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Originally Posted by cdru
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.
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Ok that gives me a clue as where to startlooking for the problem. The rear A/C unit is not working so now Im wondering if that could be where the problem is. Do you know if I could short the wires going to the damper control swicth out to bypass the unit? That swicth is not working and I have not been able to locate a replacement as of yet.