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hello , i have a 1939 chevy 1.5 ton truck , i put in a 350 sbc with an 12 volt up dated system . my question is i have ran this system for 2 years, now when i start the truck the volt meter shows charging for a few seconds then the reading on the volt gauge drops . i have searched all the wires and fuses , plenty of power from the alternator , i am missing something . in the past the alternator always showed charging .thank you 1pete1.

Are you sure you have a voltmeter & not an ammeter. That is the normal way for an ammeter to act. A voltmeter will start off low (about 12 volts) & build up to about 14 volts as the alt. or gen. replace the current used for starting.
George

If you actually have a voltmeter, check the tension on the alternator belt. Ditto on the comment about ammeters- - - -the initial charge will be high, then the amperage tapers off as the battery voltage rises.

Did you change batteries as part of the electrical upgrade? A bad battery might not charge up properly, making the charging system run a constant heavy current into the battery. That would keep the ammeter in the "charge" range all the time. Just a possibility of why you're seeing a "normal" response now.
Jerry

You do need to quantify this complaint. What voltage does it read on startup and what does it drop to?

Also, I suggest a test.
Put a multimeter, in volt meter mode, on your battery, at rest, to see it is at 12.6V. Then have a helper start the truck. See what multimeter does. (state actual values).
Depending on what it does and the recommended test/causes others have advised, you will eventually find it. A voltmeter will start high and drop also.

Some causes, IF there is a problem.

1. Bad Gauge
2. Bad battery
3. Loss of ground somewhere
4. Bad alternator
5. Cables/post clamps
6. Idle speed
7. Trailer lights?

A voltmeter will start out LOW- - - -and rise as the battery recovers its charge from the starting load- - - - -like somewhere in the neighbor hoof of 13.5 volts after startup, and 14.5-14.7 at full charge. AMPERAGE starts out high and tapers off as the battery recovers.
Jerry

Reason I asked about voltage values is suspected voltage drop, possibly normal, if heater, radio, lights turned on? Jerry, are you saying volt gauge needle won't drop with draw? Guess I'm wrong on my thinking.

The charging system supplies the accessories first- - - -then whatever is left over goes to recharge the battery. The load of the starter lowers the terminal voltage, so the charging system only has to raise its voltage a little to force the charge back into the battery. It makes no difference what other accessories are drawing current. If there happens to be a situation where high beam headlights, full speed heater blower, and a few other high-draw accessories are operating, (Booming sound systems with high wattage amplifiers, for instance) it just takes longer to recharge the battery. As the resistance at the battery posts goes up, so does the voltage to maintain the charge rate, up to the point where the voltage regulator kicks in and keeps the charging unit (generator or alternator) from overcharging the battery.

A battery acts sort of like an accumulator in a hydraulic system or a storage tank in a water system. It stabilizes the pressure spikes the pump (alternator) produces when load changes, up or down. Never, EVER disconnect a battery cable with an engine running. I've seen spikes of over 100 volts in a 12V system on an oscilloscope when someone disconnects a battery cable.
Jerry