George Murphy, the technical editor of the Mercedes Benz Club of America, wrote an article about fixing the fuel sending units. The following is quoted from the article:
DISCONNECT THE GROUND CABLE FROM THE BATTERY. A spark around the fuel level
sensor opening could cause a nasty surprise...! (This warning mostly for
gasoline cars, but it is also wise for diesels...)
I have removed several of these units and fixed them - I use a large pair
of channel-lock pliers to unscrew the unit from the tank - they are never
very tight. Access thru the first aid kit opening in the rear window shelf.
Make sure you have less than 1/2 tank of fuel (or less.)
For station wagons, lift the cover out in
the cargo area that covers the recess for the 3rd seat (if you don't have the
third seat.) Gently pull up the "fuzzy" material that lines this recess.
Underneath you'll find the top of the fuel gauge assembly. Carefully remove the
electrical connector. I used a real big Crescent wrench to remove the sender, as
this is something like a 30 or 40mm sized "nut."
Once you get the 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 the tiny
holes in the bottom. Hold the unit above the fuel level until it is
drained, then using a rag to catch drips, remove it to your work bench.
If it's not all glommed up by algae or other such crud, gently unscrew
the nut on the bottom. Once you remove the tube that surrounds the sender,
(float) you'll find two fine wires running down each side. As you see this
you'll notice the disc on the bottom of the float that contacts the wires on the
points they're soldered to. Check this circuit for continuity when the sender is
touching the contacts. I found that one of the wires on mine was broken. I
carefully soldered it back in place, allowing my little red warning light in the
fuel gauge to work again.
Screw the sender back in place, being careful not to damage the gasket.
Reconnect the electrical connector.
PMBurg, South Africa
1991 W124 230E (2.3 litre 4cyl)
Test values immersion tube transmitter in ohms
MODEL RESISTANCE: FULL RESISTANCE: RESERVE
Sedan 1.6 ± 0.7 56.9 ± 1.9
T-Sedan 3.2 ± 0.8 52.2 ± 2.1
Testing immersion tube transmitter (removal).
Connect ohmmeter to terminal G (4) and terminal 31 (3) and measure
a. In installation position (read out reserve, float below).
b. Rotate by 180° (read out full, float at top).
Testing reserve warning contact.
Connect ohmmeter to terminal W (2) and terminal 31 (3) and measure
a. Nominal value 0 ohm in installation position.
b. Nominal value inf. ohm turned by 180°.
1. Pull coupler from immersion tube transmitter and measure resistance on
jack 3 and vehicle ground. Nominal value 0 ohms (at test value inf. ohm the
grounding line is interrupted).
2. Measure resistance on terminal G (4) and terminal 31 (3) on installed
immersion tube transmitter. Value depends on amount of fuel in the tank.
Plug coupler back on the immersion tube transmitter.
3. Pull coupler from instrument cluster and measure resistance between jack
3 and jack 15.
Nominal value Sedan and Coupe: The value measured under 2. If the value is
attained, the harness is in order (slight deviation caused by length of line
Nominal value T-Sedan and special vehicles: The value measured under 2
plus 4.7 ohms. If the value is attained the harness is in order (slight
deviation caused by length of line possible)
4. If the measured value is higher or at inf. ohm, the harness couplers (on
instrument cluster, on main harness/tail harness or on immersion tube
transmitter) are having poor contact, a dry joint, or a line is interrupted.
5. If no fault is found during test exchange indicating instrument.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".