Yup. The switches are not the problem. Well, I should qualify that. All of the switches work on just one pair of wires. If the "on / off" switch causes the green "cruise" light on the dash to turn on, you know the wires including the "clock spring" under the steering wheel are okay. Each switch feeds a different voltage to the Engine Computer so it is indeed possible the "set" switch is defective, but the chances of that are very small.
First of all, try holding the brake pedal up with your foot when you press the set button. If that works, the switch is out-of-adjustment or has a bad contact. Next, find the throttle cable on the engine coming from the servo. Tug on that cable. It should pull out maybe half an inch. If it pulls out a lot more, it is broken or disconnected in the servo. That's not too common either.
The best plan is to find someone with a Chrysler DRB3 scanner. Besides the dealer, a lot of independent shops have them because they can be used on many other brands of cars built after 1996. The display will show every switch related to the cruise control and whether they are "pressed" or "released". If you see "Brake pedal: pressed" when it really isn't, the contacts in that part of the switch are pitted. You can test every switch in the system while watching the display.
Next, it will list the "reason for last cutout". Obviously, since you stopped the engine when you got to the shop, the last reason shown will be "ignition switch", meaning it was turned off. If you leave the engine running, the reason will be "speed below minimum threshold" meaning below about 35 mph. The idea is to drive at highway speed, turn the system on, press the "set" button, THEN see which reason is listed.
You can also check for stored diagnostic fault codes. I autopsied a defective servo once and found a broken solder connection. It worked fine once it was repaired, but I found it quickly because the Engine Computer detected it and set the appropriate code. The computer will disable the system because that valve could be the one that vents the vacuum when commanded and if it's not working, the system won't disengage when you tap the brake pedal.
Friday, May 6th, 2011 AT 3:14 AM