I have a 2500 Ram cummins diesel. When I turn on the key, the cruise control light comes on immediately and can only be shut off by turning the key off. The cruise will not set to a speed. The vacuum operated 4 wheel drive will engage correctly and stay in until taken out. What is the problem?
It could be the brake light switch, which is like $11. Its a fairly common problem. For further troubleshooting refer to this thread with some good troubleshooting information.
Number one cause of a failed cruise control is the vacuum line running to the servo on the drivers side under the battery. Battery acid leaks down and eats a whole in it. Next check the brake light switch on the brake pedal. Seems to be the next most common failure and cheap replacement part at the auto parts store. The clock spring in the steering wheel also will go bad and is pricey. Also the cruise button on the steering wheel gets dirty contacts that can be cleaned.