83SC Speedometer questions

[HKZ Bob]

Dear all Electrical Fanatics,
I am helping my friend to get his 911 SC Mod 83 back of the road

The Speedometer does not work

Ignition on:
On point A is 10V and the schematic only shows one wire to be hocked up.
This 10V comes from the mileage counter. Is this normal
His car has a brown black wire addtional on poind A.
I have no ID what it is used for.
Who can tell me about this mysterious wire!


I assume the common failure mode is the ried switch.
Next time I will check the resistance from this unit directly 31B and minus.

Is point A supply ried supply and is 31b the positive side of the trigger of the ried switch.
How much is a new ried switch?

Thank your in advance for the help.

I notices an error in the schematic. Interface III(10) is listed gn. In reality this is brown.

BR
Bob