To clarify what MB STARS posted, all M104 engines, like yours, have a throttle actuator, in which three separate functions are incorporated into one part.

The throttle actuator is also called the EA / ISC / CC module:

EA - electronic accelerator
ISC - idle speed control
CC - cruise control

Usually one of the three functions fails and you are forced to replace the whole part. It cannot be fixed.

what I would check first, though, is your brake light switch. It is about a $15 part, and it can for whatever reason render the cruise control inoperable. The throttle actuator retails for about $1,400....

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".