Ammeter

That isn't an ammeter light, it is the Battery light it detects voltage on Battery start and lights up because it is a higher voltage than what the alternator puts out. At the Same time the Alternator is fed a Tickler (or Exciter Voltage) via that light to tell the Alternator to Start a Field and then charging starts.

If that Light is on, and doesn't flicker then you have to make a Guess that your car is not charging the battery, and that you are running off the battery.

On your alternator there is a plug that has 3 wires. Two are 12V lines to the battery, the third is the Exciter wire that feeds directly back to the Instrument cluster.
(Blue wire in).

When the blue wire in has a Charge, then your led is reversed biased by the Higher voltage output of the alternator than the battery in and turns off. There is also a Diode in the Circuit of the lamp that takes the Reverse bias and turns off the Battery light.

So, have you had your alternator tested?

I would ring out the wire from the Alternator Plug Blue wire, to the Dash Connection to verify that it is intact or has a short between both connectors.
If the Alternator or Voltage Regulator checks out. Then you are looking at a possible bad Ground on the Instrument cluster, or a bad diode, as we know that the light works.

I would also have your Battery tested, and of course the Ubiquitous if you haven't replaced your battery to Frame and frame to engine ground now would be a good time to do it, as well as adding a ground from the engine block bolt to the Case nut on the alternator.

Ammeter is a Gauge that is in series to the Battery and detects amperage in the car, as in output of the Alternator/battery.
A Voltmeter detects the voltage at a given point and tells you the condition of the voltage being produced in that Circuit that the volt meter is across.

The VW uses a Voltmeter in the Center cluster or pod, and it is off the Switched side of the Cigarette Lighter to tell you that you are at 12,13.75,0r less voltage.
Voltage is felt in a circuit.

Amperage is the current in the wires as in 65,50,or less depends on the alternator that you have installed it is the real power and can only be measured in series in the circuit.

VW being the cost effective SOB's that they are use a volt meter as they are cheaper to make and easier to install.

E=ir or Electromotive force, ir voltage expressed as (E) is equal to the Current in Amps expressed as (I) times Resistance of the circuit to current flow expressed as (R). This is basic Ohms Law and one you can never get around or break.

This can be likened to a River. The Current is how fast the river is running, Resistance is every pebble, branch, log or DAM, that impedes the flow of the river.
Voltage is like dipping your finger in the water and you say it's wet or cold....

The size of the alternator shouldn't be an Issue to the Battery light.
The voltage that the Alternator is producing should be that it is charging the Battery when running, the difference in it being off and it goes to 12 states that you
are probably have a 12V which is normal.

I suspect that your Exciter wire or that the Circuit isn't back biasing the bulb to turn it off.
That could either be the Bulb, the Diode.

The diode has a Positive and a Negative leg, the negative or "N" junction is denoted by, the Band on one end of the diode, for the circuit, and the flat side of the LED case and a shorter leg. Diodes can regulate if the circuit is designed as such, but in this case the Power from the Cluster on Run prior to start is such that Current flows
in the circuit as the Ground comes up from the pin2 (brown) wire on the cluster through the black diode causing the lamp or led to turn on.
When the Car is started and the Alternator detects the positive voltage on the Lamp it builds a field and starts charging..The charge from the alternator being greater voltage than the 12V in from the Black wire pin 13 causes the Same voltage to be applied to the bulb and since the bulb or LED has the same voltage on both sides it turns off.

Since yours doesn't then you need to concentrate on the Diode (not the led). It could also be that your new alternator does it have a PLUG or 2 Studs and uses ring connectors? as if you had to remove the plug and convert then you may have an issue with the "blue" wire on the alternator they do go bad...

Key on in run not start there should be a voltage on that wire (blue) if you disconnect the Plug, and measure the blue wire to ground.

This diagram is for a Caddy, but I think it is wrong.





Here is a Westy Rabbit.


There isn't a Diode in the Circuit but the LED in either, so while the Diode is working then suspect that the BLUE wire has a Break in it and is floating.
Some voltage regulators use internal 12v feed to start the field to start charging via the 12V line into the regulator from the batter directly which could be your case as the alternator starts charging it sends a Sense Voltage to the LED to turn it off. I suspect that your blue sense (exciter) wire has a break in it at either the Plug on the Alternator (quite common) or at the cluster side.

One way to test is to jumper 12V to the Blue wire on the cluster to see if the bulb will turn off as I suspect it will.

Since you have upgraded to 120A Alternator you may want to upgrade the Alternator plug to Ceramic for the increased current that you are producing as the old plug which is Rated for 55 amps may not live long.

You can use insulated Female Spad connectors and do away with the plug by using separate wires to the 3 pins, I would use red for the Positive ones and a ble one for the exciter wire...

https://www.serial-kombi.com/en-GB/...cal-/-fuse-box-relays-and-wiring-looms-n38553

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