Many common OBD1 and OBD2 JDM engines do not have a VTEC oil pressure switch. The corresponding JDM ECU's for these engines do not use the pressure switch, so this is not an issue. However, when using a USDM ECU on a JDM engine that does not have a VTEC oil pressure switch, you will encounter a problem with VTEC not engaging do to the USDM ECU not seeing the signal from the pressure switch.
This can be easily bypassed with a simple wiring trick:
On your engine harness, locate the VTEC oil pressure switch plug/wire (this wire comes from OBD1 ECU pin D6). Use a common T-tap or other method and simply tap this into the VTEC solenoid valve wire (the VTEC solenoid wire comes from OBD1 ECU pin A4). Do not cut the VTEC solenoid wire, simply connect the Vtec pressure switch wire to it.
That's it, your ECU will now be fooled into thinking that it has a VTEC pressure switch and VTEC will now function.