Hello,

had a similar issue.

make sure you check the ground connections

the mercedes system is based on resistance between the gauge and fuel sender unit. if the ground is not good[additonal resistance in the ckt] it will not work right.
w124 has the gound for the sender left hand side looking from the back. have to rempove the covers in the trunk. remove the brown wires [on w124 all the brown are grd]and clean well, may have to resolder?

fixed mine after a new senders unit was purchsed and didnt fix the problem?

gavinbr