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Glow Plug Relay Replacement
 
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Pelican Technical Article:

Glow Plug Relay Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$200 to $250

Talent:

**

Tools:

10mm, 8mm socket, Philips head screwdriver

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Glow plug relay

Hot Tip:

Check the fuses first

Performance Gain:

Proper glow plug operation

Complementary Modification:

Change glow plugs

The glow plug relay sensor is a very high fail item on Mercedes models from the mid-seventies until the early nineties. The relay can fail by outright not working, it can also fail by coming on while you are driving and drain your battery. Either way if you own your car long enough there is a good change you are going to have to replace the relay, and it is not cheap.

The relay works in conjunction with the glow plug coolant sensor and the dash light to determine when the glow plugs are needed and for how long. The relay turns the plugs on as well as off. There is a lot of amperage that flows through the relay so make sure you disconnect the battery before replacing the sensor.

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 relay is located in the left front corner of the engine bay and is attached to the inside of the fender (red arrow).
Figure 1

The relay is located in the left front corner of the engine bay and is attached to the inside of the fender (red arrow).

There is a high amperage line from the battery to the relay, so make sure you disconnect the ground wire (red arrow) from the battery post and place it where it cannot accidentally come in contact with the post while you are working.
Figure 2

There is a high amperage line from the battery to the relay, so make sure you disconnect the ground wire (red arrow) from the battery post and place it where it cannot accidentally come in contact with the post while you are working.

Before replacing the relay check the fuse (red arrow).
Figure 3

Before replacing the relay check the fuse (red arrow). The piece of metal across the two posts is actually the fuse; it needs to be this size to handle the amperage.

If the metal is one piece it is good, if it is burnt or broken the fuse has tripped and needs to be replaced.
Figure 4

If the metal is one piece it is good, if it is burnt or broken the fuse has tripped and needs to be replaced. Use a Philips head screwdriver and loosen the two Philips head screws that work as the posts, remove the old fuse and install a new one. Pelican sells them.

To replace the relay, pull the connection for the coolant sensor and dash light (red arrow) off the relay and check for any obvious damage or corrosion.
Figure 5

To replace the relay, pull the connection for the coolant sensor and dash light (red arrow) off the relay and check for any obvious damage or corrosion.

Next remove the connections for the glow plug wires and also check for damage or corrosion (red arrow).
Figure 6

Next remove the connections for the glow plug wires and also check for damage or corrosion (red arrow).

Use a 10mm socket and remove the power cable (red arrow).
Figure 7

Use a 10mm socket and remove the power cable (red arrow). Make sure the battery is disconnected before doing this because you are working close to the fender and if you ground the socket you run the risk of electrocuting yourself and welding the socket to the fender.

Use an 8mm socket and remove the screw(s) holding the relay to the fender.
Figure 8

Use an 8mm socket and remove the screw(s) holding the relay to the fender.

Remove the relay from the vehicle.
Figure 9

Remove the relay from the vehicle. Installation is the reverse of removal.

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Page last updated: Fri 12/9/2016 03:00:25 AM