Circuit Test D3 - Idle Speed Control (ISC) (Bypass Air)

Fig 1: ISC Circuits

CAUTION: If engine runs or idles rough, correct these conditions before diagnosing Idle Speed Control (ISC) system. Rough running or misses may be caused by the following: ignition system, fuel system, or EGR system.

NOTE: Enter this test only when a Code 34 is displayed during the QUICK TEST procedure or when directed here from another CIRCUIT TEST. To prevent replacement of good components, be aware that the following non-EEC areas may be at fault: engine not up to operating temperature, engine hotter than maximum operating temperature, incorrect air adjust screw adjustment, faulty electrical load input signal, or sticking throttle linkage. This test is intended to diagnose only the following: ISC solenoid, ECA, and harness circuits (ISC and VPWR).

1. Checking System Integrity  - Visually check all wiring, harnesses, connectors and components for evidence of damage, overheating, shorting or looseness. If a fault is found, repair as necessary. If no faults are found, go to next step.
2. Check Voltage At ECA  - Install breakout box and leave ECA disconnected. Set DVOM on 20-volt scale. With KOEO, measure voltage between test pins No. 41 and 20 (ground). If voltage is less than one volt, go to next step. If not, go to step 6).
3. Check Voltage From ISC Solenoid  - Disconnect ECA. Leave ISC solenoid connected. Set VOM on 20 volt scale. DO NOT  pierce wire to test. With KOEO, measure voltage between "W" wire at ISC solenoid and ground. If voltage is less than one volt, go to next step. If not, repair "W" wire between ECA and ISC solenoid.
4. Checking Voltage To ISC Solenoid  - Disconnect ECA and set VOM on 20-volt scale. With KOEO, measure voltage between "R/BK" wire at ISC solenoid and ground. If voltage is less than 10 volts, go to next step. If not, replace ISC solenoid.
5. Checking Voltage From Power Relay No. 2  - Ensure power relay is connected. Set VOM on 20-volt scale. DO NOT  pierce wire to test. With KOEO, measure voltage between "R/BK" wire at power relay No. 2 and ground. If voltage is less than 10 volts, go to CIRCUIT TEST B12  , step 1). If voltage is not as specified, repair "R/BK" wire between ISC solenoid and power relay No. 2.
6. Checking ISC Circuit For Short To VPWR  - Install breakout box, leaving ECA disconnected. Disconnect ISC solenoid connector. Set VOM on 20-volt scale. With KOEO, measure voltage between test pins No. 41 and 20 (ground). If voltage is greater than zero volts, repair short to VPWR. If voltage is zero volts, replace ECA.