Back to my question. Trying to stay with the Socratic method, but it's tough for me. Think about what you were measuring when you put your meter probes on Pins 1 and 3 with C154 unplugged. With it unplugged, the Manual Transmission Jumper, which is the black loop on the mating connector, is missing. So, this is what the wiring circuit looks like with that jumper removed, and your probes at pins 1 and 3 (shown by the red and black arrows):
View attachment 536465
Looking at that schematic, what are the probes measuring? Anyone? Bueller, Bueller? (that economics teacher was definitely Socratic, too!)
Ok, it's time to discuss this post. Not a soul has responded, and of everything in this thread, I think this is the most advanced concept to understand, but it's also a very important concept. So, let me put this Socratic ball cap down, and put my didactic cowboy hat (my preferred head wear) back on...
A multimeter, when measuring resistance, applies a small voltage across the leads, the meter measures the resulting current and uses ohms law (Voltage = Current x Resistance, or V=IR) to determine the resistance, and that resistance is displayed on the meter's display. If there is no connection between the two points being measured, it's an open circuit with infinite resistance - current will not flow. In that case, the display will read something indicating open circuit (meters vary on what they display).
So, knowing that, is there a complete circuit to be measured with the probes at Pins 1 and 3 on C154? At first glance, I think most people's instinct is to say, "no, there is not - it's an open circuit. If you start at Pin 1 and follow the wires, you can't end up at Pin 3." There's no loop, right?
Well, no, not right. Yes, you do need a loop. However, there are actually many loops there, you just can't see them. Every one of those dashed boxes is a component - this schematic is incomplete. It doesn't show the other connections to those dashed boxes. Even G105 has more going on (it's not the dead end it looks to be), and that's key to understanding what you're measuring in the schematic above.
If you've been following along, you may recall that G107 is another ring eye on the same bolt to which G105's ring eye is attached. The other end of G107's cable is at the negative post on the battery. So, if you continue from G105, now off the schematic, you go directly to G107 and then from G107 to the negative post on the battery through that larger gauge main engine ground cable. Then, you continue through the battery from the negative post to the cathode plates, through the acid to the anode plates, and finally to the positive post. Still not a circuit though, right? How do we get from the battery's positive post to Pin 3 to complete the circuit we're measuring?
Through all of those dashed components, of course! Every one of them needs a complete circuit to operate, after all. Since we're doing our tests mainly with the ignition off, some of those components are open-circuit and not contributing. Others, however, are very complicated devices (like the PCM), or have their own "black boxes" on the positive sides of the dashed boxes above (like the oxygen sensors), and I have no idea what's inside those devices.
It's all these circuits that combine to make one very big, very complicated circuit, and I wouldn't even attempt to guess what resistance value to detect when testing as shown above. In fact, I would venture to guess that nobody in the world could tell you that without hours and hours of research. And that is why testing resistance as shown above doesn't really help us. Who knows what the value should be?