Ok so met a 96 F350 on a trip this weekend.
He sprays ether in the intake so it will start. Crazy

Anyway I tested gpr with test light and it passed.

Can it still be bad even though it clicks on and off?

 

Have you tested the glow plugs?

 

Yes - relays can have internal contact issues, while still clicking and appearing to work. I would measure the voltage drop on the entire system when the relay clicks - you should lose 1.5-2v due to the current draw of the glow plugs. Ultimately, you should test each glow plug at the harness.

 

You say it passed. How long did it say on and how cold is it? If it is cold, it should say on for two minutes before clicking off if it hasn't been run yet.

 

This is my old relay cut open. I started having cold start issues and found that my relay was not passing full voltage across the terminals. I could hold an old screwdriver across the terminals for about a minute and it would start right up. As you can see the contacts had gotten corroded, burned and damaged so even though the coil worked and pulled the contacts together it was not making clean contact and passing the voltage.

 

Get a 12v power port plug in meter and you can see how much voltage drop that you have when your GPR comes on.