I am guessing, the OP is helping out his neighbor who owns a Hayward swcg.Your profile says SWG Pentair IC 60 but your subject line says
Hayward flow switch...
What system are we talking about?
You need to remove the mainboard and inspect the conn jack for dirt, corrosion, bent or broken pin. Make sure pins 3 & 4 of the conn jack are making good contact with the mating plug. Please reference the link in post #4 if you need to replace the conn jack.Thanks, I'll check that tonight.
If I take the old phone plug and twist the wires together and the light stays on, what does that tell me?