2000 SLK230 Fuel sender inaccurate
 

The fuel guage on this unitreads a bit under 1/2 full and the car runs out of gas.... add5 gallons and it reads 3/4 full. I assume the fuel sending unit needs replacing. Where can I find service instructions to remove the sending unit? I found it no problem but it doesn't seem to have the same retaining ring to hold it in place like domestic model cars. Any help would be appreciated.