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2008 F250 V-10
Battery new and testing fine. Alternator working fine
Hook battery up and there is a huge draw the saps the battery within seconds so it won't start.
How do I go about finding the draw?
With that kind of draw it should look like you are arc welding when you put on the cable. Does it? That is to say, have you verified the battery voltage is actually dropping that rapidly after the cable is connected? The fully charged battery is dropping from say 12.7 to about 10 volts in seconds is a monster discharge!
My thought also, although usually with the spark that should result from so large a draw it often gives one pause to halt the connection. That was why I was wondering if the drop in battery voltage was measured.
Try disconnecting the starter and see if the "draw" goes away. If not, then disconnect the alternator and see if it goes away. If not, check your positive and negative battery cable connections to be sure that everything is OK and that they are connected correctly. There are not many things that can draw that kind of current. Make sure that the starter solenoid has the starter connected to one side and "everything else" connected to the other side.
I would start with a meter at the battery to verify the extent of the draw. If you have alligators for the meter, I would clip them on to watch voltage drop before connecting the cable.
charged the battery to 12V
unhooked starter and solenoid...no drop in voltage, left it overnight no draw down of battery.
hooked up starter and solenoid...no draw down
go to start it and it sounds like the battery is weak - barely turning the engine over.
let it run for 30 sec...shut it off and start it again...this time it starts right up like a new battery.
will a dying starter act like this?
If you measure voltage immediately after charging, you will be measuring the surface charge. To remove the surface charge, turn on the headlights for a minute and remeasure. Did you load test your battery or have it tested at someplace like Advance? This is behaving more like a bad battery than a bad starter. What is the battery voltage after you start the first time, but before you run the engine for any significant time?