Alternator voltage low on brand new alternator

Can anyone help. 

 Iv brought 2 brand new alternators and both are reading 12.30 volts. Direct from the alternator and also then from the battery. It doesn't rise when under rev load but drops when lights and fan kick in / put on and returns backs to 12.30 when switch them off. Iv tried 2 alternators and both are doing exactly the same.  Iv also changed batteries and used 3 different volt meters and all saying the same.

I really am stuck 

year and model would be nice to know as well as alternator type  motorola flush regulator with plug  or bosch with 2 studs.  did yu have both tested at the store where you bought them?

The voltage shouldn't drop with a load if the regulator is working correctly. - the indicated voltage is too low. have you got some other load in the circuit you don't know about. Have you checked the current drain from the battery with everything switched off and then with the ignition switched on, might give you a steer. The other possibility is that your meter is defective. Have you checked it on a 1.5V battery for example, which should read 1.6- 1.7 V for a new cell, regardless of the fact that you have tried 3 meters

The reason the new alternators aren't charging is usually because the exciter voltage from the battery to the clock, out the clocks via the blue wire isn't getting to the alternator.  

He didn't mention if the Battery light is on or not as well as the type of alternator that he has.

Check your grounds, here is a quick test.
https://www.volkswagenownersclub.com/threads/brianos-quick-grounds-test.134637/?post_id=765374#post-765374

Here is a parasitic draw vid.
https://www.volkswagenownersclub.com/threads/battery-parasitic-drain-diagnosis.128646/?post_id=752813#post-752813

" to the clock, out the clocks via the blue wire isn't getting to the alternator.  "..You've lost me here

The Alternator takes an input from the battery from the Instrument cluster (clocks).

In the clocks ties an  input from the battery  to a resistor and a LED and a Reverse diode, that comes out of the clocks via a blue wire that is in the plug or the Little blue wire on the Alternator as I need to know which alternator that you have.
Motorola with flush regulator or Bosch that has a Big and a Little Stud on the rear.

When you are cranking the engine, the Battery lights the light on the dash that says you are running off the battery.

Once the Engine Catches and starts, the Alternator Feels the "Exciter Wire/Blue) and that wire has 12V on it to excite the Field coils so they start producing energy. or 14Vdc

Once the output voltage of the alternator exceeds the
12V of the battery the Battery light on the Clocks is reversed biased because of the diode and turns off.  Any time that the Alternator Fails or the battery, the
Lamp in the dash begins to light up.  When you see this light on, you are running off the battery.

On some VW's MK1's the alternator has to be goosed or the RPM boosted above 1100 RPM for a minute so the under freq detect of the Voltage Regulator Circuit goes out.

Depending on the make and year and Alternator there are fixes that might make the goosing the throttle a thing of the past.

"On some VW's MK1's the alternator has to be goosed or the RPM boosted above 1100 RPM for a minute so the under freq detect of the Voltage Regulator Circuit goes out."

That explains why my batery light stays on until I rev it briefly! I was thinking my alternator was at fault, even though it charges perfectly well. Wondered if i'd got the wrong size pulley on it. I shall just accept it is one of the little quirks of the car.
Thanks!
 

Depending on the Model of he alternator.
On a VW Cabriolet with a Bosch 2 stud Alternator not the recessed plug Motorola style.

There are a couple of things you can do.
Clean all your pulleys to remove all the grease and chaff/grime…(all the Pulleys Including the Crank, water pump, intermediate shaft as well as alternator.

Using sand paper slightly scuff them up on the edges to give the belt something to Bite on.

Then Tighten normally.

On a/c and PS equipped don't forget those as well.

ON the 2 stud Bosch Alternators you will need to remove the Voltage regulator and do some house cleaning.

See this thread:
https://www.volkswagenownersclub.com/threads/mk1-bosch-posted-alternator-tune-up.135143/

Now a Given is that I had new Ground cables that had 0 Volt Drop, and 0 Ohms resistance between the Battery negative, the Engine, and Frame.

I also ran a ground wire from a Case nut on the Alternator to the Engine Block Alternator Bolt I figured that bonding that this would be better than a Bolt that could get dirty.

See the Gimmies that I found after I did a little house cleaning.  Never had to Goose my engines any more.