Q: 350SDL with no start issues on 1991 Mercedes-Benz 350SDL

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It had not been driven for a year. The Battery was replaced 6 months ago, along with the spark plugs. The glow plug indicator does not turn on on the dashboard anymore.
My issue is it will turn over (crank) but not start.
The first try it only turned over but wouldn't start so we fully changed the battery and it did start. we drove it 500 miles, turning it on and off at rest stops and to fill the gas tank up. But once we parked it for the night it would not start the next day, only turning over (crank) but not start. Any sugestions? We are currently rechanging to battery again fully. But not sure if something is draining it over night, or something else?
(2) Answers
(3) Comments
I think that you mean that you replaced the Glow Plugs, a Diesel has no spark plugs. If the glow plug indicator does not light and the engine will not start I would check the Glow Plug Relay, it may be worn out. Mercedes calls it the pre-glow time relay.
Thank you Dandd for your reply. I feel very silly right now. I used to have a 1982 300SD in school, I should know better. Guess my memory has slipped!
Yes you are correct no spark plugs. I received clarification from family that had some repairs done on it before we just got it. Just the battery was replaced. The Glow plugs were not replaced. Since my original post we have replaced all fuses and it started after the battery was charged again. It had a bit of a rough time but it eventually did started. then idoled fine. We turned it off for 15 minutes and it started right up. At the moment we have a trickle charger hooked up to it.
Could a bad Glow Pug or relay drain the battery in 10 hours? THis is what happened in past.

Thank you again Dandd for your input!
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First, the alternator needs to be checked to make sure its charging the battery. Second, check for a parasitic draw. Yes, it MIGHT be the glow plug relay, but not very likely. If it was hard to start when cold, this indicates that the glow plug system is not working. Perform amperage test on each glow plug to determine if one or more is defective. You will need someone that knows this system to do a little diagnostic work for you.
Thank you everyone for your assistance! We tested Glow plugs and Relay. Glow plugs bad and Relay working fine
Just got it back from the mechanic: replaced glow plugs and it starts right up.
Now for the drain on the battery... they traced it to Fuse 15. But we will need to take it back to have them do more research to figure out what on Fuse 15 is draining it.
With Glow Plugs replaced it is not draining it enough to cause an issue now.

Just wanted to update the forum for those who helped or run into same issue.