


The indy also told me that when the fan is replaced it has to be programmed to the car?? They called the dealership and asked them and dealer said they have the only machine that can program the fan to my car? So does anyone know if this is true or not? Any MB techs out there can shed some light? I will be able to swap the fan out on 24 jan but im hoping its just plug and go. Thanks

 

Okay so today with the help of @C230 Mulac; we replaced the entire fan assembly and everything is back to normal conditions. Did a coolant flush while we were in there and I let the car sit for 20 mins afterwards and the fan kicked on at a lower speed not full blast but the temp had just barely started to reach operating temp. I monitored the fan for about 10 mins and it kept the temp where it needed to be and shut off when the temp was at designated Celsius. So be weary of whatever the dealer says as the told me I needed to spend $3K on a new ECU when at the end of the day it was a bad fan control module.