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Pelican Technical Article:
Mercedes Coolant Temperature Sensor Replacement

Steve Vernon
 
Time: 2 hour
Tab: $10
Talent:  
Tools:
E10, E12 Torx driver, flathead screwdriver, needle-nose pliers
Applicable Models:
C320 (1998-2004)
E320 (1998-2004)
S320 (1998-2005)
ML320 (1998-2003)
CLK320 (1998-2004)
SLK320 (1998-2003)
Parts Required:
New coolant temperature sensor
Hot Tip:
Don’t work on a hot engine
Performance Gain:
Piece of mind on engine temperature
Complementary Modification:
New belt
 
   

   

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Click here to order parts for your Mercedes-Benz from our parts catalog.
   
     Replacing the coolant temperature sensor is a relatively easy job if you have a lot of patience. The sensor itself is located below the air pump and between the thermostat and coolant pump housing. The sensor is actually held in place by a small clip that sits in a channel created by both the thermostat on one side and coolant pump on the other. There is very little room to work in there and if you find yourself frustrated by the lack of space, take an extra few minutes and remove or loosen the air pump to give yourself more room.

     If you are replacing your thermostat it may be a good idea to swap out the sensor while you have it off as it is much easier to do then and the sensor is not very expensive.

     Begin by disconnecting the ground strap from the battery. I like to do this before I work on any electrical component on the car. Let the car cool so you don’t have to work around a hot engine.

     You will need to remove the two air inlet ducts as well as the front engine cover to get access to the coolant temperature sensor on the front of the engine. Remove each duct by compressing them towards the engine and slipping them off the air inlet. With the ducts off remove the front engine cover. It pulls up and away from the engine. The cover is held on by five clips and will easily come off with hand pressure.

     Next use an E10 (reverse Torx socket) and driver. Locate the tensioner and using the Torx driver turn the nut on the wheel counter-clockwise 45 degrees. You can now slip the belt off the tensioner. There is a picture below that shows you the routing of the belt that you can use when you reinstall the belt.

     You can also lock the tensioner into the open position if you need both hands to install the new belt. Turn the tensioner all the way counter-clockwise and insert a retaining pin between the rotating part and the tensioner base. If you do not have a retaining pin you can use a 5mm Allen key.

     You can now get access to the temperature sensor behind the idler pulley. Squeeze the electrical connector and remove it from the sensor. Use a flathead screw driver and slide the clip that holds the sensor in place up towards the air pump, you will not be able to pull the clip all the way straight up and out with out removing the air pump.

     Now you will be able to see the groove that is formed by the thermostat housing and coolant pump that the clip sits in. Slide the clip off to the side and pull the sensor out from the engine. The old gasket may stay in the engine; make sure you remove it before installing the new sensor.

