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Home >> Ford >> 1988 >> Bronco 5.0 N, Standard, M5OD-R2 >> Repair and Diagnosis >> Engine Performance >> System >> EEC-IV CEC System >> Quick Test >> Test D5 - Idle Speed Control (Air By-Pass)
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Test D5 - Idle Speed Control (Air By-Pass)

NOTE: Perform this test when Service Code 12, 13, 16, 17, 47 or 48 displayed during QUICK TESTS or when directed here by other test procedures. If engine is running rough or has rough idle, correct these conditions before performing test. Causes may be in ignition system, fuel system or EGR system.
Fig 1: Idle Speed Control (Air By-Pass) Circuit
G72368

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. Check For RPM Drop  - 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, go to KEY ON ENGINE RUNNING (KOER) - TEST MENU  and perform appropriate TEST. If these codes are not displayed, go to next step.
  3. Checking Other Codes  - If service codes 22, 41, 42, 33, or 34 are displayed, go to KEY ON ENGINE RUNNING (KOER) - TEST MENU  and perform appropriate 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. 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  .
  5. Checking For Internal Short  - With key off and ISC harness disconnected, set DVOM on 200K-ohm scale. Measure resistance from either ISC 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. Turn key on, leaving engine off. 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 Circuit Continuity  - Turn key off and wait 10 seconds. Leave ISC harness 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 harness 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. 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 are still present, replace ECA.
  10. Checking Processor Signal  - 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 Curb Idle  - Verify curb idle speed. If curb idle speed is within specifications, replace ISC and repeat QUICK TEST  . If curb idle speed is incorrect, check engine vacuum hoses for proper routing. Use emission label as a guide. Ensure throttle plates are fully closed. Check throttle and/or cruise control linkage for binding. Repair if necessary. If these items are okay, replace ISC and repeat QUICK TEST.
  12. Code 19 Displayed: ISC RPM Low  - 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. Perform KEY ON ENGINE RUNNING (KOER) SELF-TEST  . If 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.
  13. Code 17 Displayed: ISC RPM Low  - 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 KEY ON ENGINE RUNNING (KOER) SELF-TEST  . If code 17 is displayed, inspect throttle body and air inlet for contamination. Clean if necessary. If okay, adjust curb idle speed and repeat QUICK TEST  . If code 17 is not displayed, service other codes as necessary.
  14. Code 13 Displayed: High ISC RPM  - Turn key off. Connect tachometer to engine. Start engine. With engine running, disconnect ISC harness connector. If engine RPM drops or if engine stalls, replace ECA and repeat QUICK TEST  . If engine RPM does not drop, check engine vacuum hoses for proper routing. Use emission label as a guide. Ensure curb idle is correct. Make sure throttle plates are fully closed. Check throttle and/or cruise control linkage for binding. Repair if necessary. If these items are okay, replace ISC and repeat QUICK TEST.
  15. Code 47 Displayed: Check For Low Flow of Unmetered Air  - Check for holes, cracks, or disconnections in fuel charging system (manifold gaskets, vacuum lines, vacuum tees etc.). Check purge valve for sticking. Check injector "O" rings for leaks. Service any faults and rerun QUICK TEST  . If no faults are found, EEC-IV system is okay for metered air.
  16. Code 48 Displayed: Check For High Flow of Unmetered Air  - Check for and repair holes, cracks, and/or bad connections at air cleaner outlet tube (between vane airflow meter and fuel charging assembly). Perform KEY ON ENGINE RUNNING (KOER) SELF-TEST  . If code 48 is displayed, check for other sources of unmetered air. If code 48 is not displayed, EEC system is okay. Go to KEY ON ENGINE RUNNING (KOER) - TEST MENU  and service other codes as necessary.
  17. Code 16: High ISC RPM  - If code 48 is also present, reset throttle plate. Refer to emission decal for curb idle set procedure. Rerun QUICK TEST  . If code 48 is still present, go to step 16) of this test. If code 48 is not present, check for vacuum leaks at injector "O" rings, purge solenoid and vacuum lines, etc. Service as necessary and rerun QUICK TEST.
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