Hi Jerboa,
As I understand it, the B737NG electrical system, as far as the oil temperature within the IDG rising above 199degC goes, is as follows.
The IDG disconnect is automatic,
The 'Source Off' light only comes on when one of the transfer buses is unpowered, so that won't happen (normally), the other IDG via the transfer bus, will assume the load,
The "Gen off bus' light will come on, since the IDG is now not rotating!
The IDG now has to be removed and dismantled to replace the thermal sensor before getting back into service!!
Hope this helps,
Cheers