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Coolant Temperature Sensors Replacement
 
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Pelican Technical Article:

Coolant Temperature Sensors Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$10 to $25

Talent:

*

Tools:

22mm, 19mm deep socket

Applicable Models:

300E (1985-92)
260E (1987-90)
300CE (1988-89)
300TE (1988-91)
300E2.6 (1990-92)
300TE 24 (1989-92)
300CE 24 (1990-93)
300E 24 (1990-93)
300CE (1993-94)
280E (1993-94)
300E2.8 (1993-94)
E280 (1994-96)
320TE (1993-94)
E320 (1994-96)

Parts Required:

New coolant temperature sensors

Hot Tip:

Use care with the old wiring

Performance Gain:

Piece of mind on engine temperature

Complementary Modification:

Coolant flush

There are two coolant temperature sensors on the W124. The single pin sensor sends its signal to the temperature gauge in the dash while the two-pin (or in some cases four-pin) sensor sends its signal to the fuel injection management system.

Let the car cool so you don't have to work around a hot engine. While no coolant will escape the system while performing this job, it is a good time to consider giving the system a complete flush. Please see our article on coolant flush for additional assistance.

The sensors are located on the top of the thermostat housing and while they are easily accessible they are fragile and close together. The best way to perform this job is with deep sockets. Otherwise it is difficult to get a standard wrench in the space.

This photo illustrates the two sensors on the top of the thermostat housing; the red arrow indicates the sensor to the fuel injection management system while the yellow arrow indicates the sensor for the gauge cluster.
Figure 1

This photo illustrates the two sensors on the top of the thermostat housing; the red arrow indicates the sensor to the fuel injection management system while the yellow arrow indicates the sensor for the gauge cluster.

The wiring can get old and brittle so use care.
Figure 2

The wiring can get old and brittle so use care. Squeeze together the two clips (red arrows) on the connection and wiggle/ pull it straight up and off the sensor.

Using a 22mm deep socket (red arrow) really makes the job easier as it will fit right over the top of the sensor and down to the base nut, allowing easy removal.
Figure 3

Using a 22mm deep socket (red arrow) really makes the job easier as it will fit right over the top of the sensor and down to the base nut, allowing easy removal.

Place the deep socket over the sensor (red arrow) and remove the sensor from the housing.
Figure 4

Place the deep socket over the sensor (red arrow) and remove the sensor from the housing.

The sensor has a soft brass base (red arrow); do not over torque this sensor.
Figure 5

The sensor has a soft brass base (red arrow); do not over torque this sensor.

Gently squeeze together the clips on the harness for the gauge sensor (red arrow).
Figure 6

Gently squeeze together the clips on the harness for the gauge sensor (red arrow).

Wiggle/ pull the connection (red arrow) straight up and off the sensor (yellow arrow).
Figure 7

Wiggle/ pull the connection (red arrow) straight up and off the sensor (yellow arrow). Place a deep socket 19mm and remove the sensor.

The gauge sensor has a soft washer on its base (red arrow); do not over torque this sensor.
Figure 8

The gauge sensor has a soft washer on its base (red arrow); do not over torque this sensor.

Make sure to clean any dirt or debris around the sensor openings without getting anything into the coolant (yellow arrows).
Figure 9

Make sure to clean any dirt or debris around the sensor openings without getting anything into the coolant (yellow arrows). Installation is the reverse of removal.

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Comments and Suggestions:
Lee Comments: I have a 1993 300CE with about 177k on it. I recently bought it because it's cosmetically clean, and it ran fine when I test drove it. Just today, I noticed the temp gauge in the dash will peg at the "3" mark all the way at the top when the car is moving, but will come down to about 2.25 when the car is stopped and idling. It doesn't sound like the water pump has failed, and I've never seen a car show hot while moving and not as much while at idle. Any suggestions?
December 3, 2016
MBuck Comments: My 95 e300d actually has 3 temp sensors and Im having issues with the coolant temp no reading on the gauge. But mine has the sensor by the upper rad hose for the AUX fans. But under the intake at the back of the head it has a 4 pin sensor like the w210 om606 and also a 2 pin sensor. Ive replaced the 4 pin sensor with no luck on my gauge. Also my engine has reached 115c before without the AUX fans kicking on so im thinking I should replace the other two sensors?
September 19, 2016
  Followup from the Pelican Staff: Try unplugging one, does the fan come on? If so, that may the troublesome one. - Nick at Pelican Parts  
Roy P Comments: Hi Nick,
This is Roy P. ...I have a 1995 Mercedes E 300 Diesel. Once again,I have two auxiliary fans in front the Rad fan. The drivers side work with the A/C and I was surprised it also works with the Collant temp. So what controls the aux.fan on the passenger side? If there is a relay,whereabouts is this relay hidden?
September 3, 2016
  Followup from the Pelican Staff: Thanks for the model info. The fans in front of the radiator aer auxiliary fans. They are wired in parallel and should run at the same time. - Nick at Pelican Parts  
Roy P Comments: Yesterday, I installed a new coolant sensor on my Mercedes. My old sensor is OK.I found there was a lot of air in the system. After bleeding the air,thieving looks good. But, I cannot understand something. My right hand electric fan came on. This one usually comes on and cycles with the A/C. So what controls the Auxiliary fan?
August 26, 2016
  Followup from the Pelican Staff: What vehicle model and year are you working on? I can see if I have info on your system. - Nick at Pelican Parts  
Roy P Comments: I have a 1995 300E diesel.
What controls the aux. fan? Where is it located? Is there a relay in that circuit?
Thanks
Roy
August 22, 2016
  Followup from the Pelican Staff: Coolant sensor to the DME, then a relay controlled by the DME for the fan. I would grab a repair manual. It will have the wiring.

Give our parts specialists a call at 1-888-280-7799.- Nick at Pelican Parts
 
Bob Comments: Where is the coolant temperature for the 1990 Mercedes Benz 300E 2.6L located?
July 20, 2016
  Followup from the Pelican Staff: Rear left of the cylinder head. - Nick at Pelican Parts  
Reggie Comments: My 1994 e320 has 3 coolant sensors. 2 by the thermostat housing and 1 on the side of the water pump. The one on the side of the water pump has broke off. What is this sensor? I think it is a 2 prong.
Thank you
May 16, 2016
  Followup from the Pelican Staff: Two prong is likely for the DME.

Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
Manny Comments: My 1990 Mercedes 300e 2.6 124 body need a new coolant temp sensor with 80degree temp range do we have one available
March 28, 2016
  Followup from the Pelican Staff: I’m not the best with part numbers.

Give our parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
Daniel Comments: I own a 89 300TE 124 Mercedes , and it started to just die while driving I can here the fuel pumps working it fires but it won't start
June 5, 2015
  Followup from the Pelican Staff: I can't help without knowing what is missing from the system when you are trying to start it. When your engine doesn’t start you’ll want to check the basics. Check spark, fuel injector pulse and fuel pressure, volume, quality and engine compression. Are there any fault codes? Once you figure out what is missing, it will be easier to diagnose.
- Nick at Pelican Parts
 

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