Number one refers to number one cylinder, not number one in the chain. The first in the chain being on the back would be the early series glow plugs which rarely need replacing, just cleaning.

I learned from experience a number of years ago to replace all the pin type glow plugs and be done with it. It will usually make a big difference. Of course, in those days the plugs were only three bucks each.

Number one, the front cylinder, current is measured by the relay to decide when it is ready to start.

Good luck,