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Discussion Starter · #1 ·
What are the symptoms of a faulty or bad coolant temperature sensor ? A week ago I posted about my fan turning on and running on high for the entire drive after it reached 90 degrees ( sounds like a jet engine) and today i noticed the dash temperature display acting funny. Temp would rise to 120 and 3 seconds or less later ( I counted )it would drop back down to 80 degrees. The car wouldn't actually overheat and everything except the fan and the temp seemed normal. This whole event would take place in under 10 secs or so. But even after the car settled back down to 85-90 degrees the fan would remain on full blast. I suspect it's a sensor to do with temperature reading because of the car was truly at 120 degrees there's no way it could cool 30-40 degrees in a matter of seconds. ?
 

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How to Tell if You Have a Faulty Coolant Temperature Sensor

The coolant temperature sensor, abbreviated as CTS, in your car is an important device that enables the control unit to give alerts if the engine is overheating or if the temperature within the system is rising for some reason.

The Coolant Temperature Sensor's Function

This device works on the principle of dependence of potential difference in temperature. As the temperature of the engine changes, the potential difference output of the device also changes, and this can be measured by the engine's control unit. Thus, principally, the coolant temperature sensor is a thermistor.

The temperature of a thermistor influences its resistance in an inverse proportion. As the temperature rises, the resistance of the car's coolant drops and this in turn decreases the potential difference output. This voltage output is sent to the electronic control unit of the vehicle, which constantly measures the resistance across the car's coolant. By this continuous monitoring of coolant resistance (and the voltage output), the coolant temperature sensor sends temperature information to the car's engine control unit.

Where to Find the Coolant Temperature Sensor

Different car manufacturers install the coolant temperature sensor differently. However, it is usually in close proximity of the thermostat of the cooling system, or inside it.

There may be two temperature sensors in some vehicles, one to send information from the engine system to the control unit and another from the control unit to the dashboard. However, some vehicles dispense with the need for two temperature sensors by using just one sensor to do both tasks. If there are two sensors, one of them is the coolant temperature sensor while the other is more correctly called the coolant temperature sending unit, which sends information from the control unit to the dashboard of the car.

Symptoms of a Bad Coolant Temperature Sensor

If your vehicle starts to use a lot more gasoline than usual, or black smoke is starting to come from the exhaust pipe, these are indicators that the coolant temperature sensor in your vehicle could be defective, and needs to be replaced. If you start having trouble starting your vehicle after it has reached its normal operating temperature, this is usually a very good sign that you need to have the coolant temperature sensor checked. To confirm, you could run your vehicle through an emissions test. If everything else is in order, failure in this test should be because of a faulty coolant temperature sensor.

Another telltale sign of your coolant temperature sensor not functioning properly is if your engine is overheating frequently. This can possibly happen when the coolant is leaking, causing the temperature sensor to behave erratically.

In many vehicles, a faulty coolant temperature system will trigger a check engine light or service engine light on your car's dashboard. A quick trip to a qualified mechanic with a diagnostic computer will tell you if the check engine light has anything to do with a coolant temperature sensor or not.
 

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You are referring to the coolant temperature and not the outside/ambient temperature reading, right?
It may be a faulty coolant valve or thermostat, but you may be right in that the coolant temp sensor, as the reading moves too fast within minute. May need a session with DAS/STAR to see what faults are stored.
 
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Discussion Starter · #4 ·
How to Tell if You Have a Faulty Coolant Temperature Sensor

The coolant temperature sensor, abbreviated as CTS, in your car is an important device that enables the control unit to give alerts if the engine is overheating or if the temperature within the system is rising for some reason.

The Coolant Temperature Sensor's Function

This device works on the principle of dependence of potential difference in temperature. As the temperature of the engine changes, the potential difference output of the device also changes, and this can be measured by the engine's control unit. Thus, principally, the coolant temperature sensor is a thermistor.

The temperature of a thermistor influences its resistance in an inverse proportion. As the temperature rises, the resistance of the car's coolant drops and this in turn decreases the potential difference output. This voltage output is sent to the electronic control unit of the vehicle, which constantly measures the resistance across the car's coolant. By this continuous monitoring of coolant resistance (and the voltage output), the coolant temperature sensor sends temperature information to the car's engine control unit.

Where to Find the Coolant Temperature Sensor

Different car manufacturers install the coolant temperature sensor differently. However, it is usually in close proximity of the thermostat of the cooling system, or inside it.