     Installation is the reverse of removal. Remember if there area is too tight for you to work in, or you are getting really frustrated just take a few minute and remove the air pump (please see our article on how to remove your air pump). Take care to make sure the belt is properly seated on all the pulleys, follows the right path. When reinstalling it, we have provided a picture below to help you.
Make sure you disconnect the ground terminal (red arrow) from the battery before you begin this project.
Figure 1
Make sure you disconnect the ground terminal (red arrow) from the battery before you begin this project.
Large Image | Extra-Large Image
You will need to remove the two air inlet ducts (red arrows) as well as the front engine cover (yellow arrow) to get access to the air filter housing on top of the engine.
Figure 2
You will need to remove the two air inlet ducts (red arrows) as well as the front engine cover (yellow arrow) to get access to the air filter housing on top of the engine.
Large Image | Extra-Large Image
Remove each duct by compressing them towards the engine (red arrow) and slipping them off the air inlet.
Figure 3
Remove each duct by compressing them towards the engine (red arrow) and slipping them off the air inlet.
Large Image | Extra-Large Image
With the ducts off remove the front engine cover.
Figure 4
With the ducts off remove the front engine cover. It pulls up and away from the engine (yellow arrow).
Large Image | Extra-Large Image
The cover is held on by five clips (red arrows) and will easily come off with hand pressure.
Figure 5
The cover is held on by five clips (red arrows) and will easily come off with hand pressure.
Large Image | Extra-Large Image
Locate the tensioner and using the Torx driver turn it counter-clockwise 45 degrees.
Figure 6
Locate the tensioner and using the Torx driver turn it counter-clockwise 45 degrees. You can now slip the belt off the tensioner and install the new one.
Large Image | Extra-Large Image
You can also lock the tensioner into the open position if you need both hands to install the new belt.
Figure 7
You can also lock the tensioner into the open position if you need both hands to install the new belt. Turn the tensioner all the way counter-clockwise and insert a retaining pin between the rotating part and the tensioner base. If you do not have a retaining pin you can use a 5mm Allen.
Large Image | Extra-Large Image
This is a picture that shows you the routing of the V-belt; the tensioner is shown by the red arrow.
Figure 8
This is a picture that shows you the routing of the V-belt; the tensioner is shown by the red arrow. Note: the front of the car has been removed to give you a better view.
Large Image | Extra-Large Image
You can now get access to the temperature sensor behind the idler pulley.
Figure 9
You can now get access to the temperature sensor behind the idler pulley. Squeeze the electrical connector (red arrow) and remove it from the sensor.
Large Image | Extra-Large Image
Use a flathead screw driver and slide the clip (red arrow) that holds the sensor in place up towards the air pump, you will not be able to pull the clip all the way straight up and out with out removing the air pump.
Figure 10
Use a flathead screw driver and slide the clip (red arrow) that holds the sensor in place up towards the air pump, you will not be able to pull the clip all the way straight up and out with out removing the air pump.
Large Image | Extra-Large Image
You will be able to see the groove that is formed by the thermostat housing (yellow arrow) and coolant pump that the clip (red arrow) sits in.
Figure 11
You will be able to see the groove that is formed by the thermostat housing (yellow arrow) and coolant pump that the clip (red arrow) sits in.
Large Image | Extra-Large Image
Slide the clip off to the side and pull the sensor out from the engine.
Figure 12
Slide the clip off to the side and pull the sensor out from the engine.
Large Image | Extra-Large Image
The old gasket (red arrow) may stay in the engine; make sure you remove it before installing the new sensor.
Figure 13
The old gasket (red arrow) may stay in the engine; make sure you remove it before installing the new sensor.
Large Image | Extra-Large Image
This photo illustrates where the old gasket (red arrow) should have been on the sensor.
Figure 14
This photo illustrates where the old gasket (red arrow) should have been on the sensor. The new sensor will come complete with a new gasket. Installation is the reverse of removal
Large Image | Extra-Large Image
Need to buy parts for this project?
Click here to order parts for your Mercedes-Benz from our parts catalog.
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Comments and Suggestions:
jk Comments: Can you tell me if you need to drain off any fluid prior to performing this repair
July 28, 2014
  Followup from the Pelican Staff: Yes, I would think the coolant has to be drained. - Nick at Pelican Parts  
Bob Comments: @Nick
I did mention the issue takes place only during hot days not cold below 75F ones. It is function of the outside temperature. If Fuel trim would be faulty this would be independent of the temperature please see op. Thank you for your continuous feedback!!!!
June 11, 2014
  Followup from the Pelican Staff: Fuel trim is constant at all temperatures. However, a faulty component that could show up at certain temperatures can affect fuel trim. - Nick at Pelican Parts  
Bob Comments: @Nick
I'll do tht but if it is the fuel trim wouldn't be the issue repeating independent of the outside weather? For example today we had about 78F and the car was running great, yesterday at 89F it barely picked up. Also, I could hear an odd noise like truck's hydraulic brakes when I pressed the brake. Brake works fine except when in reverse when I need to apply more power to stop the car not sure if it is related but following the thought about the vacuum leak.
June 8, 2014
  Followup from the Pelican Staff: You mentioned the vehicle will not accelerate. Fuel trim will help to determine what the fuel system is doing. ie: is there good fuel delivery, is the exhaust restricted, is the MAF working, etc. - Nick at Pelican Parts  
Bob Comments: Probably not the best place to ask this but you are my only hope at this time.
My 2004 C240 4Matic has major problems accelerating from a stop. It does pick up relatively well considering the anemic engine on it when the weather is cold, but when is hot outside it is a matter of luck and patience to get it going the hotter outside the slower the car. Once it goes above 30 miles the acceleration works a little better meaning that it only has a one second lag between the time you press the pedal and when it actually accelerates. I have no CEL on or any codes MAF and tranny fluid are new same behavior trigger replacing the two.
Yes, I know it is a toy engine, but when I say slow acceleration it means the car starts as slow as one person would push it up to 10 mph then regardless if you press the gas 1/2 or floor it.

Any idea what could cause this odd behavior?

Thank you in advance for all your feedback!

P.S. Idling is a bit erratic but not too much more like when the gas is not a quality one I'll check today for vacuum leaks if in addition to any other ideas on where to look for the issue, you guys have any idea which vacuum hoses to focus on this engine that would be great
June 6, 2014
  Followup from the Pelican Staff: I would use a Mercedes-Benz scan tool and check fuel trim. If it is a fuel system issue, fuel trim will be skewed and provide valuable data when diagnosing. - Nick at Pelican Parts  
oldsinner111 Comments: dear Sir: is there anyway to get the wire and plug portion that plugs into temp. sender .
January 18, 2014
  Followup from the Pelican Staff: Give our parts specialists a call at 1-888-280-7799. They can help you find the right part.
- Nick at Pelican Parts
 
Eric Comments: I have a 98 e320 that I have to let warm for at least 25 min. Before the temperature is over 80 before I can drive otherwise my trans will not shift properly. Would a bad sensor effect this

August 22, 2013
  Followup from the Pelican Staff: Sounds to me like the fluid level might be a little low and you have to wait for the fluid to expand before it starts working. When the engine is cold does the transmission remain in neutral or does it slip? Try changing the fluid and filter first and see how it shifts. These units do have a conductor plate that goes bad but it usually sets speed sensor codes. You may want to have you mechanic pull codes out of the transmission control unit. - Kerry at Pelican Parts  
Ilen Comments: Forgot to mention I have a 99 c230 kompressor, similar location and procedure?
August 14, 2013
  Followup from the Pelican Staff: The procedure is similar but the location is different. It is a different engine. - Kerry at Pelican Parts  
Ilen Comments: If I don't have a Check Engine Light to replace this sensor, is there any point of doing this replacement performance wise or am I just looking for an excuse to work on my car? Thanks
August 14, 2013
  Followup from the Pelican Staff: If it ain't broke,... I'm fairly certain there are many more ways to provide the service that you vehicle actually needs. Look at the maintenance booklet in your car and correlate it with the mileage on your vehicle. - Kerry at Pelican Parts  

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  Applies to: C320 (1998-2004), E320 (1998-2004), S320 (1998-2005), ML320 (1998-2003), CLK320 (1998-2004), SLK320 (1998-2003)
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