I am asked frequently about replacing the 3 speedo bulbs with led items, especially the ignition on/charging red light.
I have tried to illustrate how simple this is. There is an anomaly that VW introduced with it`s speedo. All 3 of the "bottom" bulb sockets are co-joined and are wired in what is known as "reverse positive" function. This simply means that whereas in standard configuration the contact for the bottom solder contact of the original incadescent bulb would be regarded as the positive connection and the metal shell the negative connection in "reverse positive config` it is the opposite.
The incadescent bulbs found in the holders at the back of the speedo are not polarity conscious but standard led bulbs are and will only light up when the positive and negative wires are connected the "correct way" round.
Right, briefly, a dynamo generates current as soon as it starts spinning albiet not a lot at idle speed. A alternator gives out a higher current from a lower spin speed. An alternator will not generate a current, or measurable voltage until it "sees" a voltage of ,in general, 4 volts at it`s coils. VW utilised the incadescent bulbs inherent resistance to allow a voltage to be seen by the alternator.
An led uses such a small current alongside voltage that there is insufficient feed to the alternator. This however is easily remedied. All you need to do is to obtain a "reverse positive" led bulb of the same variant as the normal bulb and push it in the bulb holder. Push the holder into the speedo "socket". Link a incadescent bulb across the led`s positive and negative terminals and this bulb will then mimic the original bulb and the alternator will be happy. I have one on my buggy and it has been in operation on the road for coming up to 3 years now and has worked flawlessly. (Both on a engine with a alternator and with one that has a dynamo in place)
The wiring differs in that the wire from the bulb goes directly to the alternator connection. But with a dynamo the wire from the dynamo links to the voltage regulator and then links from the regulator to the speedo bulb. I use what is known "as a grain of wheat" incadescent bulb as the resistance because they are truly tiny and readily available. I have shown one on the photo. (Some of you on the forum will have seen the grain bulbs in action on radio controlled toy cars etc).
Look at the photo to see how simple this is to do......
(https://i.postimg.cc/3JW6dRcX/LED.jpg) (https://postimg.cc/bscL5z8r)
.
Bit more for those new to this....
One image shows the reverse positive connections x 3 at the rear bottom of a Beetle VDO speedo. The other image shows the gels that gives the red/green colouration to the lighting. Looking at the gel unit the red on the left is the charging indication. Centre is indicators and right side gel is oil pressure. (I have noted the speedo needle magnetic drive disc within the housing. This is spun by the speedo cable and the speedo needle reacts to the spinning disc... just for info whilst it was in front of me).
Note also the earth/ground tab on the speedo casing. VW chose to have this present despite the casing securing lugs grounding the the Beetle shell. Follow suit and give the tab a good ground link.
VW utilised the rev` pos` terminal to jump 12v plus to the regulator input and again if you new to this an important factor is that the output from the regulator is 10 volts not 12volts. (12 volts fries the fuel gauge movement).
(https://i.postimg.cc/Dw070DxM/Speedo.jpg) (https://postimg.cc/WhBBf9LG)