W108 Need help diagnosing bad fuel gauge reading.
 

The fuel gauge on my 1972 280SE 4.5 stopped working about a month ago. Now the gauge reads full whenever the key is on regardless of tank level. I have pulled the sending unit and checked the resistance, 5 OHMS when the float is at the top of the sender, 67 OHMS at the bottom, progression in between the two ends seems to be linier.

Given this, should I be looking at the gauge next? Am I checking the sender correctly by testing resistance on the workbench?

Thanks for any ideas.

The fuel sender gives the gauge a variable resistance to ground, thus at "full", there is a stronger ground reference and the gauge moves up the scale. If you have a wire to the sender that's chaffed somewhere and grounding against the chassis somewhere between the sender and the gauge, the gauge will read "Full".

Since you're getting a variable signal out of the sender, it's not likely to be the issue.

Unplug tank sensor ,

There are 3 terminals........G/W/31

With key ON, jumper W to 31....Reserve lamp should come ON.
If NO, then check fuse #5 .. [ spin it in the holder ]

Jumper G to 31..gauge should show full.

Jumper G to 31 , using a 50 ohm resistor...should show low fuel level.

If yes on all test , sender is the problem
If NO, then check that 31 is a good ground..........simply repeat the test using a jumper to a known good ground for terminal 31

Something I should have included in my first post is that the gauge reads empty when the sending unit is disconnected. Is this relevant?

Reserve light functions correctly.
G to 31 gauge shows full
G to 31 with 50 ohm resistor, gauge still shows full.
G to 31 with known good ground and 50 ohm resistor gauge still shows full.
G to 31 not connected gauge shows empty.

So now I am thinking it's the gauge not the sender.

Try a 100 ohm Resistor, just to be sure.........