Does Jehovah always refer to the Father? It occurred to me that it might also refer to the Godhead in its fullness. Does anyone know and if so, how can we know? What Biblical evidence is there for this?
This is really hard to prove since Youngs Literal Translation uses the word Jehovah 5,787 times and in the American Standard Version it occurs 5,822 times. That is a lot of passages to look up. It also only occurs in the Old Testament. Which would narrow down it's potential use. Several scholarly works (of which I am NOT one) point to it's literal meaning as "He who is". Those same works go into great detail as to how it can be interpreted in slightly different ways (e.g., creator, I am, etc) depending on its use.
Now couple that with the fact that many prophecies of the Old Testament have more than one meaning (i.e., some refer to the coming struggles of Israel and the parallel struggles of Jesus on the cross at the same time). It is therefore possible that passages in the old Testament that use the word Jehovah could mean both the Father and the Son. So, for example, take Psalm 1:6 from the American Standard Version, "For Jehovah knoweth the way of the righteous; But the way of the wicked shall perish". Does that apply to only the Father or to any of the three?
Another one to try is Psalm 27:1, "Jehovah is my light and my salvation; Whom shall I fear? Jehovah is the strength of my life; Of whom shall I be afraid?" I can easily see this as a reference to the Father, but it has potential for the Son as well. It may be that it is true for the Son because the Son mimics the Father or it may be that the intention all along was that it applied to both equally for all time.
There is one passage in which Jehovah clearly refers to the Son. ""In that day," says the LORD, "I will strike every horse with confusion, and its rider with madness; I will open My eyes on the house of Judah, and will strike every horse of the peoples with blindness. ... And I will pour on the house of David and on the inhabitants of Jerusalem the Spirit of grace and supplication; then they will look on Me whom they pierced. Yes, they will mourn for Him as one mourns for his only son, and grieve for Him as one grieves for a firstborn"" Zechariah 12:4, 10). Notice that Jevohah (LORD) says they will look on Me whom they pierced. But is referring to Jesus as stated in John 19:31-37.
With nearly 6000 passages to check, I think it would be a daunting task to evaluate each one for its potential to mean more than the Father. If you could find a passage about Jesus in the New Testament that quotes a passage in the Old Testament that uses the word Jehovah, then I think you would have clear proof. Short of that, I think the best you could say is that it could possibly apply to more than one.