The crusie control will turn on but the set button with not set the speed

What speed control sensor ? Explain . Vehicle speed sensor ?
Speed Control Self-Test Diagnostics
WARNING: This test is a key on engine off (KOEO) test only that is conducted in PARK only with the emergency brake fully engaged.
NOTE: A review of the following test steps is required before carrying out the Self-Test Diagnostic procedure. After entering the diagnostic mode, the module will time out if the mandatory switch sequence is not completed within 5 seconds. If a module time out occurs, the procedure must be reinitiated.

1. Enter the Self-Test Diagnostics by pressing the speed control OFF switch while turning the ignition key ON, making sure the engine does not start and is not running. Five additional flashes at this point indicate a defective speed control actuator. Release the OFF switch.

1. Press and release the remaining switches in this sequence: ON, RESUME, COAST and SET/ACCEL (for Expedition), ON RESUME, COAST and CANCEL (for Navigator).
   The speed control indicator lamp will flash after each switch is pressed and released. Press and release each switch in the sequence immediately after the indicator light goes out for the previous switch.

1. NOTE: There will be a slight delay when the last button is pressed and released and the lamp flashes.
   A lamp flash with the last button (SET/ACCEL) or CANCEL (for Navigator) indicates that the STATIC test passed. If the lamp does not flash with the last button and there are no additional flashes of the lamp, the switches are defective.
   If the lamp does not flash with the last button, and additional flashes occur, follow the chart below for trouble codes:

NOTE: If unable to enter self-test, Go To Pinpoint Test D
Flashes Source 2 Flashes BPP defective, circuit is defective, brake applied 3 Flashes Deactivator switch is open or circuit defective

1. Immediately after the STATIC test, the speed control servo does a DYNAMIC test by automatically actuating the throttle lever from 8 mm (0.315 in) to 12 mm (0.472 in) of travel from the idle position. During the DYNAMIC throttle pull, observe throttle movement to witness any binding or sticking of the speed control cable and correct connection of speed control cable to throttle lever. Make sure the throttle returns back to the idle position.

1. Return the ignition switch to the OFF position and proceed to the Symptom Chart.

Take it , it does not set . ? only sets when driving over about 35.
turned on light indicates on in cluster OK.. but will not set, right.??
Dealers go nuts with this one. give them a try.
brake switch on brake peddle arm often fail and go out of ajustment.

Next is big money unit under the hood. Cheaper to bye after market system..

check brake peddle cut out .switch .
sw has 2 functions: brake lights off when at rest and on for crusie. remove connector. check out the switch.. the sw tabs often fail. tierap helps. or replace switch $ 20. item any local parts store.
ajusted on install.

P2159 - Generic Type Powertrain - Fuel and Air Metering and Auxiliary Emission Controls - ISO/SAE Controlled Description Vehicle Speed Sensor "B" Range/Performance
THIS IS FOR YOU Kia trouble code, please replace the speed sensor B, any questions, pls ask the 24 hrs live technical support multidiag access, they will like to help you, good luck

if your car has eltronic crusie crontrol and your set buttons are on the steering wheel take test light and see if you are getting power to control module under the hood if you are getting power your module is prob. bad if not you more than likely have a bad switch. hope this helps

You just go to the link I attached I'm sure you will get the solution of your problem Best Manuals

Check the fuses and relays.

sounds like you have a vacuum leak somewhere or a faulty check valve. the air control and speed control are both vacuum operated. check for cracked vacuum hoses under the hood. The Jeeps also ran a check valve either near the intake manifold or near the a/c and heater core lines where they enter through the firewall or some had both. to check the valve, remove from the lines and try to blow through both sides. should only allow air to flow through from one side. if you remove pay attention to how it was installed so you can reinstall in the correct direction

the cruise control is run by the input and output speed sensors on your transmission these speed sensors tells the pcm vehicle speed and engine rpms this in turn adjust the cruise control module to adjust throtle via cable to the throttle body.

it could be stop light switch or the auto cruise control switch.

don't use cruise ctl