Overheating Issue? How does the fan control unit know how hot it is? - Mercedes Benz SLK Forum

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#1 Old 05-02-2012
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Overheating Issue? How does the fan control unit know how hot it is?

OK I know theres a lot about overheating, and I thought I had my SLK set, but a minor issue arose:

It overheats.

Before the fan wasn't working, now the fan is working but it only goes "slow". It kicks on at about 82, Even as the car heats up to 90/92, the fan stays on "slow". It should be variable speed, right? And when I connect it direct to the battery it FLIES.

So this is either the fan control unit or some sort of temp sensor unit?

Can't figure out how this cooling system works. How does the fan control unit know the temp?

Car worked fine when it was like 60 outside, but now that its 90, its having trouble, but I think its the fan speed.
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#2 Old 05-02-2012
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90/92 is pretty much normal running temp. The fan speed is veriable depending on engine temp. You can test the fan speed by turn on your AC on a hot day. The fan should run at max speed on a hot day with AC on.
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#3 Old 05-02-2012
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Then what would make the temp gauge go crazy and start bouncing around between 0 and 100 once you hit 92 or so?
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#4 Old 05-03-2012
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Headgasket

Same problem last year, for no reason it would go from 80-overheat. New headgasket fixed it.
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#5 Old 05-03-2012
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Originally Posted by Klonny View Post
Same problem last year, for no reason it would go from 80-overheat. New headgasket fixed it.
I dont have any other signs of a headgasket issue- did you?
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#6 Old 05-03-2012
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headgasket

No nothing like the normal symptoms. No oil in the coolant, no leaking coolant, no coolant in the oil! no white froth, nothing.

I did change the thermostat, no luck and changed the radiator cap (wishful thinking).

I checked the plugs, they all looked fine.

Finally, did a compression test and #2 #3 were both substantially lower than number 1 & 4. turned out that the broken headgasket was between #2 #3!

changed it and presto there you go, hasn't overheated once this summer and i've driven the crap out of it (hit 110mph, cruised for 4 straight hours on the highway as well as driving in Minneapolis for a solid hour of stop and go) and it's all good.

I hope I'm wrong, but I suspect it's the HG.
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#7 Old 05-04-2012
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Originally Posted by Klonny View Post
No nothing like the normal symptoms. No oil in the coolant, no leaking coolant, no coolant in the oil! no white froth, nothing.

I did change the thermostat, no luck and changed the radiator cap (wishful thinking).

I checked the plugs, they all looked fine.

Finally, did a compression test and #2 #3 were both substantially lower than number 1 & 4. turned out that the broken headgasket was between #2 #3!

changed it and presto there you go, hasn't overheated once this summer and i've driven the crap out of it (hit 110mph, cruised for 4 straight hours on the highway as well as driving in Minneapolis for a solid hour of stop and go) and it's all good.

I hope I'm wrong, but I suspect it's the HG.
I'll do a compression test. a HG wouldnt bother me if I was sure that was the issue.. I just hate to spend a few k not solving a problem!
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#8 Old 05-05-2012
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So when it "jumps back and forth" does it gradually sweep back and forth, or is it instantaneous, i.e. 0-100 in a split second, then 100-0 in another split second?

I'm betting on the sensor or the sensor wiring.

ed
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#9 Old 05-06-2012
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Originally Posted by efair View Post
So when it "jumps back and forth" does it gradually sweep back and forth, or is it instantaneous, i.e. 0-100 in a split second, then 100-0 in another split second?

I'm betting on the sensor or the sensor wiring.

ed
Its instantaneous, Like in a split second (looks like a sound meter or something) going between 100 and 0 and then to 40, then to 0, then to 100, like crazy.

Anyone know where this temp sensor is so I can troubleshoot it/replace it? probably a smart move to try before going to a headgasket... although I'll still get around to doing a compression test, just haven't had time.
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#10 Old 05-07-2012
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Most over heating issues have a simple remedy. Usually people jump to the most complicated solutions. Thermostats rarely go bad, and a bad head gasket is rare too.

The first thing to do is examine the radiator. You find after several years on the road all sorts of trash stuck in the mesh of the cooling fins. Clean it out. You may have to take it out and down to a radiator shop to clean the core out. It isn't expensive.

The next step is to open the block and radiator drains (you did this to remove it didn't you?). Remove the old coolant and flush the system out with some clean water. Don't forget to drain the heater too. Make sure you use the proper coolant with the recommended mix of water.

While you're at it check the hoses for leaks, and if they are very old, you may have to replace them. While you have the radiator out, you'll want to check the drive belt.

