[bestgenman]

Simon, you have stated  early that power to the relay was 86 and now you are saying power is on 30.  ?? 
30 closes to 87 when the relay coil is energized and the coil terms are 85 & 86 so, I’m guessing you are experiencing some confusion!
This solenoid requires a pretty healthy amount of current, hence the relay.  Terminals 85 or 86 will be connected to negative and the other comes from the engine control. So in other words, let’s say 85 is negative then 86 will have +12 when the control tells the engine to run.

The start / stop switch is a ground to start, ground to stop circuit.  it is likely not your problem (unless the switch is worn out).   The ground (negative) should be on the center  switch terminal.  There can be as many of those as wanted, they are parallel to each other.  One or a hundred.  When the start signal wire is grounded the engine starts.  It is only operated momentarily.   The same applies to the stop wire.

I suspect your control box is located adjacent to the set and is likely in a 8 X 10 X 6 JIC junction box.  It is full of relays.  Martin can rebuild it, I believe.  Your problem could be a simple as an intermittent switch.  If the problem exists at all stations then, it is control box or a loose connection.