Gary makes a good point and you could start there. I have an Eco-Cruise on my '98 C8.3 powered Gulfstream and it worked fine except for one day...
I had done my usual check before going on a long-weekend trip. SOAB, the cruise wouldn't work! What was going on?
I figured that the Eco-Criuse had died and I was SOL as they have long since gone belly-up...but wait...there's more...
I got back home and decided to degrease the engine and CAC (ongoing issue from the factory slobber tube routing). After I got finished, I took it out to the station to top off the tank and whaddyaknow, the cruise was working again.
I finally determined that when I was doing my usual checks, I had lubed the Pac Brake, lubed the throttle linkage... and apparently got too much on there so the cruise couldn't hold the cable. The degreaser took off just enough to make it work again.
I promptly degreased and carefully re-lubed the entire linkage and have not had any more issues.
Oh, the "green light" on my Eco-Cruise panel does not illuminate unless I am driving with the lights on AND engage the cruise. It then illuminates. If I cancel the cruise, it goes out irrespective of the headlights...and it might light up during the day but I can't tell and it might not be a good idea for ME to take my eyes off the road while driving the rig...