Vibrator not working on iPhone 4S

Hello everyone !

After changing the screen of an iPhone 4S that was dropped, I check all the different part. And I notice there is one thing which doesn't work : the vibrator.

I did not know if it's my fault, so I disassemble again the iPhone and I check if the motor contact (two little springs) are aligned with the contact motherboard, it seems good, I cleaned them with alcohol to be sure.

I reassemble the iPhone, but the vibrator still doesn't work... :/

All of the other part are ok :

Volume buttons : OK

Mute toogle : OK, when I switch, the iPhone display me a popup

Headphone jack : OK, when I plug headphones, the sound is ok.

How to be sure it is the vibrator motor that is defective ?

Is there a simple way to "test" this part, for example branching an alimentation on the two little springs ?

I just want to be sure it's not a faulty cable...

Thanks !