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SLK 230 R170 1999 M111.973
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Discussion Starter · #1 ·
Some days ago I was driving around town when the coolant temperature got over 90ºC. Usually at this temperature the fan starts working. In this case I got the low coolant level light.

First, I read the user manual. Possible causes:

1) Coolant level: good.
2) Engine temperature: not over 90ºC.
3) Engine fan not working: bingo.

I tried to force it by disconnecting the MAF sensor: nothing.
I checked the "f6" fuse in the K40 module: as good as new.
I disconnected the connector on the K40 module (there are five connectors, it is the one in the middle). Everything seemed alright.

Next thing is to check if the fan itself works. I didn't have any spare cables so what I did was to reverse the process: instead of powering the fan with the battery, I measured the voltage by spinning it with my hands. Voltage: ~12 mV (believe me there's not much room there to give it a good spin). The thing is that it generates power.

I also checked the small fuse box next to the battery. There are 4 fuses in there and I checked the continuity of all of them. There were all good.

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Find here where I measured the voltage (marked in yellow):
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Find here where the K40 connector and fuse is:

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Is this enough to discard the fan? What should I check next? I'm not getting anywhere...
 

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1998 SLK 230
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3,013 Posts
The only other thing is it could be the cooling fan module has failed. First I would unplug the connectors to it and give them a clean and re-try.

this sits next to the headlights.

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SLK 230 R170 1999 M111.973
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23 Posts
Discussion Starter · #5 · (Edited)
Well, unfortunately cleaning the module didn't work. It is as good as new. The connector was in perfect condition too.

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Any ideas?
Can it be a failure in the temperature sensor? Temperature readings are okay...
Maybe the thermostat? But if there was any issue with it the temperature would go high or low, and it is staying normal....
 

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1998 SLK 230
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3,013 Posts
To test the temperature sensor, start the car while the engine is running and unplug it. This will send the ECU into failsafe mode as it can't detect the temperature - the fan will constantly be on and at high speed.

Yes it could be a thermostat but that's a general maintenance item that should be replaced anyway
 

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1998 SLK 230
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3,013 Posts
Did you do any other tests on the fan control module?
 

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1998 SLK 230
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3,013 Posts
Yep agree with the above - from memory, I think this fan needs AC voltage to spin. so you cant just hook up a battery to it.

1) unplug the coolant temp sensor
or
2) turn on the a/c to max/high cold setting
 

· Registered
SLK 230 R170 1999 M111.973
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23 Posts
Discussion Starter · #11 ·
Yep agree with the above - from memory, I think this fan needs AC voltage to spin. so you cant just hook up a battery to it.

1) unplug the coolant temp sensor
or
2) turn on the a/c to max/high cold setting
I'm pretty sure the radiator/AC condensor cooling fan comes on with the A/C compressor. Have you tried this?
Thank you guys, I will try that.

What should I spect if it does not work?
 

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1998 , SLK 230
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696 Posts
I'd try and get access to a code reader. The R170 can get very expensive if you try fault finding by substituting parts. My old Chinese Carsoft 7.4 clone paid for itself 5 times over the first time I used it.

Maybe get some ideas from videos like this:
 

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SLK 230 R170 1999 M111.973
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23 Posts
Discussion Starter · #13 ·
I'd try and get access to a code reader. The R170 can get very expensive if you try fault finding by substituting parts. My old Chinese Carsoft 7.4 clone paid for itself 5 times over the first time I used it.

Maybe get some ideas from videos like this:
Looking at this video and the second one this guy has, everything points to the fan module. I'm having the same symptoms. I'll try to disassemble it and take a look what's going on inside.

Like someone said in a comment: better pay 3€ for a transistor than 60€ for a second hand module.

Thank you Mike!
 

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SLK 230 R170 1999 M111.973
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23 Posts
Discussion Starter · #14 ·
I'd try and get access to a code reader. The R170 can get very expensive if you try fault finding by substituting parts. My old Chinese Carsoft 7.4 clone paid for itself 5 times over the first time I used it.

Maybe get some ideas from videos like this:
Okay so I have disassembled the fan control module (A 027 545 81 32 ESG 400) and didn't quite find what I was specting. Everything seems to be okay: condensers do not have any play, nor do the transistors. Every single weld is in good condition.
I must be missing something...

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SLK 230 R170 1999 M111.973
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Discussion Starter · #16 ·
I did a little test today. I jumped the electrofan and it works.

Yesterday I was talking to a friend about the fan. He told me that the model 190E usually has faulty temperature sensors (known as thermocouple or thermocontact).

Take a look at mine:

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The previous owner was a master.

I'll buy a new sensor. The fan works beautifuly with this sensor disconnected.
 

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SLK 230 R170 1999 M111.973
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Discussion Starter · #17 ·
Finally the coolant temperature sensor arrived.

It was a really easy task. Nevertheles, coolant light is still on.

Having tested the fan, the fan module, changed the temperature sensor and checked the fuses, I think the next stop may be the coolant level sensor. What do you think?
 

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1998 SLK 230
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correct if you get a low coolant light it can be

1) the sensor which screws into the side of the expansion tank
2) the sensor which measures the level of the windscreen water tank (yes they share the same circuit, if this sensor is stuffed it can trip the coolant temp sensor light)
3) the magnet in the float in the expansion tank has come off or not sitting right throwing incorrect readings


1) doesn't tend to happen often, issue number 2) happens a lot.

for issue 3) it's easy to test, unplug the coolant temp sensor and if the light goes away then the float in the expansion tank is not sitting right. the only way to fix that is to change the expansion tank.
 
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· Registered
SLK 230 R170 1999 M111.973
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Discussion Starter · #20 ·
correct if you get a low coolant light it can be

1) the sensor which screws into the side of the expansion tank
2) the sensor which measures the level of the windscreen water tank (yes they share the same circuit, if this sensor is stuffed it can trip the coolant temp sensor light)
3) the magnet in the float in the expansion tank has come off or not sitting right throwing incorrect readings


1) doesn't tend to happen often, issue number 2) happens a lot.

for issue 3) it's easy to test, unplug the coolant temp sensor and if the light goes away then the float in the expansion tank is not sitting right. the only way to fix that is to change the expansion tank.
I read you reply wrong Subby. Today I tested the coolant level sensor. I bypassed the sensor and the light was still on. When I unplugged it, the light disappeared. I guess this sensor detects the coolant level and, if it is enough, it opens the electrical circuit.

But, as you said, I have to try unplugging the coolant temperature sensor and see if the light goes out.

Something tells me I am in "section 3)" 😅
 
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