"AndrewV" wroteclip) It will run only when you hold in the start button,
shuts down as soon as you let go (clip)
^^^^^^^^^^^^^
So we know the start button and the relay coil work. There should be a pair
of contacts on the relay, in parallel with the start switch, which start to
supply holding current to the relay, so when you let go the button, the
relay doesn't open. Find this part of the circuit, and look for burned
contacts or a broken wire. That HAS to be it.