if you don't have access to a STAR SDS system to diagnose, you could check the speed sensors.
The other common cause is if you have changed or replaced tyres that have the wrong diameter.

 

The brake switch under the dash has 2 sets of contacts. One for the brake lights and I believe the other set needs to be operational for cruise to work. Easy to check and replace it needed. The brake switch I believe to be normally open, closed on brake press. I am not sure of the correct switch position for the other smaller contacts? Hope it's something easy to fix?

 

Hi and welcome to the forum! It depends on which year/engine you have. The earlier cars use a stand alone control unit located closest to the right front fender behind the false firewall.

It is very often the culprit. Visit cruisecontrolrepair.com. He does excellent work.

 

Today I noticed a tire that appeared low, while adding air, it occurred that this might very well be my problem. Low and behold, cruise control works fine now. I have much to learn about this little gem! This is my first Benz. Thanks to all who responded to my post.