amp fluctuating

[jimr37]

my 87 firebird 350 40 over is fluctuating amperage..i have a new alternator and deep cell battery..it runs electric fans,fuel pump, car stereo and amp with whoofer..didnt have this problem before i changed the alternator..been tested and it good..any ideas where to look for cause..no problems starting the car by the way..

[Chevelle598bb]

How was the alternator tested? If it was tested at the parts store most of them only have the capability of testing for voltage and if the diodes are working. It needs to be tested on a bench that can test for amperage.

If it was fine before, then you replaced the alternator and it has a problem now. I think you found the issue.

[jimr37]

was bench tested..??..checked out...

[Chevelle598bb]

Did the bench have the ability to check amperage? Or just voltage/diode operation.

[MonzaRacer]

So your worried about amperage varying under varying loads?
THIS IS NORMAL OPERATION OF A CHARGING SYSTEM!
Let me explain, a Voltage Regulator regulates the operating voltage used to spec a 13.9- 14.6, now because of most cars are loaded with electronics and we need to make systems do more with less alternator size general rule of thumb is 15v dc or under. I like to see a steady mid 14s to about 14.8.
Then as the loads increase like wipers wiping, amplifier thumping, putting on brake lights, etc can make amps gauge jump UP if properly wired. Properly wired you will see a -0.00 amperage while car isnt running and then it will jump up after car is started and gradually come down as battery is replenished.
Generally speaking a fuel injected car will pull less an 10 amps(figure 5-6 amps for fuel pump, 1-2 amps for ignition and 1 or less for computer and small accessories) no wipers, or fan motor running.
This is one good way to verify a battery with a weak cell as the charging voltage at idle or just off idle with no options on will get higher as the dead/weak cell starts to pull MORE amperage, loading the alternator.
I had a 124 amp alternator on my 70 Monte Carlo BBC and would watch the amps gauge bounce UP as subs hit. It IS normal and is nothing to worry about.
High amps draw at idle or just off idle(where we would call old choke driven carbs as HIGH IDLE, might indicate bad battery or heavy loads like a fan or fuel pump failing BUT generally electric motors will draw LESS amps as they wear because to wear on the armature leave LESS area for brushes to contact and pass voltage, also if a section is open or shorted it will change the effect on loaded amps.
Voltage regulators should limit most cars to 15v dc or less. Unless a computer drive alternator like Chrysler has used, anything 13.8-13.9 well is weak alternator or defective regulator.
Repair manuals can give you manufacturer specified voltages for specific cars.
Also one wire or self excited alternators the hot rod set use are worthless junk for anyone who really drives their car and cares about electrical integrity. Most USED to have an internally shorted or resistor jumpered inside to keep regulator turned on, but the advent of cheap off shore designed regulators in rebuild community have made that impossible nearly.
And I just have ZERO trust and/or luck using them at all.
SO varying voltage, take car up to 1500-1800 steady and allow it to idle couple minutes, it should stabilize voltage wise.
Amperage varies according to loads placed on system.
Amps gauges are pretty much worthless now days as biggest I see are most 60 amps, have seen a few 100 amps but the appropriate wiring is so heave its worthless and highly unsafe to run into passenger compartment as ALL amperage made or used goes through the gauge and wire and 8ga to 6ga is largest would run into pass compartment, and thats pushing it. Remember this handles ALL amperage car uses, if it shorts,,,,,FIRE FIRE FIRE, happens.
Use a quality voltage gauge attached to positive of battery and negative of battery makes more sense or as close as possible.
I also do my main charge wire larger, with a fuse link and as short a lead from alternator to battery, most new cars are like this. Some use mega fuses.
I like megafuses for fusing rest of electrical system at a main junction.
good luck and ask if I confused you, Ill try to get less technical.

[MonzaRacer]

Oh yeah most older alternators had analog regulators that had two ,maybe three charge stages. most NEW regulators even in internally regulated GM/Ford style alternators NOW use a digital regulator design, it turns on as needed as high as needed.
Old external regulators had three solenoids sending more power to the stator assembly to increase the out put, kind of like a low, medium, high type switch.
Now with electronics turning on as needed is much more regulated, to coin a phrase, and it allows better mileage as it ONLY loads engine as needed.