I want to encourage you in this RScanz. You'll learn a whole new language and some important troubleshooting skills. GMC V-6s are unusual eninges and aside from your present electrical search you'll learn about how industrial engines were built. In some versions 2 351s (the larger variant of your engine and available as an option in pickups) were married together to create a 702 V-12. The one I saw was used to power a piece of backwoods logging machinery and was on skids at the antique truck show near Tacoma WA. It had a really impressive sound when they fired it up. It had been the modern replacement for a steam donkey-engine on a high-lead logging show in the Cascade Mountains pulling huge logs on cables toward the landing where they loaded them on the trucks.
But back to your present problem. Long ago there was an Italian fella named Ohm who was a scientist working with electricity. He developed the concept of electrical resistance and developed a formula used even today called Ohms Law. Ohm worked with a couple other fellas named Ampere and Volta. Scientific history has named three important electrical properties after these men: The Volt, the Amp and the Ohm. Ohms law describes the relationship between these properties: It says that one volt of electrical pressure (abbreviated as E) will push one Ampere (a measure of electrons flowing past a given point in one second) abbreviated as I, through a resistance of one ohm, abbreviated as R; Ohms law formula is I=E/R.
If you know any two properties you can find the third one using this formula. You can algebraically manipulate the formula to get any one of the three on the left side of the = sign. This formula is really handy in automotive work where you often want to find amps or volts but ohms get in the way. Take your resistance wire for instance: I takes battery voltage and reduces it to a safe level for a 6V rated coil, except when you turn the key to the start position when the coil wants full voltage to compensate for the voltage reduction caused by the high current flowing to the starter. Then the resistance wire is bypassed with a normal wire. And ohms goes almost to zero except for a few tenths of an ohm in the wire and connections allowing volts to rise almost to battery voltage (there might be a couple tenths of a volt lost in the wire and connectors).
Small amounts of resistance (Ohms) are difficult to measure accurately without specialized equipment. Some high end Digital Multimeters will measure low ohms but few people have them. So we usually must infer that there is more resistance than desirable by measuring the voltage drop between two points. For example in a starter circuit where there may normally be 100 amps in the starter cable even a few tenths of an ohm will cause a no-start condition. For example: 100A=E/0.3 Ohms; where E is the voltage drop between the battery and the starter. Solving for E we get: 100A*0.3ohms=30V. So clearly 12 V won't push 100 amps through 0.3 ohm and the starter won't rotate the engine fast enough to start, if at all. Measuring volts between the battery and starter we would see a voltage drop of nearly all of battery voltage. Time to clean the battery terminals.
