Cooling fan troubles - Mercedes Benz SLK Forum

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#1 Old 01-22-2013
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Cooling fan troubles

Hello everyone.
I have been having a radiator fan issue, where the fan does not turn on under normal temps, but does turn one with ac/defroster turned on. When it is on under these conditions it is extremely weak and does not pull enough air to keep from overheating. Originally i though it was the motor, but after checking, it is not.

I have powered the fan motor directly from 12v, and it works great and fan speed is proportional to current supplied.

I then traced the wires back to the fan relay module; as i have heard it can cause problems similar to the k40. I resoldered some iffy joints and slapped her back together with no change.

I pulled the temp sensor and cleaned it off with no change, but it could still be bad.

I guess my next cheapest step would be to get a new coolant temp sensor, but im not sure if this is the switch or the gauge sensor, or if there even is a switch in the plumbing or if the switching is triggered by an ecu setting.
Or i can assume the relay module is bad and get a new used one off the ebay or a junker.

Im looking for tips on next direction to take, or maybe there is a way to test the function of the switch/sensors like resistances/voltage testing while in heated water? Is there a way to test the relay module? Which pins should i focus on?

Sorry for the lengthy post, and thanks for any advice/help.
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#2 Old 01-23-2013
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Does the fan move at all. My car gets to about 90 on the temp gauge before the fan moves slowly. No one seemed to know if that is normal. With the AC on, it moves quicker. I was going to replace the relay, but have read that 90 is nothing to worry about. Regardless, it sounds like we have the same problem. How hot does yours get? I have never seen mine much above 90.

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#3 Old 01-23-2013
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He mentions it runs but poorly...and is running with voltage applied directly (is there a difference between these two by the way?)

Is the temp ok while running without the AC on? In stop and go traffic? That would indicate that the rad is doing its job (but you may want to give it a good flush, and blow out the grill work (bugs, crap etc).

Temp sender would be a good place to start and relatively cheap.

I think your approach is sound but as you know there are a number of links in sorting this out.


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#4 Old 01-23-2013
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With 12v 6A applied the fan moves an ish-ton of air. Im not adventurous enough to let the car get up to 90+ to see what happens, although im sure it was there a few times last summer.

The fan with a/c on or off in stop and go traffic will overheat within about 10-15 minutes. With the defroster on ful blast it will slow it down enough to get pretty much anywhere, but im not cool with this scenario during the summer.

Im going to try the temp sender, then ill go for another module. If that doesn't work ill just use a different fan/motor with any external relay system from the aftermarket.
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#5 Old 01-23-2013
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What are you calling overheating? 90 is not overheating, it is about 1/8th of the dial up from the middle. When you say it overheats, I am assuming it gets to the 120 at the top of the dial. Is it just going a little higher than normal? Did it used to stay solid at 80?

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#6 Old 02-15-2013
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fan doesnt work at all and coolant line is on

Hi all, I have a SLK320 with similar issue:
1. coolant light on permanently.
2. coolant is topped up correctly.
3. fan does not turn at all ever, even when temp at 100deg on gauge, with or without aircon on.
4. car does not overheat after long hard drive, maxes at 100deg on gauge.
5. replaced fan control/relay silver finned box, but made no difference.
Any idea what I should try next ?
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#7 Old 11-27-2013
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Slk320 CEL P0115

Hi all, I have a SLK320 with similar issues described above, and updated as follows:

1. I was able to eliminate the persistent coolant light issue by replacing the auxiliary fan and the fan control/relay (the silver box). I believe the auxiliary fan solved the issue more than the relay. This also eliminated the intermittent washer fluid level light.

2. The current problem is the CEL remains on with P0115 code for "temp. sensor circuit". Have replaced sensor, with no change. The voltage check at temperature sensor connector is 5V with engine off and key on. Update: solved problem with new OEM thermostat and OEM ECT sensor. Old thermostat was apprently stuck in open position.

3. Can anyone advise if the issue may be with a one-wire coolant temperature sender? This apparently sends the temp to the dash gauge, which currently reads a consistent 50 degrees. Is there a one-wire sender? if so, where is it located? Or, does the coolant temperature sender control the dash gauge? Update: sovled with new ECT senor.


Any ideas or solutions would be appreciated.
Ro

Last edited by romaine3; 01-14-2014 at 05:41 PM. Reason: update
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#8 Old 11-27-2013
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#9 Old 07-27-2014
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#10 Old 06-12-2015
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