I noticed my gas mileage dropping and vibrating feedback through the steering wheel one day. Looked under the hood, and there was my air conditioning compressor running, with cold dew on the pipe leading to the firewall.

Very odd! What is the solution?

Check to see that your blower isn't blowing when the fan switch is in the off position. It might be blowing quietly, so turn it to the dead zone between 0 and 1 to see if it quiets down. If it gets quieter, that means your fan is on all the time, probably because the air conditioning system is on. If the air conditioning is on, the fan will blow even in the off position. So you might check to make absolutely sure the air conditioning is completely off. In both of my 88s this means a little bit left of 12 o'clock. You have to turn it all the way until you hear the click and the fan turns off.

Was your defroster on? Must cars these days are wired that way in an effort to produce dryer air out of the defroster vents.

Worst case, your A/C compressor clutch may be seized in the engaged
position. Apart from that, make sure that the A/C dial on the
dash is turned _all_of_the_way_ off. Note that the beginning
of the blue zone is the fully off position, not the end of the
red zone (which is "fully on"). Some people may be deceived
that red=warm. Next, undo the two fasteners on each side of and
two bolts on the bottom of the center console, remove the ash tray,
and pull the console forward to access the back of the A/C dial.
Remove all of the wires going to the dial and see if that stops
the compressor from running. If it does, then your A/C switch
is defective. (Don't try to remove the white pipe going to the
dial - that pipe is an alcohol-filled heat pipe coming from the
A/C evaporator which enables the A/C switch to sense temperature).
If the A/C compressor runs with the dial disconnected, remove the
center console by disconnecting all of the wires from the various
switches and removing the A/C switch, still attached to its pipe.
Trace wires from the A/C switch to find the A/C relay, and see if
the relay is stuck closed by testing across the contacts with a
multimeter when no power is applied to the coil. Hopefully, a
pin diagram is printed on top of the relay.

Other problems may include frayed wires in the wiring harness
shorting to something. If nothing else works, you can try
disconnecting the wires from the A/C receiver/dryer, which is
a cylindrical object behind the right headlamp, to see if the
compressor stops running. The safety switch on the rec/dryer is
the final link in the "chain" of devices before the compressor
clutch coil. Disconnecting the wiring to it should pretty much
insure that no power is going to the compressor. To make doubly
sure, test for +12V at the wire going to the compressor. If no
voltage is present and the compressor still runs, you have a
seized compressor clutch which must be replaced. A kit that
allows you to bypass the A/C compressor and still run the power
steering pump is available from IPD if you aren't too particular
about having working A/C.

-b.

Volvo's (at least mine) are not wired that way, t.g.. You only need the a/c
to run in certain conditions, in particular, high humidity. If you need
to just de-ice the windshield, with the air temp. above freezing, so the
a/c is not disabled, all running the a/c does is waste gas. Worse yet are
M*rc*ry Sables with the auto. climate control, where there seems to be
no option to turn the a/c off. According to the dealer, the compressor
is engaged all of the time when the outside temp. is above 40F or so so as to
provide dryer air to the cabin. If I ever bought such an abomination, I
would add a switch in series with the a/c compressor clutch solenoid.
.
-b.