I have a Williams Grand Prix that seems to work fine for one player.
Once I put it on 2, 3 or 4, problems occurs.
The player will not advance beyond player one and will remain on ball 1.
Here is what I checked so far:

-Player unit in backbox advances when manually triggered, but coil is not firing.
-Trace it back to the player reset relay, which is also not being triggered. checked switch on relay, all ok.
-Trace player reset back to the switch stack on the score motor, and verify the switch is triggered by the score motor.
-The score motor triggered the switch stack after P1 ball 1, but the player reset doesn't fire.

Any ideas where to look to troubleshoot fix this issue?

Thanks.

I try manually closing the switch to activate the coil and did not get any result.
I also manually closed the relay to activate all of the switches in the coil with no result. I could think even manually activating the relay bank of switches would have done something..

Yes, I do have the schematic but I haven't used it much in track the problem. I did not measure he resistance on the coil (but I willl tonight). Most of the coils int he machine appear to be fairly new. I think a previous owner had a lot of work done on it at some point. I found a business card of a pinball restoration company in the machine when I was working on it.

Thanks rolf_martin_062 , that was helpful. I was able to get the players to advance. The player unit looks clean, but was not advancing as smoothly as it should when the step-up coil was triggered.

But, it exposed another issue. On 4 players games, after the four player's ball drains, it does not reset to player 1, ball 2. It remains on player 4, ball 1.

-I tested with 1, 2, and 3 players games. They all trigger the player-reset and advance the ball count until the end of game.

What keeps track of the "final player" so that it knows to fire the reset coil on the player unit?

Thanks.