Description

This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.

42 On the electrical diagrams, do the circuit numbers have any meaning?

A: Yes. VW uses Bosch numbering scheme, as do BMW & M-B. The most common ones
are:
1 = Ground (0 Volt)
15= Switched Positive (Hot when ignition is on)
30= Always hot "12 Volt" (even though it'll closer to 13.4 Volt)

[From Jens, still needs to be verified]:

    31 = Ground
    30 = +12V (always hot)
    15 = +12V (when ignition is on)
    50 = starter
     1 = ignition coil, low voltage
     4 = ignition coil, high voltage
    B+ = alternator main current
    D+ = alternator exciter (load lamp is connected to this one)
    DF = alternator input of regulator (not available from outside the alt.)

Not Bosch scheme, but ...

One important circuit for VW cars is X: it is basically the same as
15, but it is getting cold when 50 gets active, i.e. when you start
the engine. Ventilator, rear window heating and other circuits which
need a lot of current are connected to X instead of 15 to leave the
current to the starter when it needs it. You should connect your high
power amps/radios to X, also. Better no music for 5 secs. than pushing
your car in winter.

 

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