Been having an issue with alternator not charging my battery. 87’ 305 tpi. When I check the voltages at rest the alternator ground is about 1v less than the battery terminals. I did clean up the bolt a bit, however no change. All the other grounds on the vehicle I check are pulling the same volts as the battery.

the alternator was tested by a private shop and verified it’s working fine, and I just got a new battery.

Anybody have a good diagram on the wiring coming from the alternator, or any other thoughts on things to check?

 

That makes no sense whatsoever.

Voltage is between TWO points. For example, the alternator output terminal and its case, or the + batt terminal and the -.

"Grounds" don't "pull" volts. Just doesn't work like that. This is meaningless gibberish.

The alternator case is bolted to the engine castings. That is, the block, heads, etc. One can be pretty damn sure that all those things are at the same voltage, because they're all bolted solidly together. It's possible of course, that there's corrosion or whatever preventing all of that stuff from behaving as one continuous conductor, butt not likely. You can verify that by checking the voltage between the alternator case and the block; should be no more than acoupla .001 V. (a few mV) Ever, at any time, for any reason, no matter what.

A diagram would be more complicated than the actual THING. It consists of, a piece of wire maybe 18" long. It would take longer to draw a diagram than to describe it in words like I just did.

All that aside, it's a REAL SIMPLE circuit, in every way. There's no mystery of any sort involved. The - batt cable bolts to the block (or some other piece that's bolted solidly to the block as well, such as a bracket). The case of the alt, which is its - terminal, also bolts to the block. There's a piece of wire that goes from the alt output, its + terminal if you will, to the + terminal of the batt. The - terminal of the batt, the block, and the alt case, should all be at the exact same voltage. The output terminal (big stud w the nut and the big wire) should be at the exact same voltage as the batt when the engine isn't running, and should be a tenth or 2 higher voltage than the batt + terminal when the engine is running. You can put your DMM leads between the batt - terminal and the alt case to make sure they're at the same voltage, and between the alt stud and the batt + terminal to see what the voltage difference between those 2 points is, when not running and then when running.

Since you've had the alt "tested", I'll assume for the moment that it itself is OK. You probably don't need to do the measurements I just gave you, butt, maybe at some point you will need to do that. Let's hold that in reserve for the time being though.

The "Choke" light (not sure if it's still labelled that in 87, might maybe be a batt icon or something, not positive, although the 87 TPI dash I have in my 83 car still says "Choke", so I'm thinking that's what yours should be as well) should come on when you turn the key on before starting the engine, then go out once the engine has been started. Does this happen?

 

Battery power and ground cable connections can become highly suspicious, depending upon what termination methods are used, what metals are used, and the preparation. I have personally seen a voltage drop greater than 2.5V on a starter cable under load - SBC in a van. Under no load (or light load) the drop would likely be less, practically unmeasurable. Anything more indicates a problem, a.k.a., resistance.