Stephan,
The low pressure switch, could be faulty, and not allowing the
compressor to kick on.
The key to this solution, was when the technician jumped the compressor
and it worked.
The thermal tube (expansion valve) wouldn't keep the compressor from
operating.
Could possibly be the temperature switch.
Check the continuity of the switch in different positions to see if it
is functioning.

Later,

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Terry 85 GL