Does your car have the small black box with a red LED and a button on the frewall near the battery? If you hold down the button for 4-6 seconds, the LED light will flash the fault code(s). At least that's how my 89' 300SEL worked. My CEL would come on intermittently while driving, the problem ended up being a bad Air Mass Meter potentiometer.

__________________
Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles