Porsche 911 Carrera Owner's Manual page 303

2. First connect the negative cable (black) to
the negative terminal of the donor battery,
then connect it to a suitable grounding point on
the vehicle with the discharged battery. This
grounding point must lie as far as possible
from the battery. For example, a solid metal
part or the engine block are suitable grounding
points. If no suitable grounding points are to
be found on either vehicle, the negative cable
must carefully be connected directly to the
negative terminal of the battery. If a suitable
grounding point is to be found only on the
donor vehicle, the negative cable must first be
connected to the terminal of the discharged
battery, then to the grounding point of the
donor vehicle.
3. Run the engine of the donor car at a higher
speed.
4. Start the engine. An attempted start using
jumper cables should not last more than
15 seconds. Then allow a waiting period of
at least one minute.
Note
Before disconnecting the jumper cables, electrical
loads such as the heated rear window and the
heating fan blower should be switched on (the
vehicle's lights must not be switched on). This
reduces voltage peaks which may occur when
disconnecting the jumper cables.
With the engine running, remove both jumper
cables in reverse order.
Practical Tips, Emergency Service
301