What would be the GI voltage coming out of a sling socket ? Thank you ...

The General Illumination (GI) for most games is typically 6 volts AC.

The controlled lamps is another story, the supply voltage depends if the game
has a lamp matrix or not. Taxi has the lamp matrix and is 18V DC.

The Taxi schematics (available on-line via http://www.ipdb.org) indicates that
the supply voltage for the GI is 5.9 volts AC.
Thus, this is what you should read on the GI sockets, wherever they are.

According to the schematics, that supply originates from the power transformer,
connects via 7P3/7J3 plug, then connects 2P6/2J6 on the backbox interconnect
board, and leaves via 2J9/2P9. The GI for the playfield goes through fuses
F2 and F4 (5A each).

Bonne chance !
- Sylvain.

Merci Sylvain ...