Cleaning the W123 fuel sending unit

Detach the accumulator terminal. To remove the fuel tank sending unit, unscrew the two screws that hold the first aid kit body on the back shelf (closer to the back seat).

Take out the first aid kit body by lifting it from the back, where were the screws unscrewed.

Now you can get access to the fuel sending unit attached to the fuel tank. Take the three-pin plug off and place is aside.

Now use an adjustable wrench to unscrew the fuel sending unit hex-nut. The fuel sending unit itself is compacted with a rubber spacer, don’t lose it. Take it easy when removing the sending unit, let the rest of the fuel flow back into the fuel tank. On the photo you can see the fuel sending unit (with a rubber spacer), an adjustable wrench to unscrew it and a screwdriver to disassemble the sending unit.

Now unscrew the screw-nut located in the plastic socket on the bottom. You can use either an extra thin screwdriver or pliers (use them with particular care not to damage the plastic socket). Be sure not to lose the screw-nut.

After removing the big metal disc you’ll find a plastic volute.

Carefully release it (if it’s stuck) by bending it a little bit.

Unscrew the plastic volute.

Carefully take the aluminum cylinder off the fuel sending unit. Pay attention to the groove in the cylinder (a projection in the unit corresponds to it).

Here’s a larger photo:

Be careful with the stretched strings! There are three of them. The yellow one is for the fuel reserve. The gray ones are used by the float-driven potentiometer needed to determine the amount of fuel left in the tank. There’s a yellow ring on the float that makes a contact with the “fuel reserve”.

Here are the contacts of the “fuel reserve”:

Here’s a larger photo:

Here are the fuel sending unit head contacts:

And the float contacts that connect to the fuel level string:

Place the float at the middle of the sending unit and carefully release the string. Bend the float contacts towards each other.

The sending unit on these photos is clean already. Use carburetor cleaner and a toothbrush to clean it.

After disassembling the sending unit (removing the cylinder) grease all the strings, the contacts and the float with plenty of carburetor cleaner. Scrub the contacts with a toothbrush, check whether the strings are properly soldered. Carefully bend the float contacts. Take it easy otherwise the float may stick.

Do not use any abrasives (sandpaper, rasp etc.).

Don’t forget to clear the plastic volute and the big metal disc (the disc is usually covered with rust, so use fine sandpaper to clear it till you see the metal itself – here it can be used as there are no contacts on the disc).

Now check if the float can move around. Turn the sending unit upside down – the float should freely slide along the strings.

Carefully assemble the sending unit. Now start testing.

The W terminal is for the fuel reserve (connected to the corresponding indicator on the dashboard). If you hold the sending unit upside down the W terminal should connect to the sending unit body – wire identification should be performed. If the float is in any other position, a circuit break should take place.

The T terminal is the sending unit body and also one of the potentiometer contacts used to determine the fuel level.

The G terminal is the other part of the potentiometer.

The potentiometer resistance level (between the T and G terminals, used to determine the fuel level) is about 60 ohm when the float is at the bottom and about 2 ohm when it’s on the very top.

When the float is in any other position, the resistance is somewhere between 2 and 60 ohm.

Screw the sending unit into the fuel tank (don’t forget the rubber spacer).

Put on the accumulator terminal.

To check the “fuel reserve” chain start up the ignition and connected the W terminal to the T terminal with a piece of wire of with any metal item (this should be done on the plug, not on the sending unit itself). The dashboard fuel reserve indicator should be on.

Put on the plug and assemble the first aid kit body.

Good luck!

Provided by ZoMer

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6 Responses

  1. Hercu van der Walt says:

    Thank you this helped and I actually fixed it perfectly.

  2. Hany says:

    I found the wires are broken, is the wire replacement need to have certain specs, if so what is it?

  3. Oldmerc says:

    Hi Hany!
    I actually dont know, but I think you can measure the resistance of whole wire, and then try to find similar thing.

  4. Andrew says:

    You can use multimeter for resistance measurement.

  5. warren says:

    Thank You, Very Helpful,

  6. John Martin says:

    Very useful, thank you for posting. I have mine apart and in the middle of the cleaning process and doing a little re-soldering of the wires. I have reason to believe this one is going to work fine.

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