Test D5 - ISC

Fig 1: Idle Speed Control (Air By-Pass) Circuit

To prevent replacement of good components, be aware that the following non-EEC related areas may be at fault: engine temperature not up to operating temperature, engine over operating temperature, A/C input (electrical problem), improper idle speed/throttle stop adjustment or cruise control linkage.

1. Service Code 12: Check For RPM Drop. Code 12 indicates that during KOER Self-Test engine RPM could not be controlled within Self-Test minimum RPM. Possible causes are: Open or shorted circuit, Sticking or binding throttle linkage, Incorrect idle adjustment, Throttle body or ISC solenoid contamination, Faulty ISC solenoid, Faulty ECA, Non-ECC mechanical faults. Turn key off. Connect engine tachometer and start engine. Disconnect Idle Speed Control (ISC) harness. If RPM drops or if engine stalls, go to next step. If not, go to step 3).
2. Checking For EGR Codes. If EGR service codes 31, 32, 33 or 34 are displayed, reconnect ISC solenoid and go to ENGINE RUNNING SELF-TEST table and perform appropriate CIRCUIT TEST. If these codes are not displayed, go to next step.
3. Checking For Other Codes. If Codes 22, 41, 42, 91, or 92 are displayed, reconnect ISC solenoid, go to ENGINE RUNNING SELF-TEST table and perform appropriate CIRCUIT TEST. If these codes are not displayed, go to next step.
4. Measuring ISC Solenoid Resistance. Turn key off. Disconnect ISC solenoid. Set DVOM on 200-ohm scale, and measure resistance of ISC solenoid connector. See Fig 2 . If resistance is between 7 and 13 ohms, go to next step. If resistance is less than 7 ohms or more than 13 ohms, replace ISC solenoid and repeat QUICK TEST.
   Fig 2: ISC Solenoid Connector

   
5. Checking For Internal Short to ISC Solenoid Case. With key off and ISC solenoid disconnected, set DVOM on 200K-ohm scale. Measure resistance from either ISC terminal pin to ISC solenoid housing. If reading is greater than 10K ohms, go to next step. If reading is 10K ohms or less, replace ISC solenoid. Repeat QUICK TEST.
6. Checking VPWR Circuit. Leave ISC harness disconnected. With KOEO, set DVOM on 20-volt scale. Measure voltage between VPWR circuit at ISC harness connector and battery ground terminal. If reading is less than 10.5 volts, repair open in circuit and repeat QUICK TEST. If reading is 10.5 volts or more, go to next step.
7. Checking ISC Circuit Continuity. Turn key off and wait 10 seconds. Leave ISC solenoid disconnected. Disconnect ECA 60-pin connector. Inspect connector for damaged pins, corrosion, or loose wires. Repair if necessary. Install breakout box, leaving ECA disconnected. With DVOM on 200-ohm scale, measure resistance between test pin No. 21 and ISC circuit at ISC harness connector. If reading is greater than 5 ohms, repair open circuit and repeat QUICK TEST. If reading is less than 5 ohms, go to next step.
8. Checking For Circuit Short to Ground. Turn key off and wait 10 seconds. Leave ISC solenoid disconnected. Install breakout box, leaving ECA disconnected. Set DVOM on 200K-ohm scale. Measure resistance between test pin No. 21 and test pins No. 40, 46, and 60 at breakout box. If any reading is less than 10K ohms, repair short to ground and repeat QUICK TEST. If all readings are 10K ohms or more, go to next step.
9. Checking For Short to Power. Turn key off and wait 10 seconds. Leave breakout box installed. Leave ECA and ISC solenoid disconnected. Set DVOM on 200K-ohm scale. Measure resistance by connecting DVOM negative lead to test pin No. 21 and positive lead to test pin No. 37. If reading is greater than 10K ohms, go to next step. If reading is 10K ohms or less, repair short to power and repeat QUICK TEST. If code or symptom is still present, replace ECA.
10. Checking ISC Signal From ECA. With key off, connect ECA and ISC. Leave breakout box installed and set DVOM on 20-volt scale. Connect DVOM between test pins No. 21 and 40. Start engine and observe DVOM while slowly increasing and decreasing engine RPM. If DVOM reading varies, go to next step. If DVOM reading does not vary, replace ECA and repeat QUICK TEST.
11. Checking Base/Curb Idle. Verify base/curb idle speed. Remove ISC solenoid and inspect for contamination, service as necessary. Rerun QUICK TEST, if code or symptom is still present, replace ISC solenoid. If curb idle speed is incorrect, reset idle speed to specification. Rerun QUICK TEST. If unable to set idle to specification, go to next step.
12. Check Faults Which Affect Idle Speed. Check the following mechanical items for faults:
    • Throttle linkage and/or cruise control linkage for sticking or binding.
    • Throttle body for contamination.
    • Vacuum hoses (check emission decal).

    Remove ISC solenoid and inspect for contamination. Service as necessary and rerun QUICK TEST. If code or symptom is still present, replace ISC. If any of the above items are faulty, service as necessary and rerun QUICK TEST.

