F 150 SPEED CONTROL

You are not alone. It is a known issue!

https://www.google.com/search?q=1989+F-150+Horn+works+with+Ignition+off%2C+but+does+not+work+with+ignition+on

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Which fuse ? There are two that supply B+ to the speed control servo ! One is located in the under hood fuse box and the other in the fuse panel inside the vehicle > The fuse may look fine but is there B+ voltage across the fuse . Check it with a volt meter . Check B+ voltage at the speed control servo , Black with yellow trace and P/O wire , what ever color that is . Is there B+ at the control switch ? The speed control servo supply's voltage to the cruise control switch . You can check it at the speed control servo as well . They could be videos on youtube showing how to test it .
Diagnose cruise control failure on 1993 Ford 150

Year make and model helps. Older vehicles have a fuse for cruise/speed control, but not for 4X4. Newer ones have a fuse for 4X4, but not cruise. If are talking speedometer/speed sensor, The PCM/ECM is what controls them.

• 3 flashes - deactivator switch is open or circuit defective.
• • Flash code 3 - deactivator switch circuit failure
  • Central junction box (CJB) fuse 13 (20A).
  • Circuitry.
  • Deactivator switch.
  • Speed control servo.
  • Go To Pinpoint Test H .
  PINPOINT TEST H: FLASH CODE 3 - DEACTIVATOR SWITCH CIRCUIT FAILURE
• CONDITIONS DETAILS/RESULTS/ACTIONS H1 CHECK THE DEACTIVATOR SWITCH CIRCUITRY
  Speed Control Servo C122
  Measure the voltage between the speed control servo C122 pin 9, circuit 307 (BK/YE), harness side and the speed control servo C122 pin 10, circuit 57 (BK), harness side.
  
  • Is the voltage greater than 10 volts?
  Yes INSTALL a new speed control servo. REFER to Speed Control Servo in this section. REPEAT the self-test.
  
  No GO to H2 . H2 CHECK DEACTIVATOR SWITCH POWER
  Deactivator Switch C1025
  Measure the voltage between the deactivator switch C1025, circuit 10 (LG/RD), harness side and ground.
  
  • Is the voltage greater than 10 volts?
  Yes GO to H3 .
  
  No REPAIR the circuit. REPEAT the self-test. H3 CHECK CIRCUIT 307 (BK/YE) FOR AN OPEN Measure the resistance between the speed control servo C122 pin 9, circuit 307 (BK/YE), harness side and the deactivator switch C1025, circuit 307 (BK/YE), harness side.
  
  • Is the resistance less than 5 ohms?
  Yes INSTALL a new deactivator switch. REFER to Speed Control Deactivator Switch in this section. REPEAT the self-test.
  
  No REPAIR the circuit. REPEAT the self-test.

Code 15 relates to a failure in the vehicle speed sensor circuit. The job of the vehicle speed sensor is to inform the computer about the speed of the vehicle so the computer can calculate engine load and prepare the engine idle speed control system while the vehicle is decelerating. On deceleration, the idle speed control system will allow additional air flow into the engine as the vehicle rolls to a stop. Usually code 15 will be accompanied by erratic speedometer readings (or no reading), erratic AT gear shifting, TCC lockup clutch instability, and intermittent or complete failure of the cruise control.

Could be a vac leak, or a dirty throttle body, or a faulty idle air control valve. The computer controls idle speed by regulating the amount of air into the intake. Could also be something pulling on the throttle cable such as the cruise control.

This is likely due to a defective blower motor speed control resistor. The part is located near the blower motor.

bad blower motor and a bad fan switch

if it an electronic trany check the speed senor control mod. either on trany or rear end probly on rear end thewy cost about btwenty bucks. berrydalewolf

yes boot coil ,spark plug whitch ever needs to be repair to stop idle air control code idle air control is trying to compensate and con not