You replaced the compressor and it still won't come on?
the way the factory Expedition & Navigator
compressors are built, they consume more amps
than any other Ford/Lincoln compressor of comparable
size and they are extremely hard on 2 things.
One is the armature inside the electric motor
part of the compressor and the other being the
power supply, which is the compressor relay.
the armature goes bad, the power consumption
can increase and require even more power from
the relay. These relays are a solid state design
and don't do a very good job handling such a
large current load and will usually be damaged.
that being said, if you have replaced the compressor
and it won't run, check for power at the compressor
within 90 seconds of turning on the ignition.
(after 90 seconds,
the module will time-out and give up)
worry, you didn't replace the compressor for
nothing. You "may have" temporarily
gotten it working again by just replacing the
relay, but because the compressor was actually
what started the problem, the new relay would
be damaged in a short time if the compressor
hadn't been replaced.