Speed Control Diagnostic Tips - PCM
The contents of this article/image reflects the changes identified in TSB 06-8-3SPEED CONTROL DIAGNOSTIC TIPS-PCM -- TSB 06-8-3
ACTION
The vehicles in this article are equipped with a 6-pin powertrain control module (PCM) controlled speed control system. Follow the Service Procedures and Diagnostic Tips in this TSB 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
Start by running speed control self test diagnostics. The self test diagnostics are a key tool in quickly and accurately diagnosing speed control system faults. Self test diagnostics are also the basis for starting speed control trouble shooting.
NOTE: Diagnostic flash codes and/or DTCs are not available to assist with diagnosis of the 6-pin system. The dynamic throttle pull at the end of the self test is the only confirmation available to verify if the system is operating correctly at the time of service.
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: The self-test is comprised of two parts. The first part is a static check of the speed control electronics module and system. The second part is a dynamic pull-test to check the actuator motor and gear mechanism.
NOTE: The module times out if each button is not pressed within 1 second of the previous button. If a module time out occurs (speed control lamp stops flashing part way through the test), the procedure must be re-initiated.
NOTE: Review the following steps before carrying out the self-test diagnostic procedure.
1. Self Test Diagnostic Procedure - Static Test
NOTE: For 6-pin servos, the lamp flash is too slow and must be ignored. Switches must be pressed within one second of each other. waiting for the cluster lamp to flash could result in a self test time out condition. Ignore cluster lamp flashes but observe dynamic throttle pull which occurs at the end of the button sequence.
a. Connect the Diagnostic Scan Tool (DST) to a power source that is not interrupted when the ignition switch changes positions. With the ignition switch in the RUN position, set the DST to monitor the Powertrain Control Module (PCM) throttle position PID while the speed control actuator carries out the self-test.
b. Enter self-test diagnostics by firmly pressing and holding the speed control OFF switch while quickly cycling the ignition switch from RUN-to-OFF-to-RUN, making sure the engine does not start and is not running.
c. The speed control indicator lamp on the instrument panel will flash once to indicate that the speed control module has entered the diagnostic mode. Release the OFF switch. If 5 flashes are displayed at this point, a speed control subsystem concern exists. Refer to the Symptom Chart. Diagnosis By Symptom
Figure 1:
Figure 1
d. Then firmly press and release the remaining switches within 1 second of other following the sequence in Figure 1.
NOTE: Monitor the PCM throttle position PID after the last button is pressed.
NOTE: If the self-test will not start or cannot be completed after multiple attempts, Go to the Symptom Chart. Diagnosis By Symptom
2. Self Test Diagnostic Procedure - Dynamic Test
NOTE: The dynamic test occurs automatically after the static test completes. If the static test fails, there will be no dynamic pull test.
a. Within 0.25 seconds after the static test has completed, the speed control actuator carries out a dynamic pull test. The actuator automatically pulls the speed control cable 0.04 to 0.39 inch (1 to 10 mm) to move the throttle from the idle position and then releases the speed control cable returning the throttle to the idle position. You should hear one sharp clicking sound which indicates the servo clutch engagement followed by a gear sound indicating the servo motor turning (try this on a known good unit for reference). If you have one sound without the other, it will help indicate which circuits are inoperative.
b. If the throttle position PID voltage value does not change during the dynamic throttle pull, GO to the Symptom Chart for Dynamic pull test failure. Diagnosis By Symptom
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: 1 and 2, 1 and 3, and 1 and 4 should measure between 1 and 5 ohms and should be similar in value between pin sets. These are the three phases of the motor. Resistance between Pins 5 and 6, should measure between 20 and 30 ohms. This is the winding of the clutch. These readings should not change with polarity.
DST Testing Tips (TPS PID)
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.
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.
