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Intake Air By-Pass Control System (Prelude)

Intake Air By-Pass (IAB) control solenoid valve is located at lower rear of engine. See COMPONENT LOCATIONS . IAB control solenoid valve is off when engine speed is more than 4900 RPM.

  1. Start engine and allow to idle. Disconnect vacuum hose from IAB control diaphragm. See VACUUM DIAGRAMS article. Check for vacuum at hose. If vacuum does not exist, go to next step. If vacuum exists, go to step  7.
  2. Disconnect vacuum hose, located between IAB vacuum tank and intake manifold, from IAB vacuum tank. See VACUUM DIAGRAMS article. Check for vacuum at hose of IAB vacuum tank. If vacuum exists, go to next step. If vacuum does not exist, repair blockage or vacuum leak between IAB vacuum tank and intake manifold.
  3. Turn ignition switch off. Disconnect IAB control solenoid valve 2-pin connector. Measure voltage between IAB control solenoid valve harness connector terminals. If battery voltage does not exist, go to next step. If battery voltage exists, replace IAB control solenoid valve.
  4. Measure voltage between ground and IAB control solenoid valve harness connector terminal No. 1 (Black/Yellow wire). If battery voltage exists, go to next step. If battery voltage does not exist, repair open in Black/Yellow wire between IAB control solenoid valve connector and No. 14 ECU (15-amp) fuse located in under-dash fuse/relay box. See WIRING DIAGRAMS article.
  5. Turn ignition off. Reconnect IAB control solenoid valve 2-pin connector. Turn ignition on. Using backprobe set, connect a fused jumper wire between PCM 32-pin connector "A" terminal No. 9 (Brown/Black wire) and terminal No. 26 (Red/Blue wire). See PIN VOLTAGE CHARTS article. Go to next step.
  6. If IAB control solenoid valve does not click when jumper wire is connected, repair open in Red/Blue wire between IAB control solenoid valve and PCM 32-pin connector "A". See WIRING DIAGRAMS article. If IAB control solenoid valve clicks when jumper wire is connected, substitute a known-good PCM and recheck. See SUBSTITUTING PCM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or indication goes away, replace original PCM.
  7. Increase engine speed to more than 5000 RPM. Check for vacuum at disconnected TAB control diaphragm hose. See VACUUM DIAGRAMS article. If vacuum does not exist, IAB control system is okay. If vacuum exists, disconnect IAB control solenoid valve 2-pin connector. Check for vacuum again at disconnected IAB control diaphragm hose. If vacuum does not exist, go to next step. If vacuum exists, replace IAB control solenoid valve.
  8. Turn ignition off. Disconnect PCM 32-pin connector "A". See PIN VOLTAGE CHARTS article. Check for continuity between ground and IAB control solenoid valve harness connector terminal No. 2 (Red/Blue). If continuity exists, repair short in Red/Blue wire between PCM 32-pin connector "A" and IAB control solenoid valve. See WIRING DIAGRAMS article. If continuity does not exist, substitute a known-good PCM and recheck. See SUBSTITUTING PCM in SELF-DIAGNOSTICS - INTRODUCTION article. If symptom or indication goes away, replace original PCM.
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