Both voltage gauges historically indicated 13 volts. Today the port gauge is at 12 volts, while the starboard gauge reads 13 volts. Is my port alternator going out? Both serpentine belts are in good shape, no cracks and not slipping. What should I be checking? The engines have just under 1100 hours. Could the port voltage gauge be faulty?
Yes check AT the batteries with engine Off & On-running, since you could have a noticeable voltage loss between batteries and the dash gauge or MFD . Compare to the dash gauges .
And if you don't have a battery tester, I'd suggest you take them to any 12 v battery store ( auto parts, etc., ) and have them both conductance tested and resistance load tested for free.
I find this printing conductance tester handy: https://www.midtronics.com/testers/mdx-p300/ I print out , date it and tape it to the batteries for future reference. Use it on all my boat batteries, cars and those of friends.
Tip: If you get a not-fully satisfactory reading on the conductance test when connecting to the lug posts for instance, try the test again connecting to the threaded studs terminal posts, or vice versa.
Results between the 2 can sometimes vary a lot ( "Bad" vs "Good" and discrepancy between remaining CCA results) . Suggest you use the terminals with the Good results even if it requires and adapter.
Threaded posts, especially, can be compromised by over tightening the nut w/o using a thin wrench to stabilize the base hex on many batteries. But sometimes the studs work better than the lug posts.