There may be two temperature sensors in some vehicles, one to send information from the engine system to the control unit and another from the control unit to the dashboard. However, some vehicles dispense with the need for two temperature sensors by using just one sensor to do both tasks. If there are two sensors, one of them is the coolant temperature sensor while the other is more correctly called the coolant temperature sending unit, which sends information from the control unit to the dashboard of the car.

Symptoms of a Bad Coolant Temperature Sensor

If your vehicle starts to use a lot more gasoline than usual, or black smoke is starting to come from the exhaust pipe, these are indicators that the coolant temperature sensor in your vehicle could be defective, and needs to be replaced. If you start having trouble starting your vehicle after it has reached its normal operating temperature, this is usually a very good sign that you need to have the coolant temperature sensor checked. To confirm, you could run your vehicle through an emissions test. If everything else is in order, failure in this test should be because of a faulty coolant temperature sensor.

Another telltale sign of your coolant temperature sensor not functioning properly is if your engine is overheating frequently. This can possibly happen when the coolant is leaking, causing the temperature sensor to behave erratically.

In many vehicles, a faulty coolant temperature system will trigger a check engine light or service engine light on your car's dashboard. A quick trip to a qualified mechanic with a diagnostic computer will tell you if the check engine light has anything to do with a coolant temperature sensor or not.
You are referring to the coolant temperature and not the outside/ambient temperature reading, right?
It may be a faulty coolant valve or thermostat, but you may be right in that the coolant temp sensor, as the reading moves too fast within minute. May need a session with DAS/STAR to see what faults are stored.

Thanks for the help! I'll try to catch the display fluctuating in action, and post a video of it. It's just been really puzzling me recently because the whole A/C system works fine both cooling and heating elements. It's just the fan is really loud, and it's had my confused for about a week. I tried convincing myself it's normal, but I feel it isn't.
 

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..... It's just the fan is really loud, and it's had my confused for about a week. I tried convincing myself it's normal, but I feel it isn't.
You are right, this is not normal.
 

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You are referring to the coolant temperature and not the outside/ambient temperature reading, right?
It may be a faulty coolant valve or thermostat, but you may be right in that the coolant temp sensor, as the reading moves too fast within minute. May need a session with DAS/STAR to see what faults are stored.
wonder can these fault codes be read with the regular OBD2 code reader? or have to be STAR?!
 

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Discussion Starter · #7 ·
You are referring to the coolant temperature and not the outside/ambient temperature reading, right?
It may be a faulty coolant valve or thermostat, but you may be right in that the coolant temp sensor, as the reading moves too fast within minute. May need a session with DAS/STAR to see what faults are stored.
I was thinking maybe it could be air trapped in the cooling system ? Because I just swapped out the thermostat and you have to drain a bit of coolant to be able to replace the thermostat. I don't know the symptoms of air in the cooling system, but maybe that is also a possibility.
 

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My wifes Polo diesel had a similar issue, the temp gauge would read very high which was incorrect when i checked the engine and hose temp with an infrared thermometer, and the radiator fan would sometimes run for up to an hour when the engine was switched off. Faulty coolant temp sensor. Some cars may have more than one for different functions, come to think of it, so did my Ducati's!
Modern cars often have the radiator fan controlled by the engine ECU instead of a seperate sensor just to make matters worse! :surprise:
 

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I was thinking maybe it could be air trapped in the cooling system ? Because I just swapped out the thermostat and you have to drain a bit of coolant to be able to replace the thermostat. I don't know the symptoms of air in the cooling system, but maybe that is also a possibility.
It is a possibility there is some air in the system but each time you drive it and the engine reaches full temp, some of the air will be displaced. In my case it took probably a half dozen engine cycles to get all the air out. I filled the reservoir to the tab as spec'ed and then after driving it and letting it fully cool down I would check the level. Each time it would be down about 1/4 inch and I would bring the level back up until it stopped dropping. And during that period the temp gauge did read a little different but nothing as extreme as yours.

You may want to make sure the coolant is at the proper level in the reservoir and then watch it each time you drive to see if there is any drop in the level. If you still have strange gauge issues after you are sure all the air has been purged, then consider replacing the sensor.
 

