OBD1 P72 ECU IAB Polarity Vs. OBD2 IAB Polarity
Intake Air Bypass, known as "IAB's" are the secondary runners in the factory intake manifold on B18C1 GSR and H22 intake manifolds. The IAB's force the intake air to take the longer path into the engine at low RPM for low RPM power, then the IAB's open to allow the incoming air to take the shorter runners at higher RPM for high-RPM power.
The IAB's use manifold vacuum and a solenoid for control. The ECU control's the solenoid, thus determining when the IAB's are closed and open.
On OBD1 ECU's, the IAB control solenoid has +12v power one one side, while the ECU applies negative/ground to the other side of the solenoid when the ECU is holding the IAB's closed (low RPM). When the ECU does NOT apply the negative/ground, the IAB's open for higher RPM.
OBD2 vehicles are wired opposite of this. The IAB control solenoid has a constant negative/ground from the vehicle, and the ECU applies +12v power to the solenoid to hold the IAB's closed. When the ECU does NOT apply the +12v power, the IAB's open for higher RPM.
Why does this matter?
Many people are finding the need to install an OBD1 P72 ECU that is chipped/modified into newer OBD2 vehicles (ie: 96+ Integra GSR). If you intend to install an OBD1 ECU with IAB's (P72) into a newer OBD2 vehicle with IAB's, you will need to make some wiring changes for proper IAB operation and to avoid damaging your vehicle or ECU.
On your OBD2 vehicle, locate the IAB solenoid connector. It is a grey, round, 2-pin connector located under or on the side of your intake manifold. Using a continuity tester, determine which wire on the IAB solenoid connector is connected to ground/negative. Cut this wire (leaving room to work with), then solder and heat shrink a 2-3 foot length of wire onto the remaining pigtail coming from the connector that you just cut. The other free end of the new length of wire needs to connect to a +12v switched ignition source. You can connect this to the Dead-End plug which has +12v switched power. The Dead-End plug is a large Grey or Blue connector with a large number of yellow-black wires. Use a volt meter to test the connection. Your IAB's are now wired "obd1 style" and are set to be used with an OBD1 P72 ECU.