dieseldo

New user first time post
I've got a 2002 7.3 excursion
Just started having a start problem
Ohmed glow plugs there good
Changed gpr it worked for a few days
Then stopped working again
Checked voltage on the relay wires and I have 12 volts
On both of the coil wires, not the large battery and glow pug poles
But the wires that open and close the contacts
I've done some reading and it seems the relay is actuated by making and breaking ground. so it seems I should have positive voltage on both sides. If I remove (purple/ground wire) and ground it
The coil actuates I get glow plugs and it fires right up
I do get the wait to start light on dash and tach works
Batteries are good.
Any advise or wiring schematics would be great