[KevinC]

My '99 7.3L PowerStroke Diesel has a hard time starting. While both batteries measure over 12.5 and full capacity, cranking the engine (the cranking seems slow) will drain the batteries enough fairly quickly -- to the point of not being able to crank the engine.

When I am able to get it started, the truck runs fine. When I get home and measure the battery voltage and capacity, it looks like they are fully charged. I did just replace the alternator (the problem pre-dated the alternator swap), and that helped for about a week, but it has gone back to being very difficult to start. When it's running and I turn on all of the electrical components (air, lights, radio), the voltage drop is not significant.

Is this a starter problem? Doesn't seem like a fuel pump issue because if it does start, it runs just fine.

 

[Flounder_Pounder]

Running voltage\output should be around 14.4v+/-, fully charged battery should be 12.6v. Might be weak batteries, weak starter, and\or bad cables\grounds. How old are batteries and starter?
For an Alternator, I went with a 6G 140, For the starter, I went with a Denso, had a PowerMaster for years but can say the Denso out spins it by a far bit!
Free things to try and help, remove and clean all cables and grounds at the engine and adding a second one to the back of the alternator or the bracket directly to the negative post is a good thing.

 

[klhansen]

What Flounder_Pounder said!

Voltage alone won't tell you whether your batteries are good, you need to get them load tested (disconnected from each other.) If the batteries are more than about 5 years old, they may be on their last legs.

 

[KevinC]

What Flounder_Pounder said!

Thanks to both of you. I replaced the batteries about a year and a half ago for the same reason, though I’m beginning to suspect them. Trying to start it this afternoon drained them. I charged them to almost 15v and the charger said they were at 100%. I was able to start it right up after that. Ran for an hour, and they still show about 14v at 100% capacity. Will check in the morning, and will follow your advice about cleaning connections.

 

[CarParts.com]

I also agree with the others. I would have the batteries load tested.