Pinpoint Test B: The Set Speed Fluctuates Or Incorrect Vehicle Speed Signal (VSS)
Cruise Control
Pinpoint Test
Pinpoint Test B: The Set Speed Fluctuates Or Incorrect Vehicle Speed Signal (VSS)
Normal Operation
The cruise control actuator adjusts the cruise control cable, which controls the throttle body. The Output Shaft Speed (OSS) sends a signal to the PCM indicating the transmission OSS. The PCM sends an AC voltage signal to the cruise control actuator indicating the vehicle cruise.
This pinpoint test is intended to diagnose the following:
- Cruise control cable
- Accelerator cable
- Cruise control actuator
- Base engine concern
- PCM
PINPOINT TEST B: THE SET SPEED FLUCTUATES OR INCORRECT VEHICLE SPEED SIGNAL
NOTE: It is normal for the engine rpm to increase slightly and then drop off slowly when the clutch pedal is applied during operation.
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B1 CHECK THE CRUISE CONTROL CABLE/THROTTLE BODY LINKAGE
- Ignition OFF.
- Visually inspect the cruise control cable connections, the throttle body linkage, and the cruise control actuator pulley cover.
- Are the cruise control cable connections, the throttle body linkage, and the cruise control actuator pulley cover OK?
Yes
GO to B2.
No
INSTALL a new cruise control cable or REPAIR the throttle body linkage. For a new cruise control cable, REFER to Cruise Control Cable Service and Repair. TEST the system for normal operation. For the throttle body linkage, REFER to Fuel Delivery and Air Induction. TEST the system for normal operation. For the cruise control actuator pulley cover, INSTALL the cover correctly. TEST the system for normal operation.
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B2 CHECK THE SPEEDOMETER OPERATION
- Drive the vehicle and observe the speedometer operation.
- Does the speedometer needle fluctuate?
Yes
REFER to Instrument Panel, Gauges and Warning Indicators to continue diagnosis of the speedometer.
No
GO to B3.
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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?
Yes
REFER to Computers and Control Systems Information to continue diagnosis of the base engine concern.
No
GO to B4.
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B4 CHECK THE VEHICLE SPEED SIGNAL VOLTAGE
NOTICE: 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: Cruise Control Actuator C122.
- With the vehicle in NEUTRAL, position it on a hoist. For additional information, refer to Vehicle Jacking and Lifting.
- 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.
- Measure and record the AC voltage between the cruise 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?
Yes
GO to B5.
No
GO to B6.
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B5 CHECK THE VEHICLE SPEED SIGNAL FREQUENCY
NOTICE: 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: Make sure the automotive meter is set to DC hertz when measuring the frequency.
- 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.
- Measure and record the frequency between the cruise 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 an mph result and compare this to the reading of the speedometer.
- Does the frequency reading result match the speedometer reading?
Yes
INSTALL a new cruise control actuator. REFER to Cruise Control Actuator Service and Repair. TEST the system for normal operation.
No
GO to B6.
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B6 CHECK THE VEHICLE SPEED SIGNAL FOR A SHORT TO GROUND
- Measure the resistance between the cruise control actuator C122-3, circuit VMC05 (VT/OG), harness side and ground.
- Is the resistance greater than 10,000 ohms?
Yes
GO to B7.
No
REPAIR the circuit. TEST the system for normal operation.
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B7 CHECK THE VEHICLE SPEED SIGNAL CIRCUIT FOR AN OPEN
- Ignition OFF.
- Disconnect: PCM C175b.
- Measure the resistance between the cruise control actuator C122-3, circuit VMC05 (VT/OG), harness side and the PCM C175b-1, circuit VMC05 (VT/OG), harness side.
- Is the resistance less than 5 ohms?
Yes
GO to B8.
No
REPAIR the circuit. TEST the system for normal operation.
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B8 CHECK FOR CORRECT PCM OPERATION
- Connect: All Disconnected Connectors.
- 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?
Yes
INSTALL a new PCM. REFER to Computers and Control Systems. TEST the system for normal operation.
No
The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector. TEST the system for normal operation.
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