Diagnostic Aids

WARNING: This page is about a different car, the 1995 Ford Explorer and 1995 Ford Aerostar. However, it is still accessible from the selected car via links, so may be relevant.

The IAC solenoid controls engine idle speed and dashpot functions by regulating volume of air by-passing throttle plate. IAC solenoid positioning is determined by signals sent from PCM.

Perform this test when instructed by QUICK TEST or if directed by other test procedures. This test is intended to diagnose:

RPM during SELF-TEST mode.
IAC solenoid.
Wiring harness circuits (IAC and VPWR).
Powertrain Control Module (PCM).

To prevent replacement of good components, be aware the following non-EEC related areas may be at fault:

Engine temperature outside correct operating range.
A/C input (electrical problem).
Incorrect throttle stop adjustment.
Faulty throttle or cruise control linkage.

Fig 1: Idle Air Control Circuit (4.0L)

TEST PIN NO. 21 (ISC) WIRE COLOR IDENTIFICATION

Application	Wire Color
All Models	White/Light Blue

TEST PINS NO. 37 & 57 (VPWR) WIRE COLOR IDENTIFICATION

Application	Wire Color
All Models	Red

1) Code 412 Code 412 indicates engine RPM could not be controlled within upper RPM limit during KOER SELF-TEST. Possible causes for this fault are:

Open or shorted circuit.
Sticking or binding throttle linkage.
Incorrect idle airflow setting.
Throttle body or IAC solenoid contaminated.
Faulty IAC solenoid.
Faulty PCM.
Mechanical faults unrelated to EEC-IV which could affect RPM.

Turn ignition off. Connect tachometer to engine. Start engine. Disconnect Idle Air Control (IAC) harness. If RPM drops or engine stalls, go to next step. If RPM does not drop and engine does not stall, go to step 3).

2) Check For EGR Codes If EGR service Code 213, 232, 326, 327, 328, 332, 334 or 336 is displayed, reconnect IAC solenoid. Go to appropriate CIRCUIT TEST. See DIAGNOSTIC TROUBLE CODE REFERENCE CHARTS . If these codes are not displayed, go to next step.

3) Check For Other Diagnostic Test Codes If Code 126, 136, 137, 172 or 173 is displayed, reconnect IAC solenoid. Go to appropriate CIRCUIT TEST. See DIAGNOSTIC TROUBLE CODE REFERENCE CHARTS. If these codes are not displayed, go to next step.

4) Measure IAC Solenoid Resistance Turn ignition off. Disconnect IAC solenoid. Connect DVOM positive lead to VPWR terminal and negative lead to IAC terminal. See Fig 2. Measure resistance of IAC solenoid. If resistance is not 6-13 ohms, replace IAC solenoid and repeat QUICK TEST. If resistance is 6-13 ohms, go to next step.

Fig 2: IAC Solenoid Connector (4.0L)

5) Check For Internal Short To IAC Solenoid Case Turn ignition off. With IAC solenoid disconnected, measure resistance from either IAC terminal pin to IAC solenoid housing. If reading is more than 10,000 ohms, go to next step. If reading is 10,000 ohms or less, replace IAC solenoid. Repeat QUICK TEST.

6) Check VPWR Circuit Voltage Leave IAC harness disconnected. Turn ignition on. Measure voltage between VPWR terminal at IAC harness connector and battery ground terminal. If voltage is 10.5 volts or more, go to next step. If voltage is less than 10.5 volts, repair open circuit and repeat QUICK TEST.

7) Check IAC Circuit Continuity Turn ignition off. Leave IAC solenoid disconnected. Disconnect 60-pin PCM connector. Inspect terminals and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between test pin No. 21 and IAC terminal at IAC wiring harness connector. If reading is 5 ohms or less, go to next step. If reading is more than 5 ohms, repair open circuit and repeat QUICK TEST.

8) Check IAC Circuit For Short To Ground Turn ignition off. Leave IAC solenoid and PCM disconnected. Measure resistance between test pin No. 21 and test pins No. 40, 46 and 60. If any reading is less than 10,000 ohms, repair short to ground and repeat QUICK TEST. If all readings are 10,000 ohms or more, go to next step.

9) Check IAC Circuit For Short To Power Leave IAC solenoid and PCM disconnected. Turn ignition on. Measure voltage between breakout box test pin No. 21 and chassis ground. If voltage is more than one volt, repair short circuit and repeat QUICK TEST. If code or symptom is still present, replace PCM. If voltage is one volt or less, go to next step.

