Mercedes W123 Fuel Sending Unit Disassembly And Cleaning Repair Guide

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Moderate

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Time Required

                          15 minutes

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Mercedes W123 fuel sending unit disassembly and cleaning
4 steps

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Introduction

What you need

Step 1

              Mercedes W123 fuel sending unit disassembly and cleaning

With the fuel sending unit removed (if you need help with this see the fuel sending unit replacement guide) you can proceed to disassemble and clean it.
Start by wiping any fuel, grime and grit from the sending unit body to make it easier for you to work with.

With the fuel sending unit removed (if you need help with this see the fuel sending unit replacement guide) you can proceed to disassemble and clean it.

Start by wiping any fuel, grime and grit from the sending unit body to make it easier for you to work with.

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Step 2

Turn the fuel sending unit over. Handle it over a container like a bucket at all times as small amounts of diesel fuel may continue to dribble out as you turn it over.
On the bottom side of the fuel sending unit there is a small nut. This nut needs to be removed with a spanner bit as shown.
Turn it until it is removed from the base of the sending unit.

Turn the fuel sending unit over. Handle it over a container like a bucket at all times as small amounts of diesel fuel may continue to dribble out as you turn it over.

On the bottom side of the fuel sending unit there is a small nut. This nut needs to be removed with a spanner bit as shown.

Turn it until it is removed from the base of the sending unit.

Step 3

Turn the unit back over and lift the top out of the tube that makes up the body of the unit.
There are two small plates (one plastic and one metal) behind the nut you removed. Mind the order of these. The metal plate is on the outside and the plastic is on the inside. Also note the orientation of the plastic plate. There is an inside face, and outside face.
Inside you will find the float and the three wires that make the sending unit work. You can see the three wires pictured here.
Inspect these wires carefully. If any of them is broken they will need to be repaired by soldering them back on to their contact points.
If these wires are intact, proceed to clean the internal components (wires, float, etc) and the inside of the tube with brake cleaner.
Be very careful when handling the insides of the sending unit as the three wires are very fragile!

Turn the unit back over and lift the top out of the tube that makes up the body of the unit.

There are two small plates (one plastic and one metal) behind the nut you removed. Mind the order of these. The metal plate is on the outside and the plastic is on the inside. Also note the orientation of the plastic plate. There is an inside face, and outside face.

Inside you will find the float and the three wires that make the sending unit work. You can see the three wires pictured here.

Inspect these wires carefully. If any of them is broken they will need to be repaired by soldering them back on to their contact points.

If these wires are intact, proceed to clean the internal components (wires, float, etc) and the inside of the tube with brake cleaner.

Be very careful when handling the insides of the sending unit as the three wires are very fragile!

Step 4

Wait until the components are dry of brake cleaner, then re-assemble it.
Consider replacing the seal at the top of the unit, just above the threaded portion. A good seal is needed to prevent diesel aromas from permeating the trunk and/or cabin.
When done re-install the sending unit.

Wait until the components are dry of brake cleaner, then re-assemble it.

Consider replacing the seal at the top of the unit, just above the threaded portion. A good seal is needed to prevent diesel aromas from permeating the trunk and/or cabin.

When done re-install the sending unit.

To reassemble your device, follow these instructions in reverse order.

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                                                                                      10 other people completed this guide.

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                    Nicolas Siemsen

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eviestar50 - Aug 15, 2016

awesome thank you, worked out fine and easy.

nourishlight3 - Sep 21, 2016

2.6 spanner is NOT correct. You will need a # 10 spanner bit to do the job. Otherwise, all other info is correct, and very helpful!

vwnate1 - Nov 2, 2016

For the tiny nut on the bottom I simply gently use some small Water Pump Pliers….

-Nate

kieron walsh - Aug 18, 2018

I have a Mercedes 409d I removed the sender unit and cleaned it,but broke the wires,is there definitely 3 wires to get soldered

Regards Kieron

Nicolas Siemsen - Aug 28, 2018

Hi Kieron, I cannot say with certainty that this part is the same in the 409D. As you can see, this guide was done on, and written for, the W123 diesel. However, I can say with certainty that there are 3 wires on the W123 sending unit as pictured.

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