How to change the coolant temperature sensor

Tiny
LBERNADEL
  • MEMBER
  • 2002 MERCEDES BENZ ML500
  • V8
  • AWD
  • AUTOMATIC
  • 140,000 MILES
First, a location of that sensor would be great. Temperature needle sits between 90-100. It also creeps up when the AC is on. The fan does come on with the AC, so I do not think the fan clutch is a problem.

So I wanted to start with the sensor, unless you think otherwise. Please provide as much details as possible.

Thank you
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Thursday, June 15th, 2017 AT 1:50 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
First be aware that temperature sensors have an extremely low failure rate because there is just one component inside them. The circuits have more trouble with corroded connector terminals and cut or bare wires, but that would be detected and a diagnostic fault code would be set.

I assume your 90 - 100 degrees is Celsius. That would be the normal range. If your gauge is going too high, that is being reported by a properly-working sensor doing what it is supposed to do. Replacing the sensor will not fix the overheating problem. If you hear your electric radiator fan turn on, that proves the entire circuit, including the sensor, is working.

Also, most engines of this time period used two coolant temperature sensors. The single-wire sensor is for the dash gauge and has nothing to do with the radiator fan. The fact your gauge is working proves that sensor is okay. The engine computer uses a two-wire sensor. It is that one the computer looks at to know when to turn the fan on. It also has a very low failure rate.

In later years most vehicles stopped using the single-wire sensor for the dash gauge. Instead, the instrument cluster gets its information from the engine computer. Regardless if there is one or two sensors, they are usually on or right next to the thermostat housing on most engines. There are some variations, but the thermostat housing is most commonly at the engine end of the upper radiator hose.

To test the fan system on most cars, just unplug the two-wire coolant temperature sensor while the ignition switch is on. That will set a diagnostic fault code, but since the computer recognizes a problem with the circuit, it will turn the fan on by default in case the engine is too hot.

To add to the confusion, your car should have two fans. Just because you hear one turn on when the air conditioning is turned on, do not assume that is for the radiator. When there are two fans, usually one is for the radiator and the other is for the AC condenser.
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Friday, June 16th, 2017 AT 2:08 AM
Tiny
LBERNADEL
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Thank you for your reply. I can certainly confirm that I have a single fan ; definitely not the double fan system. There is a slight whining sound at about 30-45 seconds intervals when the fan is on with the AC. So not sure what that means, and could be related to fan speed -- not fast enough?
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Friday, June 16th, 2017 AT 11:04 AM
Tiny
CARADIODOC
  • EXPERT
I don't know enough about your car to know if the fan runs at multiple speeds. 30 -45 seconds is about the typical length of time a fan will turn on and cycle off, but it may run continuously when the AC is on.

If your only symptom is the dash gauge is going slightly higher than normal, that may be because of the hotter weather, but also consider that given the age of your car, the cooling fins on the radiator might be corroded to the point they can't give up their heat to the air efficiently. A clue to that would be the temperature gauge would come down when you run the heater on one of the higher fan speeds for a while.
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Friday, June 16th, 2017 AT 3:54 PM
Tiny
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  • MEMBER
I understand what you're saying, but iThe fan DOES turn on as soon as the AC is on. The whining sound is heard while the fan is spinning
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Sunday, June 18th, 2017 AT 12:35 PM
Tiny
CARADIODOC
  • EXPERT
It is common for the fan to turn on when the engine reaches a specific temperature, then cycle off, AND to run when the AC is turned on, regardless of coolant temperature. That's because it needs to cool the refrigerant. On some cars, the fan runs non-stop whenever the AC is on. On other models the fan only runs at the same time the AC compressor cycles on.

The fact you hear the fan run at times suggests it's working properly. That supports my suspicion the cooling fins on the radiator are corroded. That can be an elusive cause of running hotter than normal. One of the clues is driving at higher speeds doesn't bring the temperature down as you would expect. Natural air flow at higher speeds makes the radiator fan unnecessary. To say that a different way, if the fan is not turning on when it should, the problem will not occur at highway speed.
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Tuesday, June 20th, 2017 AT 1:08 AM

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