Porsche 911 Turbo Owner's Manual page 272

Connect jumper cables in the following
sequence:
Always observe the sequence below:
1. Connect the positive lead (red) to the
positive terminal of the discharged battery
first, then connect it to the positive terminal of
the donor battery.
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 is a suitable
grounding point.
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.
270
Practical Tips, Emergency Service
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.
f Note
Before disconnecting the jumper cables, elec-
trical 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 oc-
cur when disconnecting the jumper cables.
With the engine running, remove both jumper
cables in reverse order.