W202 Overheating!!!!!!!!!!!!!

[paulmb]

The worst thing happened to me today..........

Was driving down fulham palace road with the aircon on full blast..........lovely day........

Then, all of a sudden, the aircon started pushing out warm air?!?!?!!?!?!?!?!?

Looked at the temperature..........needle pointing at 120 degrees!

Quickly pulled over and switched of the car...........after 20 minutes, turned to ignition position and the needle was pointing at 100 degrees........but i could'nt hear the fan......it was'nt working!?!?!?!?!?!?!?!

did'nt want to risk driving in the slow moving traffic so called out recovery.......just got home.....

can you guys help or tell me what it might be...the car has not lost any water and still drives as i put it on my driveway when i got back..........

i think it might be the fan, are they viscous fans on these......c200 sport.....

someone help!!!!!!!!!!!!!!

 

[Simon]

Here's a few suggestions:

1 Thermostat might be faulty ie not opening
2 Radiator core blocked or grille restricted
3 Cooling fan or viscous coupling faulty
4 Inaccurate temperature gauge sender unit
5 Airlock in cooling system
6 Pressure cap faulty

Good luck with it and let us know how you get on please.

 

[paulmb]

thanks simon................ i thought i had done some serious damage.....

feel a bit better now.............will get these things checked out.....any other suggestions.......

 

[silverarrow]

fan control relay known fault
have you had the low coolant light or screen wash low light on ????

 

[paulmb]

no lights came on.............

Am i right in saying that the single fan runs the aircon and cools the car down? is that why the a/c cut out and only blew out warm air?

where is the fan control relay positioned? are they expensive?

 

[paulmb]

this morning, i put a 12v supply directly to the fan and it worked.....at least i know the fan is not knackered.............

any more ideas?

 

[nickmann]

Mine had three fans. Two on the rad are electric and are part of the aircon system.

The third is on a viscous coupling on the front of the motor, which clearly cannot spin if just the ignition is on. This is the engine cooling fan. If this coupling has gone then the fan is freewheeling and not really doing much cooling.

You can test it with a carrot. Get then engine up to working temp, then carefully stick a fresh carrot into the fan blades. If they stop, the viscous coupling should be inspected. If the coupling is sound it will slice the carrot.

 

[paulmb]

correct me if i'm wrong, but i've only got one fan sitting behind the rad.................

no fans on the front of the rad.........
is this the fan you r talking about nick?

 

[nickmann]

The viscous one is physically connected to the engine, and turned by the big rubber belt that also drives the water pump, alternator, compressor etc. It is partially enclosed by a big plastic shroud. This is the one for the carrot test.

 

[paulmb]

sorry for being ignorant nick, but i did'nt see a fan on the front of the engine.....................................i'll check it again tonight............

 

[Asif Kazani]

correct me if i'm wrong, but i've only got one fan sitting behind the rad.................

no fans on the front of the rad.........
is this the fan you r talking about nick?

Some of the newer ones have just one variable speed electric fan behind the radiator and none up front. If this is the case, then you might want to look at the relays or coolant/temperature/fan sensors.

Good Luck.

 

[paulmb]

thanks asif, i'll check these things out...i know the single fan behind the rad is working............matter of checking each thing.....................i think it might be the relay?....you don't know where it might be situated do you?

 

[nickmann]

Some of the newer ones have just one variable speed electric fan behind the radiator and none up front. If this is the case, then you might want to look at the relays or coolant/temperature/fan sensors.

Good Luck.

Ah, thanks for that

 

[Asif Kazani]

thanks asif, i'll check these things out...i know the single fan behind the rad is working............matter of checking each thing.....................i think it might be the relay?....you don't know where it might be situated do you?

Can't say for sure but I believe that there are some in the fuse box near the brake booster on the driver's side. There may be some inside the ECU housing box (big black box) on the left hand side. I am more inclined on it being in the fuse box on the right side. There is also a chance of the relay being the same as those of the headlamp so you may easily be able to identify it being faulty by substituting one for another if the ratings and amperage are the same. My instinct tells me that it might the temperature/fan sensor.

 

[Joe Bagnara]

If it is the single fan model - electric -simple way to check it out is to start the engine and turn on airconditioner. Fan should run (this assumes air conditioner is not U/S).

 

[paulmb]

is it worth getting a new temperature/fan sensor..........how much r they and where r they situated....thanks, asif..........

when the aircon is switched on joe, the fan is not running.....

 

[Asif Kazani]

is it worth getting a new temperature/fan sensor..........how much r they and where r they situated....thanks, asif..........

when the aircon is switched on joe, the fan is not running.....

Paul, if you ask my opinion, replacing something without proper diagnostics, is an utter waste of money, for which, we all work very hard. As far as their life is concerned, there is no telling since as I have experienced, I have had one last for 13 years and one about 2 years in the same car. Yes, they may go bad and are not very expensive to replace and the relay, fan sensor, or a fuse may be the only things that you or your indi may have to look at. It should not be much. As for location of these, the temperature, coolant and the fan sensors are all locate either on the block or on the water flanges with one or two wires sticking out of them. My educated guess would be that the fan switches usually have just one wire sticking out of them. If you identify the correct one, you may disconnect it and get the wire connected to ground by sticking it on the block/flange with a piece of wire. This should turn the fan on and let you know if the relay is ok and the switch is bad. Replacing them is easy and you do not need to be an engineer, just a little coolant loss. Just be patient and e gentle with them. Also, have the engine oil and filter changed immediately before you put the car back on the road.

 

[silverarrow]

you need a new fan control relay which is under the wheel arch on the passengers side its about 4inc by 3 inch and silver in colour with one electrical connector coming to it from the above.near the lights about10to 12 wires from memory
mercedes will want your chassis num before they will let you order one
because they fail often and can damage the engine due to over heating the relay controls both the air con and electric fan the w202 does not have a conventional fan switch it has the control relay cost with discount about £125 and 5 mins to fit not the 1.3 hrs mbz want.
they can be changed without taking the front wheel off
reason why they fail is water ingress on the pcb in the relay you may be able to get yours repaired BUT IF THEY FAIL AND YOU ARE NOT WATCHING THE TEMP THEN MEGA ££££££ ON THE ENGINE.
ask mbz why relay in a wheel arch when so much water is present daft design or clever because more parts sold and diagnostic fees umm ummm
no need to reset the obc either no fault codes are stored

 

[paulmb]

i don't think the engine has been damaged in any way, however, my friend is a technician at my local mercedes benz dealership. He is going to come over and check on the diagnostics computer.

is it definately the fan control relay silverarrow?

and its got to be ordered via mb with the chassis number.................

well, i'll wait and see what happens.....thanks to all you guys for your input..........will let you know what it was......

 

[silverarrow]

the star ie obc diagnostic computer will not store a fault but i have had 2 w202 models have the same fault
the reason why the relay / control unit fails is due to water ie it is because its in the wheel arch mud salt water ans jet washing is not good for any electrical system
i would say 90% correct in it being the relay
second hand they are £60 but how long will it last get a new one post back when sorted it was 6 weeks for my last one to arrive