Description

Idle Speed Control Description

1. Fault Codes:
Idle Control System RPM Lower Than Expected P0506
Idle Control System RPM Higher Than Expected P0507
Idle Control System Circuit Open P1508
Idle Control System Circuit Shorted P1509

2. Monitoring Procedure

Summary
The open and short circuit tests check for hardware fault events by sampling the stepper motor coils' fault status lines of the Serial Output Devices.

The closed loop mode test monitors the cumulative opening and closing idle speed control steps, during closed loop control, a fault event being registered when the cumulative steps applied in either an opening or closing direction exceeds threshold values, without causing a corresponding increase or decrease in airflow rate.

Closed Loop Test
At the start of any closed loop idle speed control, the airflow is sampled, ISC DIAG INIT AIRFLOW, and from that point, the count of the cumulative opening and closing idle speed control steps, ISC CUM STEP SUM CNTR is monitored. No further action is taken until ISC CUM STEP SUM CNTR is greater than ISC DIAG CLOSING STEP THRESH in the closing direction, or is greater than ISC DIAG OPENING STEP THRESH in the opening direction. Then the airflow is sampled again, ISC DIAG FINAL AIRFLOW, and the difference between ISC DIAG FINAL AIRFLOW and ISC DIAG INIT AIRFLOW calculated.

If ISC CUM STEP SUM CNTR is greater than ISC DIAG CLOSING STEP THRESH in the closing direction, and the airflow decrease is less than ISC DIAG CLOSING AIRFLOW THRESH, then a closed loop fault event has occurred. If the airflow decreases by more than ISC DIAG CLOSING AIRFLOW THRESH, then the test was passed.

If ISC CUM STEP SUM CNTR is greater than ISC DIAG OPENING STEP THRESH in the opening direction, and the airflow increase is less than ISC DIAG OPENING AIRFLOW THRESH, then a closed loop fault event has occurred. If the airflow increases by more than ISC DIAG OPENING AIRFLOW THRESH, then the test was passed.

If a fault event has occurred the fault event counter, ISC DIAG CL FAULT CNTR, up to a limit of ISC DIAG CL FAULT LIMIT is incremented. If the test was passed then the closed loop fault event counter is decremented, down to a limit of zero.

If the fault event counter reaches ISC DIAG CL FAULT LIMIT then a fault is present. If ISC DIAG CUM STEP CNTR is greater than ISC DIAG OPENING STEP THRESH in the opening direction then an ISC engine speed low fault is present, otherwise an ISC engine speed high fault is present.

If the fault event counter equals zero, there are no ISC engine speed low, or high, faults present.

If ISC STABILITY OCCURRED indicates that the stepper motor adaptive offset has been updated during closed loop control, then re-sample ISC DIAG INIT AIRFLOW, and calculate the airflow difference from this new value. The ISC CUM STEP SUM CNTR will have been reset, so monitor the cumulative step count from this new value.

Short Circuit Test
The short circuit test checks for short circuit fault events by sampling the stepper motor coils' fault status lines, ISC DIAG STATUS [A, B, C, D], of the Serial Output Device, which indicate the fault status of the coils.

To check for short circuits, ISC DIAG STATUS [A, B, C, D] are sampled during the three most recent stepper motor steps, which were stepped in the same direction, guaranteeing that at some point the potential differences across both coils will reverse. For any stepper position, one of the ends of a shorted coil will indicate a fault on the fault status lines. Using the three most recent samples of a coils' fault status lines, if for each sample, either end of the coil indicates a fault, there is a short circuit fault event.

At the start of stepper referencing, as indicated by STEPPER REFERENCING IN PROGRESS being set, the fault event counter, ISC DIAG SHORT FAULT CNTR is cleared. If a test results in a short circuit fault event on either coil then the hardware fault event counter is incremented up to a limit of ISC DIAG SHORT FAULT LIMIT. If, during the ISC position referencing, the fault event counter reaches ISC DIAG SHORT FAULT LIMIT, there is an ISC short circuit fault present.

Open Circuit Test
The open circuit test checks for open circuit fault events by sampling the stepper motor coils' fault status lines, ISC DIAG STATUS [A, B, C, D], of the Serial Output Device.

During the open circuit test, compare the fault status signal at each end of a coil.
If the ends of a coil have different values for the fault status, then there is an open circuit fault event. Thus, sample ISC DIAG STATUS [A, B, C, D], and if either A and B are not the same, or C and D are not the same, then there is an open circuit fault event.

When the coils go tristate, as indicated by STEPPER COILS TRISTATE being set, clear the fault event counter, ISC DIAG OC FAULT CNTR. If a fault event occurs on either coil, increment the fault event counter, ISC DIAG OC FAULT CNTR. At least ISC DIAG OC FAULT LIMIT consecutive open circuit tests should occur during one power down test.
If the fault event counter reaches ISC DIAG OC FAULT LIMIT, there is an ISC open circuit fault present.

3. Primary Detection Parameter

Closed Loop Test
Difference in airflow with respect to cumulative opening and closing of the stepper motor, i.e. slope of airflow Vs stepper position.

Short Circuit Test
Hardware fault status from stepper motor drive ASIC which monitors line voltage before low side drive transistor.

Open Circuit Test
Hardware fault status from stepper motor drive ASIC which monitors line voltage before low side drive transistor Hardware fault status from stepper motor drive ASIC which monitors line voltage.

4. Fault Criteria Limits

Closed Loop Test
ISC DIAG CLOSING STEP THRESH 30 steps
ISC DIAG OPENING STEP THRESH 50 steps
Airflow Per Step = 0.3kg/hr/step
ISC DIAG CLOSING AIRFLOW THRESH 0.3 x 30 = 9Kg/hr @ 30 steps
ISC DIAG OPENING AIRFLOW THRESH 0.3 x 50 = 15Kg/hr @ 50 steps

Short Circuit Test
ISC DIAG SHORT FAULT LIMIT = 63; limit used to define that there is a fault present with the stepper motor.

Open Circuit Test
ISC DIAG OC FAULT LIMIT = 50; limit used to define that there is a fault present with the stepper motor.

5. Monitoring Conditions

Closed Loop Test
Closed Loop idle Speed Control is active.
Engine coolant temperature is above 70° C.
The engine speed is above a limiting value: 800rpm
The road speed sensor does not indicate a fault.
The ISC open circuit test does not indicate a fault.
The ISC short circuit test does not indicate a fault.

Short Circuit Test
The vehicle status is power down.
The ISC power down referencing routine is occurring.

Open Circuit Test
The vehicle status is power down.
The power is not removed from the ISC circuitry.
The ISC coils are tristate, as indicated by the internal flag, STEPPER COILS TRISTATE.

6. Monitoring Time Length / Frequency of Checks
The frequency of the ISC diagnostic routine is 10Hz.

7. Criteria for Storing Fault Code
Two successive trips where the diagnostic routine indicates a failed stepper or stepper circuit.

8. Criteria for Illuminating MIL
Two successive trips where the diagnostic routine indicates a failed stepper or stepper circuit.

9. Criteria for Determining Out of Range Input Signals
The airflow meter is subject to out of range limits of 350mv and 4.5V.