1996 UD 2300

[UD2300]

I've got a 1996 UD 2300 with the FE6-TA straight six diesel.

If I leave the key in the on position for about 20 minutes and wait to hear a 2-3 buzzing sound, it then will fire right up. It will NOT START no matter what until I hear that 2-3 second buzzing sound.

Can anyone point me towards what this might be so ? This phenomenon started after the ground strap from the batteries to the chassis broke while going down the road. It always started fine before that.