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I have a 2016 F-250, 6.7L with just shy of 60,000 miles on it. I've noticed the past week, it has started to have intermittent weak/slow cranks the first few cranks, then will speed up to normal right before it starts. It doesn't take any longer to crank before it starts, just the first few cranks sound weak. It doesn't do it every start either.
The batteries are about a year old and measure a little over 12v with the engine off. With the engine on, they measure about 14.4v, so I feel the batteries and alternator are fine. Is the starter starting to go out?
Sounds more like the batteries....What the batteries carry for volts at rest and running does not tell you what they are doing with a load, so with your digital meter...I assume you have one, when it's cold in the morning, put your meter on the battery then as you crank the motor watch the volts, if they drop down to the 9v or 10v range causing the motor to turn slow for a moment, I would think you probably have a weak cell in one or both batteries, or you can go to the auto parts store and see if they'll do a load test for you.