Here is a long answer if you really want to know what is going on. Trace out the wiring in the service manual and you will see how it works.

Some correct answer some incorrect. All current is not routed through the amp meter. Basics: Battery can source more amps then alternator. 700 Cold Cranking Amps etc... The alternator is limited to a design current based on mechanical conversion of rotating magnetic field to create current. Rectified through diodes. So early amps from alternator limited to about 37, 50 amps. If you look at the wiring diagram, the alternator does not have fusible links, like the battery does in later cars. A short in the wiring before the car would start, while just ignition switch on, could source that 700amps from the battery which would quickly melt your 12, 14, 16 gauge wire. The alternator could not sustain that high a current.

The output of the battery goes directly to one end of the amp meter (and starter). The output of the alternator goes to a splice joint to feed ignition, fuse panel, etc, while one of the splices feeds the other end of the amp meter. The only thing going through the amp meter normally when you start the car, is the alternator feeding amperage to recharge the battery. Based on the voltage difference between alternator and the battery. Current flow from high voltage to low voltage. The alternator when good, will have a higher voltage then a new battery. The alternator runs everything via the splices before it ever gets to the amp meter. A normally running car, would have a trickle charge of the battery after it levels out from starting the car. (Yes battery can be sourcing some current since it is in parallel with alternator and two uneven power sources will load share to a degree, but the one with a higher output voltage will source the majority of the load).

Weak links are the terminal connections at the bulkhead. Poor connections as mentioned on the amp meter. Bad voltage regulator, or bad diodes on the alternator which would reduce alternator voltage. NOW the battery is feeding current via amp meter, back to the splice and even to the alternator. The other weak link is that splice. They route the alternator output to the center of the dash on a 70 B body, then back to the column for ignition switch. All that excess wire bundled together creates heat, raised resistance, more heat.

So the only thing the amp meter monitors is how much the alternator is charging the battery. Or with car not running, how much current the battery is sourcing for anything on.

A simple 10 gauge wire from alternator output to battery simplifies the battery charging and prevents sending the current into the dash, and then back out to engine compartment, when both the battery and alt are in the engine compartment. A volt meter placed on the alternator output to ground, tells you what you need to know. Alternator voltage output is higher then normal battery voltage.

So even with a 100 amp alternator your not at risk other than your amp meter does not have a fusible link to limit the now much higher amperage available for your original wiring. Yes can overload the amp meter and wiring now, if the battery is dead. Will act like a short to the alternator. If your interior wiring remains the same, and all your new loads are in the engine compartment with separate wiring, and fuses and relay (electric fans, water pump, separate wire to fuel pump). That amp meter is not at risk. Current is not going through it as long as the battery is good.

Current kills, so running wire from alternator through bulk head, back to battery, just so you can have a gauge tell you if your battery is charging or discharging is a little archaic with more modern alternators and regulators. A Voltmeter can do it and does not jeopardize the wire.



But the alternator is still beneficial where the voltmeter will not protect you:, Let say you developed a short on the ignition wiring laying across the engine. You turn on ignition but not start. You see amp meter peg battery discharging because of the direct short. You immediately turn ignition off and investigate. Early cars without any fusible link your ignition is not really protected from the battery. A voltmeter would not really tell what is going on. Voltage would droop, but not enough to warn you. 100 amps to fry a 16 gauge wire not a problem for the battery.

I left my amp meter on, but what I did do was double up my bulk had connectors, and I also sent the alternator output to the ignition switch directly, not to the splice and then back. I have fusible links on the battery output. When I start the car, I watch amp meter. I know where it indicates and how fast it comes down, even when sitting for a month.

My car prior to purchase had the engine harness burn up. I found that that overheated the ignition wire under dash at the splice and had to repair all that wiring. Had nothing to do with amp meter, but a shorted engine harness allowed all that current to go into dash harness to get to the ignition switch and back out. Bulk head connector melted too. Repaired it all and modified it for a more efficient electrical wiring. Without an amp meter you rely on blowing a fuse, breaker, or fusible link to know you have a problem. An Amp meter can show you a developing high current situation that a voltmeter can not. Just like watching oil pressure.