Troubleshooting Truck electrical

[maxhifi]

Monday night I went to start the truck, and was greeted with a run down battery. Not enough juice left to crank it fast enough to start.

Checked out electrical system, and figured out there was no output from alternator. Bought a new alternator, but before I changed out the whole thing I decided to try just the voltage reg. And brushes from the rebuilt alternator.

Sure enough it works now, battery charges, back to normal

But: charge voltage is 13 at idle and 13.5 with engine revved up high. It's within spec but seems to me kind of low, and I am at a loss as to why. I would much prefer to see 13.8 - 14.5 with engine revving.

I am thinking for next steps: check battery terminal of alternator with scope to verify if I have any bad rectifiers

Check charge current at idle to verify if I have any parasitic loads

Unfortunately regulators for this alternator are pricey so I can't easily swap in a new one just for the sake of testing.

Truck is an 86 Toyota pickup, with 2.4 Diesel engine.