On a 91 300SE, there are maybe a dozen reasons for a check engine light. I got a check engine light for starting the car with the air temp sensor disconnected... or whatever that thing is at the snorkel of the air cleaner.
You can check the code behind the check engine light. Check the archives for the website that tells you how to check the code and how to erase the check engine light when you've fixed the problem. I went to the dealer and they were kind enough to check for me (no tools required). They were also kind enough to shield me from what they were doing.
81 300SD ... buy me