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s55 amg, slk 230, e55amg, E63amg, 450slc
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Discussion Starter #1
With more than 900 pages of R170 SLK forum, I could not find an answer to my problem. Please help. I have a 1998 SLK 230 with 30K miles. Recently the car has been overheating. I have replaced thermostat, coolant, spark plugs and cooling fan control module, but still no improvement. When outside temperature is about 75 degrees and the reading of the temperature gauge reads 105 -106, the cooling fan does not turn on. When I jump the fan directly to the battery the fan runs at high speed. I have opened the k40 relay looking for defective soldering but all looks fine. I have read in this forum that there are other owners with same problem but could not find a solution or guidance to help solve the problem. Could somebody please help me. Thank you in advance.
 

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2001 SLK 320(217K Miles), 2002 E320 Special Edition(183K Miles)
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Does the fan turn on when you switch the aircon on? The aircon will also switch on the fan depending on the aircon gas temperature/pressure, so it may be few minutes before the fan goes on.

It may have a separate temperature sensor for the fan, not the same one that is used for the instrument panel - but I may be wrong. That at least was how it was on my 190E.

Also, is the temperature higher than in the past? Normally if it does not go into the red, it's fine.
 

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3 other possibilites, general automotive, and not specifically related only to SLKs.

1. The radiator may be plugged, or low cooling capacity through only a few channels.
In my experience with other cars, flushing does not work. The only thing that works is replacing the radiator. You can buy aftermarket radiators cheaper than factory. On other cars you can check by feeling or using a heat sensor on different areas of the radiator fins and seeing if all are hot and at the same temoerature. But with an SLK, you do not have much access.

2. The water pump may be bad. Often a bad circulating water pump leaks water. Sometimes it doesn't leak, but still doesn't circulate much water.

3. The head may be warped or cracked from being overheated previously.
EXPENSIVE to fix. $2000+ for a / my 190E 2.3. Probably (much) more for an SLK.
Fairly difficult to DIY.

If you can't get enough answers here to diagnose it yourself, it can be worth the trip to a MBZ specialist to pay for a diagnosis. There are good MBZ repair places other than the dealer who charge less. An excellent factory trained person is in Benicai, Northeast SF bay, where I live.

Star Motors, Benicia, Toni Gauman owner (707) 745-5606

I do much of the work on my 4 MBZs, Toni does the rest.
If you call, reference me, maybe a discount. ( I don't get anything except good will.)

Good luck, hope it's a cheap fix., Mark Wininger
 

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s55 amg, slk 230, e55amg, E63amg, 450slc
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Discussion Starter #4
Thank you Mark for your thoughts. The problem of my overheating is that the fan is not turning on when temperature hits 105 degrees. If the fan turns on then is cools down the engine. I still cannot figure out what trigers the electric fan to go on when the temperature goes up over 105. The air conditon works well. There are some that says that the AC controls the electric fan. I would let the engine idle for about half an hour and then the temperature start going up. I do not think I have a cyclinder head gasket problem nor a clogged radiator. If I can make my fan turn on, my overheating problems will be solved.
 

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1998 SLK 230
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i know this isnt a DIY way but you can do this in STAR DAS (diagnostics) at the dealer or independent workshop.

you can set the fan to come in -10c earlier or +10c later. this option is there to accommodate different climates.

you can also test the fan in STAR DAS to trigger it on/off. i got mine set to come on -10c earlier. my engine temp ALWAYS stays at 80c, regardless how hot it is outside or how hard you push the motor. when idling in traffic on a very hot day it may hit 83 but no more. as soon as you start moving drops to 80c.
 

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s55 amg, slk 230, e55amg, E63amg, 450slc
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Discussion Starter #6
Thank you Subby, I will bring the car into the dealer tomorrow. At least I know what my options are. Will keep you posted.
 

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Yellow '99 230 slk
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Overheating

Similar problem in my 99 230. Cooling fan never coming on. Replaced the radiator sensor/sender, no change. Independent workshop checked out and said that the fan should be switched to 5 different speeds dictated by the ECU, but this was not sending the correct signals. So he by-passed the ECU and now the fan is on or off (almost always off as I live in the north of Scotland). All I see now is a few deg. variation in the coolant temperature from time to time.
 

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2001 SLK 320(217K Miles), 2002 E320 Special Edition(183K Miles)
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That was why I was asking what happens if he switch on the air-con. On my car, the fan starts running at a low speed shortly after switching the air-con on, even if the temperature guage has not gone higher than 80 degrees.
 

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08 ml320 cdi,slk 230,01 boxster S,04 carrera911, 99 S320,67 GT350mustang,02G500L,96 s600 w140
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how about fan relay? im not sure if its connected in the k40 but Other cars if the relay doesnt work the fan will not come on or if the relay gets stuck on the off position.
 

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1998 SLK 230
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the fans on the MB arent just on/off. You confirm they actually dont come "on" at all or because you cant hear them come on?

the have a few variable speeds and you can hardly hear the fan running at all... even in peak hour traffic the fan is quiet..

since i made the fans come on earlier thru STAR they become louder (as they are spinning faster) to maintain the desired operating temp.
 

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2001 SLK 320(217K Miles), 2002 E320 Special Edition(183K Miles)
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the fans on the MB aren't just on/off. You confirm they actually dont come "on" at all or because you cant hear them come on?

the have a few variable speeds and you can hardly hear the fan running at all... even in peak hour traffic the fan is quiet..
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Subby, you are quite correct. The fan is very quiet and I've never heard mine run. Most of the time, the fan run so slow that there is no indication that it is running, unless you open the hood and have a look. Even in Las Vegas in the hight of summer with the aircon running, I had never heard the fan.

However, you are wrong to state that the fans are not just on/off. :D
They are exactly that. Where some other cars uses multiple circuits and brushes to control the speed, the MB fan uses an electrical pulse action to control the speed. To put it simple, the power to the fan is switched on and off in a rapid succession. For a slow speed, the on/off interval is quite long,for a fast speed it is cycled rapidly. This in theory provides an infinite range of speeds, although in practice there is probably just a few profiles that are used. This is one of the reason why you cannot just hook on a meter to read the current. You should look for the pulses.
 

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1998 SLK 230
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my mistake.... it is PWM (pulse width modulated) signal.

see WIS screenshot. it takes the coolant temp sensor as an input. guessing if your fan comes on late it could make sense to replace the coolant temp sensor...

if it doesnt turn on at all (which i doubt as the car would get hot verrrrry fast) then might need to check the controler N76. its the big silver thing with heat sinks near the headlight. follow the wires and you will see it lead to the fan
 

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s55 amg, slk 230, e55amg, E63amg, 450slc
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Discussion Starter #14
Hi Subby, finally the overheating of my car was solved. The mechanic in Autobahn had to trace the wirings of the cooling fan. He found out that the wirings in the power distribution box (located at the left side where the fuses are) there was one bolt that was stripped. So the wires were not grounded properly, therefore not giving enough voltage. How the bolt was stripped confuses me as I know no one has opened the box before. Anyway I drove tested the car today with outside temperature above 80 degrees and the engine did not overheat even with stop and go. Thank you all, this is a great community.
 

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Glad it worked out well. :) And another novel problem to make a mental note of. :D
 

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SLK 230 Kompressor, 2001
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Hi Subby, finally the overheating of my car was solved. The mechanic in Autobahn had to trace the wirings of the cooling fan. He found out that the wirings in the power distribution box (located at the left side where the fuses are) there was one bolt that was stripped. So the wires were not grounded properly, therefore not giving enough voltage. How the bolt was stripped confuses me as I know no one has opened the box before. Anyway I drove tested the car today with outside temperature above 80 degrees and the engine did not overheat even with stop and go. Thank you all, this is a great community.
Do you have any idea how did the mechanic trace the problem? Did he measure the voltage of the fan?
I have the same problem, the engine gets hot, the fan is working, but turns at a very low speed.
 
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