I got the 98 service manual in front of me and the DTC P1639 is indeed for 5 Volt Reference (B) Circuit. The 5 volt circuit is used for sensor feed for the A/C refridgerant pressure sensor as well as the fuel tank pressure sensor according to the wiring diagram. The DTC is set when the PCM detects a voltage out of tolerance on the circuit for more then 10 seconds. The tolerance is 5 +/-.5 volts. Check connections, as well as engine ground points.