91 f-250 idi turning slow warm. Good starter, new batteries
 

Well guys, I've owned her for about a year now. Love the truck, seems like one thing after another though. So anyway, I'll attempt to make a long story less long, truck sat most winter due to bad block heater plug in, put that in a few days ago. Also has brand new starter, had old one tested was bad, also new batteries (850cca) it was a cold 30 degrees yesterday in PA and couldn't get the truck to fire up. It's a hard HARD cold start. Fires up first or second twist of the key in 70 degrees +. So I killed the batteries, took them to get charged. Didn't even put them in until today. I turned the key one time this morning. Didn't want to risk killing the batteries so I decided I'd plug it in for awhile. I left it plugged in for 45 minutes and I felt around the block plug to see if it was working, it was. So I unplugged my extension cord. And go to start the truck. The motor turns fast and fine, fired up but died out. So I shut the key off and go to turn it again. It turned extremely slow! Like completely dead batteries, so far I have checked batteries both reading 12.3 with multimeter/ test light. Have tried to jump the starter, still turns slow. I cleaned all battery terminals and leads. I looked at the grounds from the negative leads. They both ground into the front end of the block, however that's where I stopped because I ran out of daylight! Please any info or ideas would be greatly appreciated. I am fairly new to diesel. So I'm not sure what could have caused this! Thanks in advance.
Nick

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I'm sorry, I think I posted this in the wrong forum.