Why does L7905cv have an output voltage of -10V instead of 5 when inputting -12V ?

[QYape.1]

I' m sure GND has been connected. 

[s__it]

Have you fitted the necessary capacitors (IN to GND and OUT to GND)? Also note that the negative regulators (79xx) have a different pinout from the positive ones (78xx). See datasheet for details.​

[Peter BENSCH]

Welcome, @QYape.1​, to the community!

Did you set up your circuit according to Fig. 4 of the datasheet of the L7905CV?

Are the capacitors present and located close to the L7905?

Regards

/Peter