You sure it's #18?
If so, using data from this site
http://pages.prodigy.net/jforgione/MB_S500.html
I'm assuming that when your CE turns on, it's because a code has been set with the Diagnostic Module. If so, looks like it is: "Adjustable camshaft timing solenoid, open/short circuit"
Other modules using the same code, so just in case it could also be: "Magnets for crankshaft position sensor (L5) not recognized"
or
"No CAN data transmission with DI control module (N1/3)"
or
"Circuit for left front lock switch (S86/1), right front (S87/1), trunk lid lock switch (S88/2): closing time exceeded"