I have a 2.8 CRD and did not have this issue, but for sure had this issue with my 6.2 detroit diesel.
I bought the 1984 CUCV (Military blazer) from someone, and I could only start it with ether, but it would NOT start on it's own. When I did start it with starter fluid (don't do that BTW) black smoke would pour out for about a minute and then be totally clear.
The previous owner had cranked it so much and on dead batteries (24Volt system) that it ground the flex plate teeth down and the starter would just spin when it hit the bare spots. So I changed the flex plate (and rear main seal, and mechanical fuel pump).
Still wouldn't crank without starting fluid. I checked everything from the fuel, to filters, used clear lines to make sure no bubbles, checked the injection pump, everything....then...
Turns out the glow plus were totally dead. I tested each one and not a single good one.
I changed them and the glow plug relay, and it fired up (and still fires up) with a flick of the key like it just came off the C130 in 1984.
I'm guessing that's the issue, or they aren't getting power.
Your voltage is also going to drop when turning the key because your glow plugs have to heat. There is usually a temp sensor on diesels that will indicate to the glow plug "job" on the computer to shut off the glow plugs once a certain temp is reached. It that never reaches temp because the plugs are faulty, it'll just keep draining, unless these plugs don't allow that to happen (there are different glow plugs that are self-regulating for protection for this)
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