I have a 98 E300 Turbo diesel with 220K miles.
I turned the key to glow the plugs this morning. The check engine light and glow plug light stayed on. After 15 seconds, I turned the key. The engine turned over but would not start. There was a humming sound like the plugs were getting power and when turning over, it sounded like it was getting fuel. Could this be the glow plug relay? I have seen some threads about a 'K40'. What is the 'K40'.

Thanks for the help.