Main.TrailIndexPage | Fuel System | Fuel Sender Unit

Fuel Sender Unit

This component is part of the Fuel System.

Definition

The Fuel Sender Unit

  • Its technical name & common name(s)
  • part # - start year & end year
  • which area it belongs to - engine, transmission, body, injection etc, link back to the relevant section

Function

The Fuel Sender Unit transmits the Fuel level in the Fuel Tank to the Fuel Indicator.

Two types of fuel sender units are used on the Pagodas. Early 230 SLs have a sender unit with an external float barrel at the end of a rod (Part # 111 542 11 04). As the fuel level changes, the float moves up or down with the fuel level in the tank. An arm moves against a wound resistor wire and changes the fuel level reading on the instrument gage. With this type of sender unit, you get the indicator needle 'dancing' when braking hard or going round corners, as the fuel shloshes about in the tank. From Chassis # 008954 (Manual Transmission) and 013303 (Auto Transmission) on the 230 SL and all 250 SL and 280 SL, the sender unit was changed to a tube type(Part # 000 542 12 04), which is cylindrical in shape and the float is inside the tube.

Picture showing sender unit with external float:

This fuel sender unit is designed as an aluminium tube, inside of it a float that rides along a metal rod/shaft. Along this float are also two small resistance wires (actually one wire which forms a "U" in the sending unit) that contact the float.As the float descends, the resistance wire path gets longer and the resistance increases. (Resistance is ~83 W at bottom and 4 W at the top.) This is the transducer that sends the signal to the fuel gauge in the instrument panel.

When the float touches the bottom, there is a metal contact that connects two wires together. The other wire goes to the low fuel light; when it's connected to ground, the light comes on. The rod that the float runs along provides this ground.

Picture showing the later cylindrical sender unit with internal float:

Note: The fuel indicators are matched to the sender units and are NOT interchangeable.

Fuel Sender UnitFuel Indicator in Instrument PanelChassis Type
111 542 1104000 542 8003230 SL Before Chassis # 008953 (Man) 013302 (Auto)
110 542 1204000 542 8803230 SL after Chassis # 008954 (Man) 013303 (Auto), 250 SL, 280 SL

Maintenance

The fuel gauge sending unit in the tank sometimes gets gunked up in the bottom due to alge or dirt. This can cause both your gauge to read incorrectly and prevent the reserve light from coming on. Clean it, very carefully.

Disconnect The Ground Cable From The Battery! A spark around the fuel level sensor opening could cause a nasty surprise...!

Once you get the fuel sender unit loosened - Don't Pull It Out Of The Tank Too Fast! It will be filled to some level with fuel that will squirt out the tiny holes in the bottom. Hold the unit above the hole in the fuel tank until it is drained, then using a rag to catch drips, remove it to your work bench. Carefully unscrew the round plastic "nut" on the bottom of the unit and remove the nut. Slide off the alloy sleeve , being very careful not to damage the two very fine wires along which the float slides. Clean all parts very carefully, then re-assemble and reinstall. Tiny plastic "maze" at bottom is there in order to dampen fuel flow (prevent indicator from dancing). If one of the wires is broken near the bottom, re-solder it to its lug - you may have to adjust the bottom fitting upward so the wire can be re-soldered. Installation is the reverse of removal, as they say... Use fuel-proof RTV Silicon to seal the little round nut so it won't come off.

You might want to check the actuation of the little red "reserve" light on your dash, via the electrical plug. Connect the battery, turn on the key and jumper the pins on the connector at the sender unit, to see if the red light by the fuel level needle works. 06 December 2007, at 09:03 EST


Do any here have experience with the fuel indicator, mine is stuck at the bottom and it doesn't move. I tried to look at the sensor at the back but I did't know how to take it off to clean it. Can I take the sensor off without dismantling the whole fuel deposit? How? when I unscrewed it I wasn't able to pull it up, is any specific tool necessary?

Hans Strom says: the fuel level sensor can be removed out of the fuel tank for cleaning and repairs. Loosen all the small nuts, and pull the unit up. Replace the cork seal when you re-assemble. I enclose some lines by George Murphy with some comments of mine included into it: "in the W113 the float in the tank operates both the needle and the "near empty" light. The red reserve light in the dash is turned on by mechanical contacts when the float reaches the bottom of the sending unit. The fuel sender unit is designed as an aluminium tube, inside of it a float rides along a metal rod/shaft. Along this float are also two small resistance wires (actually one wire which forms a "U" in the sending unit) that contact the float. As the float descends, the resistance wire path gets longer and the resistance increases. (Resistance is ~83 W at bottom and 4 W at the top.) This is the transducer that sends the signal to the fuel gauge in the instrument panel. When the float touches the bottom, there is a metal contact that connects two wires together. The other wire goes to the low fuel light; when it's connected to ground, the light comes on. The rod that the float runs along provides this ground. The fuel gauge sending unit in the tank sometimes gets gunked up in the bottom due to algae or dirt. This can cause both your gauge to read incorrectly and prevent the reserve light from coming on. Clean it, very carefully. NOTE! Disconnect The Ground Cable From The Battery! A spark around the fuel level sensor opening cold cause a nasty surprise...! Once you get the fuel sender unit loosened - Don't Pull It Out Of The Tank Too Fast! It will be filled to some level with fuel that will squirt out the tiny holes in the bottom. Hold the unit above the hole in the fuel tank until it is drained, then using a rag to catch drips, remove it to your work bench. Carefully unscrew the round plastic "nut" on the bottom of the unit and remove the nut. Slide off the alloy sleeve, being very careful not to damage the two very fine wires along which the float slides. Clean all parts very carefully, then re-assemble and reinstall. Tiny plastic "maze" at bottom is there in order to dampen fuel flow (prevent indicator from blinking). If one of the wires is broken near the bottom, re-solder it to its lug - you may have to adjust the bottom fitting upward so the wire can be re-soldered. Installation is the reverse of removal, as they say... Use fuel-proof RTV Silicon to seal the little round nut so it won't come off. You might want to check the actuation of the little red "reserve" light on your dash, via the electrical plug. Connect the battery, turn on the key and jumper the pins on the connector at the sender unit, to see if the red light by the fuel level needle works.

Early 230 (now I checked it: up to VIN 008953) have a different fuel indicator (111 542 11 04) instead of the later one with the aluminium sleeve (111 542 12 04). The latter one seems to be more reliable. The early one is completely different in that it has an approx. 10 cm long wire arm with a buoy that swims on top of the fuel surface. You can take it out from the tank in the same way as the newer one, only make sure to be carefully enough not to damage anything. Didn't repair my one so far (was not necessary yet) but to me it seems to be easier than for the later type. Both types are still factory new available with the earlier type being slightly more expensive. Furthermore it can be that not your tank indicator unit is damaged but the fuel indicator/sensor in the instrument cluster. Of course you can disassemble this unit and take the instrument out to check it or to replace it. Unfortunately only the newer instrument (from VIN 008954 on) which goes with the later fuel indicator is still factory available but not the earlier type. You should consider this if you want to exchange your tank unit.


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