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2005 SLK350
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Discussion Starter · #1 · (Edited)
Hi all,

I am having a REALLY annoying problem that is making me start hating my car.

So I have a 2005 SLK350 (R171) with 118k miles and a few months ago it started throwing the "Coolant visit workshop" message on the dashboard.
My mechanic scanned it and said that he could not pinpoint it to the thermostat but that I should have replaced it along with the water pump that was underperforming.
I replaced both and changed the engine coolant as well.

It all worked well for a month, the thermostat was URO brand. After one month, the same message on the dashboard popped up. I thought it could be the thermostat and I replaced it with Borg Warner. Again it all worked normally until a few weeks later I pushed the engine a bit harder for 1min, just taking off at first, second, and third gear. Right after, that same message popped up and it has been like that.

The fan seems to be working and kicking in around 96 degrees, the coolant level is above the plastic fin, wiper's fluid is topped.
The main problem is that when the dash message pops up, and that's everything the car reaches around 90 degrees, the A/C stops blowing cold air and the message intermittently pops on the screen.

This is driving me crazy. Does this look like a thermostat going bad again? Could it be something that burns or kills the thermostat?
I touched the top hose coming from the thermostat when the car was hot and the hose was hot too. if the hose were cold I guess it could be a thermostat shut problem.

Any ideas what the cause could be?


Thank you, guys!
 

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Discussion Starter · #4 ·
A little more information too. When I replaced the parts I bled the coolant by leaving the reservoir cap off and having the heat on for a while.
I thought it could be an air gap inside the system but I can't get any more bubbles out.
 

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Founding Member 2006
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low or high, since sensors windshield washer and coolant share

following from Dave2302 Jeff edit :D
*
What Jeff means is that they share the same wiring - different sensor and different warning lights. Because they share the same wiring if one goes faulty (usually the windscreen sensor) if can bring up the other warning light. Of course this doesn't happen if both are working properly

The Coolant Light is supposed to come on if you have a Cooling Fan issue or Low Coolant level, but it can come on if there is some other fault as discussed by jbanks15 such as Washer Level Sensor, Coolant Level Sensor, the shared Wiring or even an Instrument Cluster issue.

In this case as BaD_81 correctly says, if the Temperature isn't getting to circa 97 / 99 then the Fan is not programmed to cut in below that Temperature.

Fan is controlled Speed wise in percentage increments by the Fan Controller Module which selects the appropriate Speed in percentages by computing from a PWM signal sent from the Motor ECU.
The actual percentage Fan Speed PWM steps the ME sends to the Fan Controller are dependent upon exactly what temperature the ECT Sensor is sending to the ME.
For Example when the Coolant reaches the initial pre programmed 20% figure of ~ 97c if the ECU sees no reduction in ECT, or even a rise, it will step up to 40% etc etc.
****
 

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2008 SLK350
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following from Piet:
Did I write that? Uhmm...I wished I was that smart but credit due where credit is due @Dave2302....... 😁 😇

 

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2008 SLK350
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But maybe try an OEM thermostat instead of an aftermarket? Not saying it is your problem for sure but it is not your oldfashioned thermostat that opens at a certain temp and closes below a certain temp.
Thing is controlled by the ECU and depending on several variables ECU controls it with a PWM signal to open or close it a certain amount. Maybe something goes wrong there?
Read this for an overview
 

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Discussion Starter · #9 ·
Th aka you for the information.
I don’t think it could be coolant level too high. I could be wrong though.
It could be something to do with the ECU, but then why it was temporarily fixed when I replaced the Thermostat? Cause right now every single time I drive the car’s malfunction IC message pops up. Just like before when I replaced it the other thermostat.
I think I am going to test the thermostat resistance as per @Sneaky Pete documentation.
 

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2007 SLK350
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@FloridaMerc
I had the same issue and covered the fix here..

As both the washer level and coolant level use the same wire with a different resistance for each sensor, if one goes bad it can trigger an alert for the other.
It does seem that the coolant sensor goes bad most of the time but Is very easy to replace.
 

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Discussion Starter · #11 ·
@FloridaMerc
I had the same issue and covered the fix here..

As both the washer level and coolant level use the same wire with a different resistance for each sensor, if one goes bad it can trigger an alert for the other.
It does seem that the coolant sensor goes bad most of the time but Is very easy to replace.
I guess I could try replacing the sensor too.
My IC does not say low coolant nor low washer fluid though. Could this still be the case?
 

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Discussion Starter · #13 ·
You have not mention the radiator as having a flow check.
How can I check flow in the radiator?
another interesting think is that, in order to drain the old coolant I was not able to open the radiator valve. I turned it with a screw driver but I wasn’t able to open it up. Nothing came out of it. I had to disconnect one of the low hoses instead.
 

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2005 SLK55 AMG /2005 SLK350
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Without seeing the codes it's hard to say what part it is, themostat, level switch, temp sensor, etc.
You need to scan it again, preferably with SDS, and post the exact codes here. Find a mechanic with SDS if possible - accept no substitutes. There are some "almost as good" scanners that may help in this case, but a mechanic with actual SDS is going to be your best bet.
 

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Discussion Starter · #15 ·
Without seeing the codes it's hard to say what part it is, themostat, level switch, temp sensor, etc.
You need to scan it again, preferably with SDS, and post the exact codes here. Find a mechanic with SDS if possible - accept no substitutes. There are some "almost as good" scanners that may help in this case, but a mechanic with actual SDS is going to be your best bet.
I’ll take it to my mechanic to scan it this Saturday. I’ll share the codes with you.
Thank you all.
 

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**Registered From: New Mexico
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How can I check flow in the radiator?
another interesting think is that, in order to drain the old coolant I was not able to open the radiator valve. I turned it with a screw driver but I wasn’t able to open it up. Nothing came out of it. I had to disconnect one of the low hoses instead.

You shop should know how to do a flow check. We had a pail about 14 qts that you put the upper radiator hose from the radiator to and from the pail back to the radiator. Crank up the vehicle and watch the flow and check your hoses to ensure that they were not hard. If they are going bad then they would swell up and slow down the flow. Here is a link. Also I would check the temp on the liquid when it is moving to ensure that those sensor are working and not bad. This also checks the pump and thermostat.

flow check
 

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2007 SLK350
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I guess I could try replacing the sensor too.
My IC does not say low coolant nor low washer fluid though. Could this still be the case?
From what I understand, yes.
Im 99% sure that the coolant message you are getting is the message from the coolant level sensor. You can easily check this by removing the sensor from under the plastic fluid tank, leave it plugged in and start the car.
But note that the same message can be caused by a faulty washer fluid sensor.
with mine it was the coolant sensor, an easy fix and I’ve not had the alert since.
 
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Discussion Starter · #19 ·
I little update.

Replaced both coolant level and temperature sensors; drove almost 20miles then same problem again: "coolant visit workshop".

I noticed that sometimes just before the fan engages, it kinda "tries" a couple of times before fully engaging. Could it be a problem with the controller?
 

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2005 SLK55 AMG /2005 SLK350
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Have you had it on SDS yet, and if yes what codes were present? Not much sense in arguing about it until we have those. With that message on the IC there will almost certainly be codes present in SDS.
 
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