308--replacement "power distribution block"?

AHAH, I think you've got a lead on a big part of the problem, battery voltage shouldn't be dropping 1.5V with the A/C load. Also, if the fuse block voltage was measuring ~10V at the same time, then you're loosing another 2V between the battery & the fuse block.

What does engine RPMs lug down to when you do this?

What battery voltage do you measure if you increase engine speed to ~1000RPM?

Do you measure any voltage between battery -terminal and chassis w/AC on?

Yes,
normal for RH radiator fan to run w/AC on.

Other than measuring the A/C clutch current as suggested earlier. I don't think voltage readings for A/C relay, etc. are significant at this point in time.

Didn't mean to lecture, but realized that I several posters hadn't picked up on info in the other thread & were repeating suggestions.

 

Suspect a couple of problems:

I think that at least part of the problem is that the A/C is slowing the engine down enough so that the alternator can't keep the voltage up. The test with the engine @1000-1200 RPM should show a significant improvement in battery voltage w/the A/C on. If so, then you'll need to increase the engine idle speed during A/C season.

However, I think you may still have too much resistance between the battery & the fuse panel, or possibly in the alternator or battery ground path.

 

For alternator -ground solution, see my earlier post.

The fix will depend on the problem. Once you've established that the loss is occurring between two points, then start measuring between points along that path to see where the loss is occurring.

For example : If you measure 2V between the battery & fuse panel input connection, next measure between the battery & input to distribution block, also measure between distribution block input & fuse panel input to see which section of the wiring the 2V drop in. When you find a section with a loss, find a connection in the middle of it & measure between the ends. Repeat until you find a single connection or wire that's dropping the voltage.

Be suspicious of crimp-on connectors, they can oxidize internally. You may even find that you've got 3 or 4 crimp-on connectors each dropping 0.25 or 0.5 volts each. Check each end of the battery cable as well, those connections often degrade like any other crimped connection.

Often you can fix a lossy crimp-on connector by re-crimping it. Next try soldering the connection with electrical solder, or replacing the connector.

BTW, if you're dropping something like 2 volts between the battery & fuse block input, something's likely to be getting noticably warm or even hot, so you may be able to find the problem by touch.