Find a mechanic with the Chrysler DRB3 scanner or their newer Starscan scanner. It will list the "reason for last cutout" for the cruise control. At first it will list "ignition switch turned off" because you turned it off when you got to the mechanic's shop. If you leave the engine running, it will list something to the effect of "speed below minimum threshold" meaning you slowed down below about 35 mph to turn into the parking lot.
You need to drive the truck at highway speed with the cruise control activated, then read that reason for last cutout when it occurs. If the ignition switch comes up again, suspect bad contacts in the switch or a wiring problem. I doubt the minimum speed will come up. That happened more on the older vehicles with speedometer cables and was caused by failing speed sensors.
The next clue has to do with the operation of the dash light. Does it turn on just when the cruise is activated and holding the speed for you, or is it also on after you tap the brake pedal? If the light normally goes off when you hit the brakes, it could be going off now because the brake light switch is out of adjustment or has pitted contacts. The scanner will list "brake pedal applied". It might help to understand that one part of the switch turns on to turn on the brake lights, but it's a different part that must remain on for the cruise to work. That part of the switch turns off when you press the brake pedal. Pitted contacts mimic pressing the brake pedal.
If the dash light only goes off when you turn the on / off switch off, suspect the clock spring under the steering wheel. That is a wound up ribbon cable in a round housing. The scanner will list "on / off switch".
Monday, January 17th, 2011 AT 6:42 PM