10) Check IAC Signal From PCM Turn ignition off. Reconnect IAC solenoid. Connect PCM to breakout box. Connect DVOM between test pins No. 21 and 40 at breakout box. Start engine. Observe DVOM while slowly increasing engine speed to 3000 RPM. If DVOM voltage reading is 3.0-11.5 volts, go to next step. If DVOM voltage reading is not 3.0-11.5 volts, remove IAC solenoid to confirm that it is not stuck open. Repair or replace if necessary. If IAC is okay, replace PCM and repeat QUICK TEST.

11) Check Base Idle Verify base idle speed is correct. See ADJUSTMENTS - 4.0L article in the ENGINE PERFORMANCE section. If base idle speed is within specification, remove IAC solenoid and inspect for contamination. Repair or replace if necessary. Repeat QUICK TEST. If code or symptom is still present, replace IAC solenoid. If base idle speed is not correct, reset idle speed to specification. Repeat QUICK TEST. If unable to set idle to specification, go to next step.

12) Check For Faults Affecting Idle Speed Check following mechanical items for faults:

Throttle linkage and/or cruise control linkage (sticking or binding).
Throttle body (contamination).
Vacuum hoses. Check Vehicle Emission Control Information (VECI) label.
Check for leaks around IAC solenoid (gaskets, etc.).

If any of these items are faulty, service as necessary, and repeat QUICK TEST. If all of these items are okay, remove IAC solenoid and inspect for contamination. Repair or replace as necessary. Repeat QUICK TEST. If code or symptom is still present, replace IAC solenoid. Repeat QUICK TEST.

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 12) to step 15). No test procedures have been omitted.

15) Code 411 Code 411 indicates engine RPM could not be controlled within lower RPM limit during KOER SELF-TEST. Possible causes for this fault are:

Incorrect idle airflow setting.
Vacuum leaks.
Sticking or binding throttle linkage.
Throttle plates open.
Incorrect ignition timing (Distributor ignition only).
Throttle body or IAC solenoid contamination.
IAC circuit shorted to ground.
Faulty IAC solenoid.

If listed items are okay and idle is set to specification, remove IAC solenoid and inspect for contamination. Repair or replace as necessary. Repeat QUICK TEST. If code or symptom is still present, replace IAC solenoid. If idle speed is not within specification, adjust as necessary and repeat QUICK TEST. If idle speed cannot be set to specification, go to next step.

16) Check For Conditions Affecting Idle Speed Check following mechanical components:

Vacuum hoses. Check Vehicle Emission Control Information (VECI).
Throttle linkage and/or cruise control linkage (sticking or binding).
Ensure throttle plates are fully closed.
Induction system (vacuum leaks).
Throttle body (contamination).
Ensure CANP solenoid is not stuck open.
Ensure base ignition timing is to specification on emission decal.

If everything checks okay, go to next step. If fault is found, service as necessary and repeat QUICK TEST.

17) Check For Internal Short To IAC Solenoid Case Turn ignition off. Disconnect IAC solenoid wiring harness connector. Measure resistance from either IAC terminal pin to IAC solenoid housing. See Fig 3. If reading is more than 10,000 ohms, go to next step. If reading is 10,000 ohms or less, replace IAC solenoid. Repeat QUICK TEST.

Fig 3: IAC Solenoid Schematic (4.0L)

18) Check IAC Circuit For Short To Ground Turn ignition off. Leave IAC solenoid disconnected. Disconnect 60-pin PCM connector. Inspect terminals and repair if damaged. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance between test pin No. 21 and test pins No. 40, 46 and 60. If any reading is less than 10,000 ohms, repair short to ground. Reconnect components, and repeat QUICK TEST. If all readings are 10,000 ohms or more, go to next step.

19) Check IAC Signal From PCM Turn ignition off. Connect PCM to breakout box. Reconnect IAC solenoid. Set DVOM on 20-volt scale. Connect DVOM between test pins No. 21 and 40. Start engine. Observe DVOM while slowly increasing and decreasing engine RPM. If voltage is 3.0-11.5 volts, remove IAC solenoid and inspect for contamination. Repair or replace as necessary, and repeat QUICK TEST. If voltage is not 3.0-11.5 volts, replace PCM and repeat QUICK TEST.