Serving Our Country!
Join Date: Aug 2007
Location: Manor, TX
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You're in the ballpark, but there are a lot of if's that come with that.
If both batteries are at 750cca, then they should be 1500cca together.
850cca is a good size for these trucks, but it doesn't hurt to go higher.
12.3v is the surface charge, you need to load test them to see the actual voltage.
Is the 57% SOC after charging the batteries? If so then your batteries are only holding half of their original capacity which would explain the 1120cca and hard starts.
60a for the alternator and 13.29v is a bit week, but could be because the batteries are drawing it down or could just be the RPM it was tested at. Realistically, I would prefer to see 13.6-14.2v from the alternator and at least 90a from a stocker.
The current the starter draws depends on its condition and the load. The starter on my 2000 is marked 3600w which would be 300a loaded. If the truck is cold, the fluids will be harder to move and require more from the starter.
You also have glow plugs that come on every time you turn the key to run for up to 120s depending on temperature. So that's another ~100a draw.
My guess is your alternator is getting weak and not fully charging your batteries.