Golf 1 Alternator

Good day, I need  help…my 1984 GTs (1.4 Carb previous owner changed the engine) alternator seems to be giving issues.
I've replaced battery.
Reconnected wires  with new lugs from alt to battery.
Blue wire works…when disconnected from alt… battery light by dash goes off. When connected back light goes back on…only when driving or revving past 3k rpm light goes out and I see the on my multi meter it's above 13.5 volts.
If I disconnect ground cable from battery car stays on, but when I put a load i.e fan, lights and radio…car dies out
I've replaced the regulator with Bosch one ….
Am I missing something?

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Check to make sure the belt is tight. Doesn't take much for it to slip a little and the alternator not spin to a sufficient speed.

What I have found over the years to insure that your alternator lives a good old age.

Proper tension on the belt.

Take the Pulley, and clean it with about 120 grit sand paper, it will clean oil and crud out of the "v" groove, and leave a bit of roughness to grip the belt.

On a/c cars, it is directly related to the tension on the a/c compressor belt tension, which is why the Barrel bolt tensioners on 84 Golfs is far superior to the 80 Golfs/Rabbit/Cabriolet belt and crow bar method, and the 90 Cabriolets Star gear.



Cleaning of the voltage regulator is a must.
see:
Alternator Tune up (Bosch 2 wire) | Volkswagen Owners Club Forum

Ground effects and your car, replace the grounds:
Ground effects and the repair of your car. | VW Vortex - Volkswagen Forum

Cabby-info.com has the earlier CIS grounds.

Add a ground on the alternator case nuts to the engine block.

This way you don't have the pivot bolts taking the ground who can get crud on any of the parts reducing the ground path to the alternator.

Make sure that your Clocks have a good ground to them, check the brown wire for 0 ohms to bare frame if not then splice a wire from the connector side of the harness to the frame behind your clocks, make sure that it is long enough to allow you to get your clocks out.

The Blue exciter wire on the motorola connector 3 wire, and or the single wire attached to the small stud on the bosch alternator has 12V Battery on it when the key is in the run position.

It's sole purpose is to start the field coils to charge, once the Charge is bigger than the input from the battery, it turns off the light by reverse biasing it.  

In the Voltage regulator there is a under frequency detect, so that is why at a higher rpm, blip the accel to get the rev above 1000 for a second that will turn off the light, since yours seems to need a bigger RPM, I would suspect that you either need to clean your grounds, add the ground to the alternator case, Tune your voltage regulator up (disconnect your Positive wire from your battery first). or tighten your belt and clean the pulley.

The Motorola Voltage regulator still could have the contacts cleaned up, sorry I don't have a good shot of it as the last one I saw was in 2000 when I sold my 81 Rabbit Diesel 4dr LS after 20 years of ownership.