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Rear Window Defroster Relay Replacement
 
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Pelican Technical Article:

Rear Window Defroster Relay Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$90

Talent:

**

Tools:

Philips head screwdriver driver, Mercedes tool 140598023300, 10mm wrench

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Relay

Hot Tip:

Do pull hard on the cluster

Performance Gain:

Working wipers

Complementary Modification:

Check all your gauge bulbs

If your rear window defroster stopped working because of a failed relay you are probably thinking, "Great, it's only a relay." While relays themselves are simple to replace, the location of the rear window defroster relay is interesting to say the least. If you work on cars long enough, from time to time you will find things that make you scratch your head and wonder just what were the engineers thinking when they decided on the placement of the relay. For some reason the relay is located behind the instrument cluster on the left side up under the dash. To replace it you will first need to remove the instrument cluster. There are a couple of things you need to be aware of; it is a tight fit behind the cluster so do not try and yank the cluster forward when removing it, you will need to reach your hand behind to release a few things to get some working room, also if your cluster is old and you are using the cluster removal tool make sure to make sure it is seated on the cluster and not the front plastic shield as you can pull this straight off the cluster if you do.

Make sure that you disconnect the battery ground cable before you begin as you are going to be with the cars electronics.

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.

While you do not technically need to remove the steering wheel to perform this work, I highly recommend it as it is a very tight work space. Please see our article on steering wheel removal for additional assistance.

Disconnect the negative battery cable (red arrow).
Figure 1

Disconnect the negative battery cable (red arrow). Make sure it cannot make accidental contact with the battery while working.

Remove the five screws holding the under dash panel and remove the panel, please see our article on under dash panel removal for additional information if you need it.
Figure 2

Remove the five screws holding the under dash panel and remove the panel, please see our article on under dash panel removal for additional information if you need it.

With the lower under panel removed you can reach up underneath and push the cluster forward and out from the dash.
Figure 3

With the lower under panel removed you can reach up underneath and push the cluster forward and out from the dash. I prefer to use the cluster pulling tool that Mercedes sells or you can make your own by bending a couple of long pieces of sturdy metal 90 degrees at the end.

Insert the instrument cluster removal tools between the dash and the cluster and turn them 45 degrees so they engage the rear of the housing taking care that they do not damage the material on the dash.
Figure 4

Insert the instrument cluster removal tools between the dash and the cluster and turn them 45 degrees so they engage the rear of the housing taking care that they do not damage the material on the dash. Make sure that the tools are hooking on the side of the housing and not the front glass as this gets brittle over the years with age and if you are not careful you can pull the front shield away from the housing rather than the cluster out from the dash.

Gently pull the cluster forward until you can separate the electrical connections on the left side (red arrow).
Figure 5

Gently pull the cluster forward until you can separate the electrical connections on the left side (red arrow).

Reach your hand in behind the cluster and unscrew the speedometer cable from the rear of the housing.
Figure 6

Reach your hand in behind the cluster and unscrew the speedometer cable from the rear of the housing.

You can now tilt the housing forward and use a 10mm wrench to remove the tach cable (yellow arrow) if your vehicle is equipped with one.
Figure 7

You can now tilt the housing forward and use a 10mm wrench to remove the tach cable (yellow arrow) if your vehicle is equipped with one. In this picture you can also see where the speedometer cable has been separated (red arrow).

Remove all of the other electrical connections and bulb holders and remove the cluster form the dash.
Figure 8

Remove all of the other electrical connections and bulb holders and remove the cluster form the dash.

The relay is in behind all of the sound proofing on the left side slightly under the dash (red arrow).
Figure 9

The relay is in behind all of the sound proofing on the left side slightly under the dash (red arrow).

With the sound proofing removed you can see the relay and how it is mounted to a bracket on the firewall (yellow arrow) and where the wiring connects on the bottom (red arrow).
Figure 10

With the sound proofing removed you can see the relay and how it is mounted to a bracket on the firewall (yellow arrow) and where the wiring connects on the bottom (red arrow).

Use a short handled Philips screwdriver and remove the single screw holding the relay to the mount and then disconnect the wiring (red arrow) by pulling the connector straight off.
Figure 11

Use a short handled Philips screwdriver and remove the single screw holding the relay to the mount and then disconnect the wiring (red arrow) by pulling the connector straight off.

You can now remove the relay from the dash; the new relay should come with the metal mount already attached to the top of it.
Figure 12

You can now remove the relay from the dash; the new relay should come with the metal mount already attached to the top of it. Installation is the reverse of removal.

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