i have a 1997 gmc 3500 with a 6.5 litre in it and it almost seams like it has a short.when i turn the key on the dash lights up for a second then gets really dim when the glow plug light goes on and when it goes out try to start and the starter just clicks like the batteries are dead.pulled the battery leads and cleaned them and charged the batteries but still the same. the glow plug relay was chattering really bad and sounded like it was shorting out so i replaced it but no luck and not sure where to turn next i have noticed the last few days that the batteries seemed weeker or that there was and added load when starting but after a charge they test fine

any help would be greatfull as i dont have alot of diesel experiance im a gasser

 

Check your ground going from frame or firewall to passenger side head also make sure starter is tight and all wires to starter are tight.

 

*2 bad connection at ground from the batteries to the block or positive at the cable terminal end,or corroded cables under the insulation.