GMC 1998 Savana Van Owner's Manual page 206

If you have a diesel engine vehicle with two batteries
(or more) you should know before you begin that,
especially in cold weather, you may not be able to
get enough power from
a
single battery in another
vehicle to start your diesel engine.
If your vehicle has more than one battery, use the
battery that is under the hood
of
the vehicle
-
this
will reduce the electrical resistance.
2. Get the vehicles close enough so the jumper cables
can reach, but be sure the vehicles aren't touching
each other. If they are, it could cause a ground
connection you don't want. You wouldn't be able to
start your vehicle, and the bad grounding could
damage the electrical systems.
To avoid the possibility of the vehicles rolling, set
the parking brake firmly on both vehicles involved in
the
jump start procedure. Put your automatic
transmission in PARK (P).
3. Turn off the ignition on both vehicles. Turn off all
lamps that aren't needed, and radios. This will avoid
sparks and help save both batteries, and could save
your radio.
NOTICE:
- 1
If you leave your radio on, it could be badly
damaged. The repairs would not be covered by
your warranty.
1
4. Open the hoods and locate the batteries. Find the
positive
(+)
and negative
(-)
terminals on each battery.
An
electric fan can start up even when the engme
is not running and can injure you. Keep hands,
clothing and tools away from any underhood
electric fan.
5-4