The accumulator has a pressure switch, looks like an oil pressure sensor.
Unplug plug, and use a short heavy jumper wire to bypass switch, now try
just a short burst please ! The switch keeps the compressor from engaging
if the freon pressure is too low.
Without this switch, the compressor would burn up, due to poor / no oil
flow. Assuming the compressor starts, and sounds OK with switch jumpered,
you could simply be low on Freon, or you have a leak. If it were my truck,
I'd convert to R 134A. Much cheaper in the long run,
if you plan on keeping truck a long time. Post back if you need more info
on the conversion.