W108 Need help diagnosing bad fuel gauge reading.

[billhard]

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.

[Diseasel300]

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.

[Arthur Dalton]

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

[billhard]

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

Originally Posted by Diseasel300

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.

[billhard]

Originally Posted by Arthur Dalton

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

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.

[Arthur Dalton]

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