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Glow Plug Coolant Sensor Replacement
 
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Pelican Technical Article:

Glow Plug Coolant Sensor Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$15 to $40

Talent:

**

Tools:

19mm wrench, rags

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Glow Plug coolant sensor

Hot Tip:

Make sure to install a new crush washer

Performance Gain:

Working glow plugs

Complementary Modification:

Coolant flush

Diesel engines do not have spark plugs instead they work on the principle of self-ignition by spraying atomized fuel into a highly compressed, and therefore a highly heated, combustion chamber; this causes the fuel to self-ignite.

On cold engines the self-ignition temperature may not be attained by the compression and need some help bringing up the temperature of the compressed air. This is where the glow plug comes into play. In the older engine, the temperature is taken from a coolant temperature sensor and in newer engine it is monitored by ambient air temp sensor. Our 300TD had a glow plug coolant sensor that works with the glow plug relay and this article will show you how to replace it.

Due to the placement of the sensor under the fuel lines, I recommend draining the coolant to at least a level below the opening for the sensor. It is a very tight area to work in and there really is no way to get the senor out, install a new crush washer and sensor and install them without losing the coolant to below the opening level so just go ahead and drain the system. If the coolant is fresh be sure to drain it into a clean container and you can reuse it. Please see our article on coolant flush and refill for additional assistance.

Over the years your car may have been serviced by multiple people including previous owners and this can lead to parts replaced with different size fasteners and hardware. 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. If something is different on your vehicle please let us know and share your info to help other users. If you have any questions or comments or even a different procedure you would like to share please leave it below and please leave your vehicle information.

The sensor is located on the front left side of the block under the fuel injection lines and between the glow plugs (red arrow).
Figure 1

The sensor is located on the front left side of the block under the fuel injection lines and between the glow plugs (red arrow).

Carefully remove the wiring connection from the end of the sensor (red arrow).
Figure 2

Carefully remove the wiring connection from the end of the sensor (red arrow). Use care here as these wires have seen a long life and most are very brittle at this point.

The sensor is located close to the fuel pump and can be difficult to get loose.
Figure 3

The sensor is located close to the fuel pump and can be difficult to get loose. We had to use a 19mm socket (red arrow) to get it started and then finish removing it with a 19mm wrench. This another reason to drain the coolant.

Always install a new crush washer (red arrow) with the new sensor, even if you reinstall the old sensor.
Figure 4

Always install a new crush washer (red arrow) with the new sensor, even if you reinstall the old sensor.

While the sensor is out, check the mounting surface and give it a cleaning.
Figure 5

While the sensor is out, check the mounting surface and give it a cleaning. Make sure that the old crush washer is not stuck to the engine (red arrow). Installation is the reverse of removal. Do not forget to refill and bleed the coolant.

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