Intermittent Speed Control Operation Tips
Intermittent or irregular speed control operation may be traced to poor pin contact between the PCM pins and the mating wiring harness connector terminals. Indication of this condition is that the speed control indicator lamp on the instrument panel will illuminate but the servo will not actuate the throttle. This can be tested by using a test lamp at the servo connector while running the self test. Pins 2, 3, and 4 to B+ the lamp should flicker slightly at the end of the test for 4 seconds. This indicates the phases of the motor firing in sequence. Pin 5 top B+ the lamp should glow solid at the end of the test for 4 seconds. This represents the speed control clutch circuit operation. Pins 1, and 6 to ground should glow brightly when the ignition is on.
ADDITIONAL REFERENCE INFORMATION
Speed Control Servo Operation and Function
PIN 1 - Motor Common: Hot in run or start. Load test circuit to ensure current carrying capability. Disconnect C122 then with the key in the run position, measure voltage from Pin 1 to ground, then re-measure with a test lamp (1156) from Pin 1 to ground. The difference between the two measurements should be less than 0.3 VDC. If the voltage drop test fails, check the speed control system related terminals at the PCM connector to make sure they are not spread too far. If the terminals are okay, check the wiring.
PIN 2 - Motor Phase C: The PCM completes the circuit to ground as each motor phase is sequentially fired. This circuit carries the same current as the motor common. It is important to load test this circuit without back feeding the PCM or risk damaging the internals. With the key off, remove C122 and PCM connectors and conduct a voltage drop test on the Pin 2 circuit. Apply B+ (with a fused jumper) to the terminal on the servo end of the harness and with a DVOM set to DC Volts record the voltage at the PCM terminal to ground. Next re-measure voltage while a test lamp is connected from the PCM terminal to ground. The difference between the two measurements should be less than 0.3 VDC, otherwise check the wiring.
PIN 3 - Motor Phase B: The PCM completes the circuit to ground as each motor phase is sequentially fired. This circuit carries the same current as the motor common. It is important to load test this circuit without back feeding the PCM or risk damaging the internals. With the key off, remove C122 and PCM connectors and conduct a voltage drop test on the Pin 3 circuit. Apply B+ (with a fused jumper) to the terminal on the servo end of the harness and with a DVOM set to DC Volts record the voltage at the PCM terminal to ground. Next re-measure voltage while a test lamp is connected from the PCM terminal to ground. The difference between the two measurements should be less than 0.3 VDC, otherwise check the wiring.
PIN 4 - Motor Phase A: The PCM completes the circuit to ground as each motor phase is sequentially fired. This circuit carries the same current as the motor common. It is important to load test this circuit without back feeding the PCM or risk damaging the internals. With the key off, remove C122 and PCM connectors and conduct a voltage drop test on the Pin 4 circuit. Apply B+ (with a fused jumper) to the terminal on the servo end of the harness and with a DVOM set to DC Volts record the voltage at the PCM terminal to ground. Next re-measure voltage while a test lamp is connected from the PCM terminal to ground. The difference between the two measurements should be less than 0.3 VDC, otherwise check the wiring.
PIN 5 - Servo Clutch Control Return: The PCM will complete this circuit to ground when speed control is active. This circuit carries the same current as the deactivator switch. It is important to load test this circuit without back feeding the PCM or risk damaging the internals. With the key off, remove C122 and PCM connectors and conduct a voltage drop test on the Pin 5 circuit. Apply B+ (with a fused jumper) to the terminal on the servo end of the harness and with a DVOM set to DC Volts record the voltage at the PCM terminal to ground. Next re-measure voltage while a test lamp is connected from the PCM terminal to ground. The difference between the two measurements should be less than 0.3 VDC, otherwise check the wiring.
PIN 6 - Brake Deactivator Switch: This is a redundant shutoff switch. Without depressing the brake pedal, measure voltage on Pin 6, and re-measure with a test lamp (1156) from Pin 6 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.
Speed Control Switch Operation
NOTE: Performed with key in OFF position.
Figure 2:
Figure 2
Steering wheel speed control switch operation is checked at the PCM connector. Determine PCM pins which correspond to Control Switch Input and Control Switch Return. Remove PCM connector, using a DVOM set to resistance, individually press each button and measure the resistance between control switch input and return pin. See table of resistance readings (Figure 2) for each switch when it is depressed.