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Coolant Temperature Sensor Replacing
 
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Pelican Technical Article:

Coolant Temperature Sensor Replacing

Nick Czerula

Time:

2 hours2 hrs

Tab:

$50

Talent:

****

Tools:

10mm socket, flathead screwdriver

Applicable Models:

Mercedes-Benz W211 (2003-09)

Parts Required:

Engine coolant sensor

Hot Tip:

Work with a cool engine

Performance Gain:

Remedy a check engine light or incorrect engine temp gauge

Complementary Modification:

Replace coolant at same time

The coolant temperature sensor on Mercedes-Benz W211 models with a 6-cylinder engine is mounted in the coolant housing in the center of the engine, next to the thermostat. It monitors the engine coolant on the engine side of the thermostat. This provides an accurate reading of both banks of the engine. The sensor is a negative temperature coefficient (NTC) sensor. This means as engine coolant temperature increases, sensor internal resistance decreases, causing a drop in sensor voltage. The engine coolant temperature sensor is responsible for sending the temperature of the engine to the DME and then shared with the instrument cluster via bus signal. This data is used in conjunction with other signals to determine the correct engine timing, fuel injector pulse along with other adaptable features. It is the main sensor used to operate the electronic cooling fan.

It is a two-wire sensor. One wire is the reference voltage monitored by the DME to determine coolant temperature. The other a shared ground wire, shared with the engine oil sensor and the engine pressure sensor. If you have a coolant sensor fault code stored in your DME and your engine isn't running right, I suggest replacing the sensor before digging too deep. You may also have an inaccurate temperature gauge in your instrument cluster. This can also be from a faulty sensor.

Remember that your car may have been serviced before and had parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve, as they grow older. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

In this article, I'll go over the steps involved with replacing the engine coolant temp sensor on W211 models with a 6-cylinder engine.
Figure 1

In this article, I'll go over the steps involved with replacing the engine coolant temp sensor on W211 models with a 6-cylinder engine. Be sure to work with a cool engine. The coolant temp sensor is located at the center of the engine, below the secondary air pump (red arrow). Place a drain pan under the center of your engine. You can drain the coolant. However I find very little leaks out during the procedure.

Working at the engine cover (red arrow), pull off the two front sir duct hoses (green arrows).
Figure 2

Working at the engine cover (red arrow), pull off the two front sir duct hoses (green arrows).

To detach the ducts, pull them straight off the engine cover/air filter housing.
Figure 3

To detach the ducts, pull them straight off the engine cover/air filter housing. Then pull the front of the duct out of the radiator support and remove it from the engine. Repeat this step for each duct.

Lift up and remove the front engine cover.
Figure 4

Lift up and remove the front engine cover.

Then, pull the engine cover/air filter housing straight up to remove.
Figure 5

Then, pull the engine cover/air filter housing straight up to remove. Four metal clips that grab onto rubber mounts hold on the cover. The front two clips are shown (red arrows). The rear of the cover has two as well. Once detached, remove the engine cover/air filter housing from the engine.

Remove the secondary air pump.
Figure 6

Remove the secondary air pump. This makes it easier to get to the sensor. I have been able to replace a few without removing the pump.

Working at the coolant temperature sensor, press the release tab (red arrow) and pull the electrical connector straight off to remove it.
Figure 7

Working at the coolant temperature sensor, press the release tab (red arrow) and pull the electrical connector straight off to remove it.

Clean the area surrounding the coolant sensor.
Figure 8

Clean the area surrounding the coolant sensor. I find this area can become quite corroded. You want to prevent dirt from entering the engine once the sensor is removed. I use a flathead screwdriver wrapped in a rag (red arrow) and soapy water spray.

Once clean, use a small flathead screwdriver and remove the coolant sensor retaining clip (red arrow).
Figure 9

Once clean, use a small flathead screwdriver and remove the coolant sensor retaining clip (red arrow). Lever the clip straight up. Then pull off the sensor.

This image shows the sensor, viewed from above.
Figure 10

This image shows the sensor, viewed from above. Pull the sensor straight out of the coolant housing, toward the radiator (red arrow), to remove it. If it doesn't budge, spray the surrounding area with penetrating oil.

Here's the tricky part.
Figure 11

Here's the tricky part. I am located in a salt belt, so these sensors are almost always stuck in place. The red arrow points to the sensor, showing it broken from trying to remove it. This is likely to happen to you. Not to worry, it is not that bad to extract the broken sensor. Once the sensor breaks, pull the front electrical portion off. Then use a small pick to remove the center electrical terminals from the sensor and the surrounding plastic. When removing the sensor, be very careful not to damage or mar the O-ring sealing surface in the housing. If you do, the O-ring will not seal again, causing a leak.

Once you have the center of the sensor semi-hollow, use a pick, and gently bend the sensor body toward the center (red arrow).
Figure 12

Once you have the center of the sensor semi-hollow, use a pick, and gently bend the sensor body toward the center (red arrow). Spray a bit more penetrating oil. Then lever the sensor out gently, using a small pick.

Once you have it free, slide the sensor out of the coolant housing.
Figure 13

Once you have it free, slide the sensor out of the coolant housing. Be sure the sensor body (red arrow) and sealing O-ring (green arrow) both come out. If they do not, remove the thermostat and either push the sensor body from the inside or remove them from the housing through the thermostat bore. See our tech article on thermostat replacing, if needed.

Once you have the sensor out, clean the mounting bore thoroughly.
Figure 14

Once you have the sensor out, clean the mounting bore thoroughly. Install a new sensor with O-ring. Then install the retaining clip. Top up and bleed the cooling system.








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Page last updated: Fri 12/9/2016 02:44:26 AM