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Replacing Your Fuel Sending Unit in a E300 Diesel
 
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Pelican Technical Article:

Replacing Your Fuel Sending Unit in a E300 Diesel

Steve Vernon

Time:

2 hours2 hrs

Tab:

$150

Talent:

**

Tools:

Voltmeter, 46mm socket or large adjustable wrench, flathead screwdriver or trim removal tool

Applicable Models:

Mercedes-Benz E300 (1995)

Parts Required:

New Sending unit and O-ring

Hot Tip:

Clean around the tank opening

Performance Gain:

Knowing how much gas you have

Complementary Modification:

Clean your trunk of all the junk you have been hauling around

If you have been recently amazed at the fabulous mileage you have been getting, right up to the point that you ran out of gas with the gauge still showing full, you probably have a bad fuel level sending unit.

This article will show you how to test and replace your fuel level sending unit.

The first thing you will need to do is clean out your trunk. You are going to be removing the trunk lining between the tank and the trunk so you will need lots of room to work. Remember you are going to be working around gas, so wear eye protection, gloves and work in a well-ventilated area, away from sparks or an open flame.

There is a large panel at the rear of the trunk that protects the gas tank from everything you throw into it. The panel is held in place by two "push pull" clips. Simply and gently pry this out using your trim removal tool. The clips can be reused, just pull the center part away from the base and reinstall, then push the center back in. Once the clips are removed, pull the panel forward and down.

You will now be looking at the gas tank with the wiring harness and sending unit attached. You are going to be removing the sending unit, but first make sure the area around it is clean so no dirt or debris will get into the tank. Remove the plug from the top of the unit and inspect it for damage it or corrosion. The plug just pulls straight off the top of the unit and is held on to the tank by two retaining clips. Remove the harness from the clips as you are going to be testing the unit and you will need access to the harness outside of the trunk area.

With a 46mm socket, or a very large adjustable wrench, unscrew the flange that holds the sending unit in the tank. It is a little recessed, so if you are using a large adjustable wrench it can be tricky. After the flange is off, slowly remove the unit from the tank. If you take your time doing this, most of the gas should drain from it, but have a rag standing by just incase, so you don't get gas spilled in the trunk.

Once the fuel level sending unit is out of the car, inspect it for damage. Ours had a large crack in it.

To test whether the fault lies in the unit or the gauge, plug the harness into the unit, and turn the ignition on but do NOT start the car. If the gauge gives a reading with the unit plugged in, and immediately shows empty when you unplug it, there is a very good chance the wiring and gauge is fine and the unit is bad. The next test is to slowly turn the unit upside down while plugged in. There is a small device in the unit that changes the resistance output signal as it moves up and down inside the tube. With the ignition on but not started, stand by the rear drivers side trunk and slowly turn the tube upside down. If the fuel gauge moves erratically or not at all, it is another sign the sending unit has gone bad. For a final check, you can attach your voltmeter to the top of the unit, set it on Ohms, and turn the unit. You should see the resistance move in slow and steady increments. If it jumps around, shows nothing or doesn't change as you turn it upside down then level sending unit is definitely bad and needs to be replaced.

Replacing the unit is simple. Install the new unit along with a new O-ring into the fuel tank with the tube facing downwards. The sending unit will only fit one direction so if it doesn't, you probably have it installed wrong.

Reinstall the sealing ring and tighten it down. Plug the harness back in and set it back in its clips along the top of the tank.

Reinstall the trunk cover and the two push pull clips and you are done.

There is a divider between the trunk and the fuel tank.
Figure 1

There is a divider between the trunk and the fuel tank. It is held in place from above by two "push pull" clips (red arrow), one on each side. Remove these using your trim removal tool or a flathead screwdriver. Save these for reinstallation (insert lower right). Pull the divider forward and down to reveal the tank.

This photo illustrates the driver's side top of the fuel tank.
Figure 2

This photo illustrates the driver's side top of the fuel tank. There is a wiring harness running to the fuel level sending unit. Unplug the harness from the unit (yellow arrow) and unclip the harness from its retainers (red arrows).

Inspect the harness plug for any damage or corrosion.
Figure 3

Inspect the harness plug for any damage or corrosion.

Using a 46mm socket, or a large adjustable wrench, unscrew the retaining flange from the tank.
Figure 4

Using a 46mm socket, or a large adjustable wrench, unscrew the retaining flange from the tank.

After removing the unit from the tank, inspect it for damage.
Figure 5

After removing the unit from the tank, inspect it for damage. Over years of use, the constant vibrations can cause all kinds of damage, including cracks (yellow arrow).

You are going to be running a series of test described in the article above, so make sure you have the unit and harness outside of the trunk area.
Figure 6

You are going to be running a series of test described in the article above, so make sure you have the unit and harness outside of the trunk area. You will want to be able to turn the unit while watching the gas gauge and not worried about spilling fuel in the trunk area. I recommend you place a rag between the unit and the paint to help protect both.

Install the new unit and O-ring into the fuel tank, long end down.
Figure 7

Install the new unit and O-ring into the fuel tank, long end down. The unit will only fit one way so if you are forcing it you have it in the wrong way.

Install the sealing flange.
Figure 8

Install the sealing flange. The flange has two rubber gaskets (red arrows) to help keep the fumes and fuel inside the tank. Make sure these are in good shape before you put everything back together

Reinstall the plug, clip the harness back in and reinstall the divider and you are good to go.
Figure 9

Reinstall the plug, clip the harness back in and reinstall the divider and you are good to go.

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