The fan is thermostatically controlled. You should be able to observe its operation on a hot day. If it just doesn't come on, then you'll probably need to take it to a repair shop to have it checked out. In my experience, I have never seen one fail. They are pretty much bullet proof due to their critical nature.
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#11 Old 05-07-2012
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Mustang_Sally,
You must work on a better class of cars than I do! I've seen three dead electric cooling fans, two broken thermostats, and two broken temp sensors in my driveway over the last 10 years. Cleaning the radiator and flushing the coolant is always a good idea though -- no argument there.
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#12 Old 05-07-2012
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Originally Posted by efair View Post
Mustang_Sally,
You must work on a better class of cars than I do! I've seen three dead electric cooling fans, two broken thermostats, and two broken temp sensors in my driveway over the last 10 years. Cleaning the radiator and flushing the coolant is always a good idea though -- no argument there.
Most problems are lack of simple periodic maintenance.

Radiator caps and thermostats seldom fail since they are such simple devices. Anyway, they can be easily checked out. The thermostat dunked in some hot water, and the cap with a pressure tester.

As I seem to recall, there is a simple test for temperature sensors using a volt-ohm meter?
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#13 Old 05-08-2012
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Originally Posted by Mustang Sally View Post
Most problems are lack of simple periodic maintenance.

Radiator caps and thermostats seldom fail since they are such simple devices. Anyway, they can be easily checked out. The thermostat dunked in some hot water, and the cap with a pressure tester.

As I seem to recall, there is a simple test for temperature sensors using a volt-ohm meter?
I dont think its overheating. I drove 1 hour, raced the car in autocross, drove back, not one hint of a problem. The gauge just dances like crazy right after the temp gets to about 92. No other signs of a headgasket leak are present. I've flushed the coolant, its clean, I've checked the fan, its running. I think I may have identified the coolant sensor. When looking from the I've seartched everywhere though and can't seem to find where the temp sensor is.

efair- you mention you've seen 2 broken temp sensors "in your driveway"- either in an SLK? know where it is?
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#14 Old 05-08-2012
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No, I haven't seen any broken sensors in the SLK (VW and Volvo yes).

Idunno where the sensor is in your car, post or PM your VIN and I'll see what EPC says.

Does this help?
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#15 Old 05-09-2012
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Originally Posted by efair View Post
No, I haven't seen any broken sensors in the SLK (VW and Volvo yes).

Idunno where the sensor is in your car, post or PM your VIN and I'll see what EPC says.

Does this help?
Thats the 240, which is a different motor, right? I think I "may" have identified the temp sensor, but I dont know... I've never replaced a temp sensor in a car.

Vin is PMed.

I found two potential spots for a sensor, but I dont know which one is which. If that 240 has the same motor, then I do know which is which based on that diagram.

Thanks!
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#16 Old 05-09-2012
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When I look in EPC (Engine; section 15.120) it shows the identical diagram so yes -- item 29 here is your coolant temp sensor.

What's your second option BTW?
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#17 Old 05-09-2012
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Originally Posted by efair View Post
When I look in EPC (Engine; section 15.120) it shows the identical diagram so yes -- item 29 here is your coolant temp sensor.

What's your second option BTW?
someone tried to fix this already.. I have bare wire soldered to the terminals on the sensor. I think I'm going to just swap this bad boy out. too bad I have to tear apart EVERYTHING to get to it... ergh.

the other sensor was on the cylinder head. Not sure what its for, but sometimes i'm impressed with my own stupidity, and I know I'm going to say "duh" when someone tells me what iti s...

Thanks so much- you've been a huge help!
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Take some pics of the disassembly -- I'm sure others will be interested.
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#19 Old 05-10-2012
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Originally Posted by Arachnyd View Post
I dont think its overheating. I drove 1 hour, raced the car in autocross, drove back, not one hint of a problem. The gauge just dances like crazy right after the temp gets to about 92. No other signs of a headgasket leak are present. I've flushed the coolant, its clean, I've checked the fan, its running. I think I may have identified the coolant sensor. When looking from the I've seartched everywhere though and can't seem to find where the temp sensor is.

efair- you mention you've seen 2 broken temp sensors "in your driveway"- either in an SLK? know where it is?
If the temperature sensor were bad, you wouldn't get a reading. I assume we are talking about an analogue gauge? When you get a fluctuating reading, the problem is a poor ground or other loose wire. Check the wire's attachment to the sensor which will be some place on the engine.
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#20 Old 05-10-2012
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Originally Posted by Mustang Sally View Post
If the temperature sensor were bad, you wouldn't get a reading. I assume we are talking about an analogue gauge? When you get a fluctuating reading, the problem is a poor ground or other loose wire. Check the wire's attachment to the sensor which will be some place on the engine.
I'll jump on this this coming weekend. Seems weird that its been messed with by the previous owner though?
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