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Coolant symbol came on although everything seems fine

4066 Views 22 Replies 4 Participants Last post by  alpac
Hi Folks,
Today was a pretty hot day over here, 94 degree. My wife took the 1999 E320 to go downtown. After about 25mns of driving in stop and go traffic the engine coolant symbol came on. She said the car never overheated, the temperature gauge went close to 90 but not more. The engine coolant symbol stayed on until she came back home. As soon as she was home I checked the car and could see that indeed the coolant symbol was on but no engine light. I stoped the car, checked if there was any error code in memory with my OBD2 reader but there was nothing. When I started the car again after only a couple of minutes not running the coolant symbol was gone and never came back even after letting the car iddle for 15mns in hot weather. There is plenty of coolant in the reservoir so it is not a low coolant problem. The fan seems to run fine so I am wondering if there is a sensor somewhere that is sending a wrong message. Any idea?
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The coolant sensor is located under the coolant expansion tank (plugged into the deep end of the white plastic part of the tank. You may have a flaky sensor, or electrical connection to the sensor. The windshield wiper tank level sensor and the coolant sensor are connected in parallel with different resistor values in series. So it is possible to have a wiring problem to cause a false sensor reading. I would replace the sensor as well as the expansion tank as the plastic of the tank deteriorates in time and develop micro cracks leaking small amounts of coolant.

See the attached for removal and replacement of the tank.

Mercedes-Benz W210 Expansion Tank Replacement (1996-03) E320, E420 | Pelican Parts DIY Maintenance Article
The coolant temp sensor is different from coolant level sensor, but this is not your problem. Once you explained what the warming message is, it is more clear that the fan controller is flaky.
check the link out. it is figure 6. The item is N76.

STAR TekInfo

The pricing (remove the part and make sure you have the right part number that matches in the following link)

It could be a loose electrical connection too. Make sure the connector is tight and clean the contacts with electrical connector cleaner, or MAF cleaner.
You could also check the fan operation by pressing BOTH AUTO buttons on the a/c control units simultaneously for 15 to 20 seconds. The fan should turn on at the highest speed.

The fan module on your E320 is mounted under the car at the left front frame rail.

If you had a MB scanner or equivalent, you could have exercised the fan unit at different speeds selectively.

And the tip for getting a better picture is to click on the "printable version" button at the right top of the diagrams. You will have the PDF version which is clearer.


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When you start the car with engine cool the fan should not turn at all (ECT < 70 degrees C and a/c off). As the engine starts warming up the fan gains speed up to the full speed. If you observe the variable fan speed behavior, the fan should be fine. As far as I can see there is nothing that monitors the speed of the fan for your car (2 wires from the controller to the fan). There are some fan versions that the fan rpm is monitored and and a fault is generated if the fan rpm doe not meet the spec., however, I do not think yours is one of them.

Have you checked the fault codes with a scanner ? There is a P0801 code regarding the suction fan controller, but I am not sure how the SAM unit detects the fan / fan controller issue, or whether it applies to our cars. Maybe when the fan is turned on to a full blast the SAM unit checks the PWM line voltage level which should be at ground level. Unfortunately I do not have access to the internal logic of the SAM.

You can try your scanner, or if you have an access to a scanner like MB Star, it would make the diagnosis much simpler, as you can read the internal codes from the SAM and the ECU, and also test the fan unit at various operational stages. My car has fans in the font of the radiator, so it is different from your car's single electrical fan. With my scanner, I could check the fan speed in percentage, and who requested the fan operation (engine coolant temp, or a/c). I can turn the fan on at different speeds so I can test the controller and the fan.

I tend to agree with you, based on what you are posting the wiring or the fan controller are the main suspects. To some degree there may be a problem with the SAM but it is difficult to diagnose this without the SDS or equivalent.

Do you observe the coolant temperature at the instrument panel or the a/c controller menu display ?
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I believe they are not two directly related issues. The stalling sounds like the CPS issue, and the CPS issue is engine heat related.

The fan error is more like the fan issue from what you are describing. The fan motors get old and tired, and do not properly respond to higher speed requests when the brushes get worn out.

I believe there is a monitoring function of the current based on the fan speed level. Higher the speed higher the current demand. So when the AUTO keys are pressed you are asking the fan to turn at maximum speed, which is probably not attained due to worn brushes.

Try driving with the EC button on the a/c controller lit (compressor off) or turn off the a/c by pressing the - button, and see if you are getting any fan errors at all. The fan speeds due to engine coolant temp are generally in stage 1 and 2 (low speed) unless you have overheating problem with the coolant.
200 psi is a lot for the low side. works out almost 14 bars which is typically what you see at the HIGH side. That is what happens when the compressor is turned off due to overfilling and when the expansion valve is fully open.

Check your sensor values through the hidden menu on the a/c controller. especially for the refrigerant pressure when the a/c is running (#7). How to do it should be in the stickies (press and hold REST button).

When the high side is so high due to overfilling, the a/c controller asks for maximum cooling to remove heat from the condenser, and to reduce the high side pressure. That is why when you turn on the a/c, you get the maximum airflow demand, which generated the fan fault. When you reduced the pressure, the airflow demand wendt lower, so the fan message did not show up. I guess you can still get the fan message by pressing the AUTO buttons simultaneously to demand highest fan speed, for testing and confirmation.

So, the demand from the refr. overfill is more likely the symptom, rather than the cause, if the fan test action confirms it.
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