this is from a DIY in the how to portion, seems a bit more condensed as a guide, although still not sure what voltage should be seeing:

went on to see what all of them did.The following from 750i.de :
"Turn ignition to pos. II
Push

right button in the instrument cluster and keep it pushed. You will
first see "Check Control OK", some seconds later you will see "Test
Nr.01"
Push the left button on the instrument cluster to see the
car's identification number. Add the 5 digits together and write down
that number, you will need it for unlocking all the functions. Example:
DG23278 will lead to 2+3+2+7+8 = 22
Push the right button until you see "Test Nr.19"
Push left button until you see the number that is equal to the one you calculated above
Push right button again "
This will give you access to all the tests.