The clue to look for is whether it will go back to the set speed if you hit "resume". If it does, it's not losing memory, so that leaves out the power and ground wires. Suspect a brake light switch out of adjustment. To prove it, hold the pedal up with your toe while hitting "coast". When you release the button, it should hold the new current speed.
If you have to hit "Set" again for it to work, look at the "Cruise" light on the dash. If it goes off, I'd be looking at the switches themselves in the steering wheel. I did have one on an older minivan that had a cracked circuit board. That was a different design and different symptom, but it was an easy fix. The way to find this is to connect Chrysler's DRB3 diagnostic computer. It will display every function of the cruise control and whether each switch is "pressed" or "released". In addition, it will display the reason for the last cruise system cutout. This can be caused by pressing the brake pedal, turning off the power or ignition switch, or vehicle speed fell below around 35 mph. It can display this data while you're driving. If you see, for example, that it kicked out because the brake pedal was pressed, but you didn't really hit the brakes, you know it's a defective or out-of-adjustment switch. If it says it cut out because the vehicle speed was too low, but you were going 55 mph, you'd suspect a bad speed sensor. You could watch the display when you press the Coast button to see if other functions are activated. That would suggest a problem with the switches.
To my knowledge, there are no aftermarket computers, (scanners), that will read cruise control information. They deal mostly with emissions-related stuff, although more systems are being included as they figure them out. You'll get he most useful information with the DRB3. I have one. By any chance, are you near central WI?
Alternately, you can measure the resistance across the two wires coming from the switch assembly. I'd have to find the chart in the service manual. Each switch inserts a different resistance into the circuit. The engine computer determines the function you selected by reading the value of the resistance. One of those values is "On / Off". Every resistor has a tolerance. It's not impossible that the resistor for Coast is off just enough that the computer confuses it for "Off". That's why observing the "Cruise" light on the dash is an important clue. At the same time, if hitting the "Coast" button turns the cruise light on, check the switches.
Monday, December 7th, 2009 AT 4:52 AM