Speed Control Diagnostic Tips - SCP
The contents of this article/image reflects the changes identified in TSB 06-8-4SPEED CONTROL DIAGNOSTIC TIPS-SCP -- TSB 06-8-4
ACTION
The vehicles in this article are equipped with a 10-pin speed control servo and Standard Corporate Protocol (SCP) communication control system. Follow the Service Procedures and Diagnostic Tips in this article to assist with accurate diagnosis and repair of speed control issues.
Disclaimer:
Disclaimer
GENERAL SERVICE PROCEDURE
1. Visually inspect the vehicle. Any after market modifications, including but not limited to those listed below, may cause speed control to not operate correctly:
- Any wiring or lamp modifications affecting brake lamp operation
- LED brake lamps
- Non-factory installed trailer wiring.
- Radios (speed sensitive, auto mute)
- Remote starters and alarms
- Lighting and electrical accessories modifications
2. Visually inspect the servo and accelerator controls:
- Visually inspect speed control cable without removing and ensure smooth cable operation. Inspect cable connection to the throttle body
- Visually inspect accelerator cable without removing and ensure smooth cable operation. Inspect cable connection to throttle body. Also check for interference with carpet, bulkhead grommet, insulation and instrument panel wiring
SERVICE PROCEDURE
1. Run Diagnostic Scan Tool (DST) speed control on demand test. The DST on demand test diagnostics is a key tool in quickly and accurately diagnosing speed control system faults. DST diagnostics are the basis for starting speed control system trouble shooting.
WARNING: This test is a Key On Engine Off (KOEO) test that is conducted only while parked with the parking brake fully engaged. Failure to follow these instructions may result in personal injury.
NOTE: Do not use the servo built in self test diagnostics.
a. Use the DST speed control on demand test to retrieve diagnostic trouble codes. Follow diagnostics for any PIDs found.
b. Use the DST to confirm steering wheel button operation by pressing each button while monitoring PIDs. If buttons are found to not function follow diagnostics.
SPEED CONTROL DIAGNOSTIC TIPS
Servo Pin-Out Test Tips
NOTE: Measuring the resistance of a servo will indicate that a servo is bad if readings are outside the ranges specified below. However, a servo can have resistance readings within the specified ranges and still be faulty.
The following readings are the expected values for a good servo.
Resistance between Pins 6 and 10 should measure less than 5 ohms. This is the ground circuit through the module. Resistance between Pins 7 and 10 will vary with polarity, the type of meter, and source voltage of the meter. Also measurements between Pins 7 and 10 can be any of the following: an open circuit, increasing reading (as a capacitor charging), or changing value (re-apply leads and different value indicated), or a stable reading of 10,000 ohms or greater are all valid results as this is a solid state circuit.
DST Testing Tips (TPS PID)
1. Pinpoint test: The Speed Control Does Not Disengage When The Brakes Are Applied.
With the vehicle speed above 30 MPH (48 Km/h) engage the speed control. Then check to see if the TPS PID returns to base voltage when the brakes are applied. If it does return to base voltage, then the speed control system is working. Need to advise customer that tapping the brakes deactivates speed control and that the brakes must be applied to make the vehicle slow down.
NOTE: Check for calibration service messages relating to dashpot or idle speed control updates.
2. Pinpoint test: The Speed Control Does Not Disengage When The Clutch Is Applied.
Check to see if the TPS PID returns to base voltage when the clutch is pressed. If it does then the speed control system is working. Need to advise customer that pressing clutch deactivates speed control and that the brakes must be applied to make the vehicle slow down.
NOTE: Check for calibration service messages relating to dashpot or idle speed changes. Also a slight engine rpm flare may occur on some vehicles when speed control is disengaged which is a normal condition.
Deactivator Switch Inspection Tips
A faulty deactivator switch can result in intermittent or inoperative speed control. There are two types of deactivator switches; a brake line pressure switch which is integrated into the brake master cylinder, and a pedal travel switch, which is connected to the brake pedal arm.
Proper switch function should be confirmed. Inoperative switches of both types should be inspected for connector corrosion or pin push-out. Pedal travel switches should also be inspected for proper mechanical adjustment relative to brake pedal travel.
ADDITIONAL REFERENCE INFORMATION
Speed Control Servo Operation and Function
NOTE: All functionality listed below is performed with connectors connected and key in run position unless otherwise noted.
PIN 1 - SCP+ Standard Corporate Protocol Bus. Receives vehicle speed, BOO/BPP, throttle position and gear selector position information. It outputs: cluster telltale signal, speed control button status and DTCs. To confirm operation, use a DST to retrieve any diagnostic trouble codes using the on demand self test then confirm steering wheel button operation by pressing each button and observe that the appropriate PID changes.
PIN 2 - SCP- Standard Corporate Protocol Bus. Receives vehicle speed, BOO/BPP, throttle position and gear selector position information. It outputs: cluster telltale signal, speed control button status and DTCs. To confirm operation, use a DST to retrieve any diagnostic trouble codes using the on demand self test then confirm steering wheel button operation by pressing each button and observe that the appropriate PID changes.
PIN 3 - Not Used.
PIN 4 - Not Used.
Figure 1:
Figure 1
PIN 5 - Control Switch Input. (Note: Performed with key in OFF position) Remove connector C122, using a DVOM set to resistance, measure the resistance between Pins 5 and 6. See table of resistance readings (Figure 1) for each switch when it is depressed.
PIN 6 - Control Switch Return. This is the return side of the switches, which are grounded internally to the servo to Pin 10. If test for Pin 5 checks good then Pin 6 is functioning normally.
PIN 7 - Power. Remove connector C122, Using a DVOM set to DC volts, measure the voltage between pin 7 and ground. You should read greater than 10 volts with the key in the run position. Record the voltage from pin 7 to ground, then re-measure with a test lamp (1156) from pin 7 to ground, to ensure current carrying capability. The difference between the two measurements should be less than 0.3 VDC. If voltage drop test fails then check wiring.
PIN 8 - Not used.
PIN 9 - Brake Deactivator Switch. This is a redundant shutoff switch. Without depressing the brake pedal, measure voltage on Pin 9, and re-measure with a test lamp (1156) from Pin 9 to ground, to ensure current carrying capability. The difference between the two measurements should be less than 0.3 VDC. The system requires enough current at this pin to engage properly. If voltage drop test fails, check for corroded wiring or deactivation switch. You should measure 0.0 volts at Pin 6 when the brake pedal is depressed firmly.
An alternate to the voltage drop test would be to disconnect the switch connector and measure the switch resistance. It should be less than 5 ohms. If the switch is suspected, a bypass test may also be done: On the harness side of the BPS connector, connect a jumper between the two deactivation switch pins and drive vehicle above 30 MPH (48 Km/h). If speed control engages then the brake deactivation switch was faulty, otherwise wiring is at fault.
PIN 10 - Ground. Using a DVOM set to DC volts, measure the voltage between B+ and Pin 10. You should read greater than 10 volts. Record the voltage between B+ and Pin 10 , then re-measure with a test lamp (1156) from B+ to pin 10. The difference between the two measurements should be less than 0.3 VDC. If voltage drop test fails then check wiring. This verifies circuit can carry proper load as well as continuity to ground.