13. Service Code 13: Verify Idle Speed is Within Specification. Service code 13 indicates that during KOER SELF-TEST, engine RPM could not be controlled within the SELF-TEST lower limit range. Possible causes are: Incorrect idle setting, Vacuum leaks, Sticking or binding throttle linkage, Throttle plates open, Incorrect ignition timing (TFI ignition only), Throttle body or ISC solenoid contamination, ISC circuit shorted to ground or faulty ISC solenoid. If all of the above components are okay, and idle is set to specification, remove ISC solenoid and inspect for contamination. Service as necessary and rerun QUICK TEST. If code or symptom is still present, replace ISC solenoid. If idle speed is not set to specification, reset and rerun QUICK TEST. If idle speed cannot be reset to specification, go to step 6) of this test.
14. Check For Conditions Affecting Idle Speed. Check the following mechanical components for faults:
    • Engine vacuum hoses (refer to emission decal).
    • Throttle linkage and/or cruise control linkage for sticking or binding.
    • Check throttle plates are fully closed.
    • Check for vacuum leaks in induction system.
    • Check throttle body for contamination.
    • Ensure base ignition timing is to specification on emission decal (TFI vehicles only).

    If all above checks are okay, go to next step. If a fault has been found, service as necessary and rerun QUICK-TEST.

15. Checking For Internal Short to ISC Solenoid Case. With key off and ISC solenoid disconnected, set DVOM on 200K-ohm scale. Measure resistance from either ISC terminal pin to ISC solenoid housing. If reading is greater than 10K ohms, go to next step. If reading is 10K ohms or less, replace ISC solenoid. Repeat QUICK TEST.
16. Checking For Circuit Short to Ground. Turn key off and wait 10 seconds. Leave ISC solenoid disconnected. Install breakout box, leaving ECA disconnected. Set DVOM on 200K-ohm scale. Measure resistance between test pin No. 21 and test pins No. 40, 46, and 60 at breakout box. If any reading is less than 10K ohms, repair short to ground and repeat QUICK TEST. If all readings are 10K ohms or more, go to next step.
17. Checking ISC Signal From ECA. With key off, reconnect ISC. Leave breakout box installed and connect ECA to it. Set DVOM on 20-volt scale. Connect DVOM between test pins No. 21 and 40. Start engine and observe DVOM while slowly increasing and decreasing engine RPM. If DVOM voltage reading varies, remove ISC solenoid and inspect for contamination, service as necessary. Rerun QUICK TEST, if code or symptom is still present, replace ISC solenoid. If DVOM voltage does not vary, replace ECA and rerun QUICK TEST.
18. Service Code 47: Check For Low Flow of Unmetered Air. Code 47 indicates that measured airflow at base idle was lower than ECA specification. Possible causes are: air/vacuum leaks in fuel charging system or faulty or leaking purge solenoid and/or injector "O" rings. Service any faults as necessary and rerun QUICK TEST. If no faults are present, EEC-IV system is okay, service other codes as necessary.
19. Service Code 48: Check For High Flow of Unmetered Air. Code 48 indicates that measured airflow at base idle was higher than ECA specification. Possible causes for this fault are: air/vacuum leaks in fuel charging system or loss of ignition or fuel. Check for and repair holes, cracks, and/or bad connections at air cleaner outlet tube (between vane airflow meter and fuel charging assembly). Check for loss of ignition or injection on one or more cylinders. If any faults are found, service as necessary. Rerun QUICK TEST. If no faults are found, EEC-IV system is okay for metered air.
20. Service Code 16: High ISC RPM. Code 16 indicates that with ISC off, engine RPM was above Self-Test specification limit. Possible causes for this fault are: incorrect idle adjustment, faulty purge solenoid, air/vacuum leaks. If Code 48 is also present, reset throttle plate. Refer to Vehicle Emission Control Decal for curb idle set procedure. Rerun QUICK TEST. If Code 48 is still present, go to step 19) of this test. If Code 48 is not present, go to step 13) of this test.
21. Service Code 19: ISC RPM Low. Code 19 indicates that with ISC off, engine RPM dropped below Self-Test limit (about 600 RPM). Possible causes are: engine below normal operating temperature, throttle body/air inlet contamination or incorrect idle speed adjustment. Turn key off and wait 10 seconds. Deactivate SELF-TEST. Start and run engine at 2000 RPM for 2 minutes or until top radiator hose is hot and pressurized. Turn key off and wait 10 seconds. Rerun KOER SELF-TEST. If engine stumbles and/or Code 19 is still displayed, inspect throttle body and air inlet for contamination, service as necessary and adjust curb idle. Rerun QUICK TEST. If Code 19 has been cleared, service other codes as necessary.
22. Service Code 17: ISC RPM Low. Code 17 indicates that with the ISC off, engine RPM was below Self-Test limit. Possible causes for this fault are: excessive engine accessory load, engine temperature below normal operating temperature, throttle body/air inlet contamination or incorrect idle adjustment. Check and correct excessive engine load conditions, cooling fans, lights, etc. Run engine at 2000 RPM for 2 minutes. Turn key off and wait 10 seconds. Rerun KOER SELF-TEST. If Code 17 is displayed, inspect throttle body and air inlet for contamination and clean as necessary. If okay, adjust base/curb idle speed and repeat QUICK TEST. If Code 17 is not displayed, service other codes as necessary.