boost sensor and starter wiring
 

There is a wire connected from the boost sensor to the starter signal wire.
On my buddys car I dont see this wire.
It looks a bit self fabricated by the previous owner.
My car wont start for a couple of weeks now.Got feul spark and compression , set timing.
The wire seems to operate a black relay when i connect it to the starter +. just below the boost sensor.
Wich relay is this? And is it suposed to hook up to the starter signal wire or + wire?

I kind of dont wanna scrap a car that seems to have every part working I'm sure its something smal why it wont start.

I would appreciate any ideas and sugestions.

Sjef from holland.