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Could it be a problem with the front SAM?
Fan is controlled by the front SAM and the error codes as mentioned in the other thread all point to components that may be controlled by the front SAM to. Maybe some bad wiring or loose or faulty connector there?
I had more or less the same weird behaviour when the front SAM on my car was replaced. Temp gauge continuously jumping between high and low temp and random message on the MFD that the fan was not working. Gremlins finally came to rest after the car was shut down for a few hours, and they never woke up again....
Just thinking out loud.
 

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Could it be a problem with the front SAM?
Fan is controlled by the front SAM and the error codes as mentioned in the other thread all point to components that may be controlled by the front SAM to. Maybe some bad wiring or loose or faulty connector there?
I had more or less the same weird behaviour when the front SAM on my car was replaced. Temp gauge continuously jumping between high and low temp and random message on the MFD that the fan was not working. Gremlins finally came to rest after the car was shut down for a few hours, and they never woke up again....
Just thinking out loud.
It does have some characteristics of a failing SAM. And we hear more of front SAM problems than coolant temp sensor problems. A session with DAS will help to confirm. Fingers crossed it is the sensor as no children have to be sold to cover the cost of that repair (just kidding).
 

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I was thinking maybe it could be air trapped in the cooling system ? Because I just swapped out the thermostat and you have to drain a bit of coolant to be able to replace the thermostat. I don't know the symptoms of air in the cooling system, but maybe that is also a possibility.
I am not familiar with the procedure but I think those knowledgeable will chime in on how to get rid of air trapped in the cooling system.

Faulty temp sensor is still a possibility. I am not familiar if your model has one or two temp sensors.
 

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Probably not caused by the Thermostat replacement

I just replaced the thermostat housing last week and did the expected drain and refill to accomplish it.
I replaced it because I had gotten the engine too cool ODB code a few times and the replacement did fix the problem. It runs at 90C pretty consistently now.
(was running at 75 or so)
I have seen no unusual temp readings after doing the replacement, although when putting the coolant back I did initially notice that the level was high. After running it a few times it dropped back to a normal level.
Must have been some air in the system but it seems to displace it on its own,with out any other action on my part
 

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Discussion Starter · #14 ·
Earlier this week (Monday) I took the car to a Indy mercedes shop for a diagnosis using the MB software. I had multiple short circuits, and also the same fan problem. They fixed the shorts, but couldn't find any problems relating to the fan on their computer. My first guess was either coolant temp sensor, coolant itself, or old motor oil (getting close on being due for change) not properly absorbing engine heat. It's a bit odd because the fan does turn on when it reaches a certain temperature the only problem is it stays on, and on full blast. If I turn the heater to 2.5 on the dial the fan slows and returns to normal. I don't want to be melting in the car to keep the annoying turbine sound away.
 

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Earlier this week (Monday) I took the car to a Indy mercedes shop for a diagnosis using the MB software. I had multiple short circuits, and also the same fan problem. They fixed the shorts, but couldn't find any problems relating to the fan on their computer. My first guess was either coolant temp sensor, coolant itself, or old motor oil (getting close on being due for change) not properly absorbing engine heat. It's a bit odd because the fan does turn on when it reaches a certain temperature the only problem is it stays on, and on full blast. If I turn the heater to 2.5 on the dial the fan slows and returns to normal. I don't want to be melting in the car to keep the annoying turbine sound away.
Are you talking about the engine radiator fan or is it the blower fan for the HVAC for the car interior?
 

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Discussion Starter · #16 ·
Earlier this week (Monday) I took the car to a Indy mercedes shop for a diagnosis using the MB software. I had multiple short circuits, and also the same fan problem. They fixed the shorts, but couldn't find any problems relating to the fan on their computer. My first guess was either coolant temp sensor, coolant itself, or old motor oil (getting close on being due for change) not properly absorbing engine heat. It's a bit odd because the fan does turn on when it reaches a certain temperature the only problem is it stays on, and on full blast. If I turn the heater to 2.5 on the dial the fan slows and returns to normal. I don't want to be melting in the car to keep the annoying turbine sound away.
Are you talking about the engine radiator fan or is it the blower fan for the HVAC for the car interior?
The radiator fan, it turns on when the temperature goes up to 90-95 degrees Celsius or above. Before that it's not on like a turbine. Maybe the engine is running slightly hotter than usual or it's a sensor giving false readings causing the fan to kick in.
 

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Did you ever figure out what this problem was? My glk350 is doing the exact same thing after a water pump and thermostat replacement by dealer, and they can't figure out what is going on with the fan....
 
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