Pinpoint Test B: The Set Speed Fluctuates

Normal Operation 

The speed control actuator adjusts the speed control cable, which controls the throttle body.

This pinpoint test is intended to diagnose the following: 

• Speed control cable
• Accelerator cable
• Speed control actuator
• Base engine concern
• PCM

PINPOINT TEST B: THE SET SPEED FLUCTUATES 

1. B1 CHECK THE SPEED CONTROL CABLE/THROTTLE BODY LINKAGE 
   • Key in OFF position.
   • Visually inspect the speed control cable connections, the throttle body linkage, and the speed control actuator pulley cover.
   • Are the speed control cable connections, the throttle body linkage, and the speed control actuator pulley cover OK?  
   1. YES  : Go to  B2.
   2. NO  : INSTALL a new speed control cable or REPAIR the throttle body linkage. To INSTALL a new speed control cable, REFER to Speed Control Cable . TEST the system for normal operation. To REPAIR the throttle body linkage, REFER to FUEL SYSTEM - GENERAL INFORMATION article. TEST the system for normal operation. To REPAIR the speed control actuator pulley cover, INSTALL the cover correctly. TEST the system for normal operation.
2. B2 CHECK THE SPEEDOMETER OPERATION 
   • Drive the vehicle and observe the speedometer operation.
   • Does the speedometer needle fluctuate? 
   1. YES  : REFER to INSTRUMENT CLUSTER (IC) & WARNING CHIMES article to continue diagnosis of the speedometer.
   2. NO  : Go to  B3.
3. B3 CHECK FOR A BASE ENGINE CONCERN 
   • Drive the vehicle and determine if there is a base engine concern.
   • Is a base engine concern present? 
   1. YES  : REFER to the Introduction - Gasoline Engines article to continue diagnosis of the base engine concern.
   2. NO  : Go to  B4.
4. B4 CHECK THE VEHICLE SPEED SIGNAL VOLTAGE 
   CAUTION: Make sure the vehicle is secured correctly before attempting to place the transmission range selector lever in DRIVE (D), or in 1st gear if equipped with a manual transmission.
   • Disconnect: Speed Control Actuator C122
   • Jack up the drive wheels and chock the other wheels.
   NOTE: For vehicles equipped with a manual transmission, place the transmission into 1st gear.
   • Start the vehicle, place the transmission in DRIVE (D), and with the engine at idle, allow the drive wheels to spin.
   Fig 1: Checking Vehicle Speed Signal Voltage
   Courtesy of FORD MOTOR CO.
   • Measure and record the AC voltage between the speed control actuator C122-3, circuit VMC05 (VT/OG), harness side and ground with the vehicle drive wheels spinning a minimum of 8 km/h (5 mph).
   • Apply the brakes until the drive wheels come to a stop. Place the transmission in PARK (P), fully engage the parking brake and turn the engine off.
   • Is the AC voltage greater than 4.5 volts? 
   1. YES  : Go to  B5.
   2. NO  : Go to  B6.
5. B5 CHECK THE VEHICLE SPEED SIGNAL FREQUENCY 
   CAUTION: Make sure the vehicle is secured correctly before attempting to place the transmission range selector lever in DRIVE (D), or in 1st gear if equipped with a manual transmission.
   NOTE: For vehicles equipped with a manual transmission, place the transmission into 1st gear.
   • Start the vehicle, place the transmission in DRIVE (D), and with the engine at idle, allow the drive wheels to spin.
   Fig 2: Checking Vehicle Speed Signal Voltage
   Courtesy of FORD MOTOR CO.
   • Measure and record the frequency between the speed control actuator C122-3, circuit VMC05 (VT/OG), harness side and ground with the vehicle drive wheels spinning a minimum of 8 km/h (5 mph). Record the speedometer reading.
   • Apply the brakes until the drive wheels come to a stop. Place the transmission in PARK (P), fully engage the parking brake and turn the engine off.
   • Divide the frequency reading by 2.2 to get a mph result and compare this to the reading of the speedometer.
   • Does the frequency reading result match the speedometer reading? 
   1. YES  : INSTALL a new speed control actuator. REFER to Speed Control Actuator . TEST the system for normal operation.
   2. NO  : Go to  B6.
6. B6 CHECK FOR CORRECT PCM OPERATION 
   • Disconnect the PCM connector.
   • Check for:
     • corrosion
     • damaged pins
     • pushed-out pins
   • Connect the PCM connector and make sure it seats correctly.
   • Operate the system and verify the concern is still present.
   • Is the concern still present? 
   1. YES  : INSTALL a new PCM. REFER to ELECTRONIC ENGINE CONTROLS article. TEST the system for normal operation.
   2. NO  : The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector.