 | Battery - The battery on the 928 is located in the rear of the car
under the spare tire. It is not
uncommon to see a new battery when a car is offered for sale. This is
always a nice feature but can also be a sign of previous electrical
problems. All
of the connections should be checked for corrosion. Significant corrosion
or low voltage below 11.5 volts indicates
a less than fully charged or bad battery.
(Deduct 3 points for a bad battery)
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| Grounds
- Many of the electrical problems on the 928 can be traced to poor
grounds. The battery voltage should be measured at several points on
the car - at the battery, at the jump point at the on the right side of
the fender well and at the fuse panel. Any difference in voltage at
these points indicates poor grounds.
With the motor running measure the voltage between the front jump point
with the ground on the engine and then the frame. A difference in
voltage indicates a bad ground strap on the motor.
The main ground strap is connected to the frame by a wing
bolt at the rear of the car. The wing bolt is designed to allow
the system to be disconnected and shut down quickly in case of short.
Measure the voltage with the ground on the battery and the chassis.
A difference indicated indicates a bad main ground.
The positive lead is a long cable that runs
under the car and up to the starter. Measure the voltage at the battery
and the starter. A difference indicates a a bad main cable.
(Deduct 10 points for bad ground measurements)
|
| Current
drain - Shut off the engine and all of the accessories
including the dome lights. Disconnect the negative lead to the battery and
connect an amp meter between the lead and ground. The amp meter
should rear less than 200 Milliamps. Greater than this and the
system is draining power from the battery that will cause a dead battery
after a few days of sitting.
(Deduct 10 points for excess current drain)
|
| Fuse Panel - Most or all (depending on model year) of the fuses in the
car are located on a single relay fuse panel under foot kick in the front floor
of the passenger seat. The panel is accessed by removing the carpet and a
wooden panel. The fuse panel should be inspected closely for
corrosion. The location of the fuse panel makes it venerable to water
leaking from the air vent of the ventilation system. There is an outside
drain that flushes rain water away. The drain can become plugged with
leaves and over flow on to the fuse panel. The connections to the car are made
at the bottom of the fuse panel with a series of color coded plugs. One or
more of these plugs should be pulled and the contacts inspected for corrosion -
generally blue green in color. The same procedure should be done with a
few of the relays and fuses.
The fuses should be checked visually for blown
fuses and the panel checked for non factory wiring or relays that have been by
passed. The owners manual is accompanied by a card that defines what each
fuse and relay does. Each year has a slightly different layout so it is
important to have the correct card. Not every fuse is used on all models
so missing fuses are not an immediate indication of problems.
(Deduct 15 to 30
points for a corroded or hacked fuse panel depending on the extent of the
damage)
|
| Wiring Harness - The heat of the engine and water from the road
combine and attack the wiring harnesses under the hood. The insulation
should be checked carefully for discoloration and stiffness. The wires
running from the alternator can fray and corrode dropping the output from the
charging system.
(Deduct 10 points for bad or cracked engine harnesses)
|
| Alternator - The alternator in the pre 1985 cars was under rated for
the demands of the 928 accessories that are standard equipment. The
alternator was upgraded to a heavy duty unit in 1985 which used a wide belt. The alternator is mounted on the lower part of the engine and as
such is subject to water and dirt from the road. To compensate for this
Porsche installed a cooling shroud and flex hose that runs up under the left
fender. It is very common for mechanics to leave this hose off as excess
baggage. A common alternator upgrade for the pre -1985 cars was a Bosch
unit. The Bosch unit does not have a cooling shroud and thus should not be
used on the 928. The cooling hose on the 1980 and up with climate control
is connected to the outside temperature sensor. Often when the hose is
left off so is the cooling sensor that then disables the climate control. The
voltage at the alternator should measure 13+ volts with the car running. This should be the
same reading on the internal volt gauge when the car is running dropping down to
just over 12 volts when the engine is shut off.
(Deduct 15 points of a low
output alternator or an incompatible brand like the Bosch. Deduct 2 points for a
missing shroud or cooling hose)
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