Bosch drill switch

[ThomasWheeler]

Hi, I'm Thomas. I have a Bosch 0601915720 cordless drill. It uses a 7.2 V battery. I had replaced the drill's battery and was using for about ten minutes when the motor slowdown a lot until it stop completely, but the voltage of the battery is still 7.2 V. So I measure the voltage of the drill's circuit. When I press the switch the voltage drop from 7.2V to 1V. So I try connecting 7.2V directly to the motor and it spin normally with the voltage dropping to 5.7V. So I check the continuity of the switch, and it seem to be fine, the resistance is 0 Ω when I press the switch. I opened up the switch and clean the contacts inside it, but it still doesn't work.
How can I figure out what the problem is, I want to try to fix the switch.

 

[Bluejets]

Those switches are fairly complex units with forward/ reverse and variable speed. Not the sort of thing one dismantles and "cleans" the contacts.