Chapter A: General Diagnosis
26
Due to the higher parasitic current drains on late model
vehicles, the old test of removing a battery cable con-
nection and tapping it against the terminal while looking
for a spark is both dangerous and no longer a valid
check for excessive current drain. Furthermore, every
time the power source to computer modules is inter-
rupted, module memories are lost. Information pro-
grammed into memory by the vehicle owner, such as
radio presets, door lock combinations, seat position
memories, and climate controls, all have to be reset
when the battery is reconnected. On engine manage-
ment systems with adaptive learning capability, drive-
ability also may be affected until the computer relearns
the engine calibration or transmission shift modifica-
tions that were erased from memory.
A clean battery top prevents any drain from the negative
to positive battery terminals. Periodically clean the bat-
tery top and terminals with a mixture of baking soda and
water applied with a brush. Do not allow the solution to
enter the battery cells.
To test for abnormal battery drain, disconnect the neg-
ative battery cable and connect an ammeter in series
between the negative terminal and the cable, figure
1-31. On many late-model vehicles it is necessary to
wait up to one hour before taking the reading. Many on-
board computers have timer circuits which must time
out before they shutdown.
To isolate the source of a draw, disconnect each acces-
sory system one at a time until the meter reading drops
into the normal range. This locates the offending circuit.
Consult a wiring diagram to determine how power is
routed through the circuit, then systematically eliminate
components to determine which one was remaining on
and draining the battery.
STARTING SYSTEM SERVICE
The starting system includes the battery, ignition switch,
safety switch, starter relay, solenoid, and starter motor,
as well as the circuitry that links everything together. A
failure at any point of the system can prevent the engine
from starting. When an engine fails to crank, perform
these preliminary checks:
• Inspect the battery—check for loose, corroded, or
damaged terminals and cable connections. Also
check the battery state-of-charge
• Inspect the ignition switch—check for loose mount-
ing, damaged wiring, sticking contacts, and loose
connections
• Inspect the safety switch—check for proper adjust-
ment, loose mounting and connections, and dam-
aged wiring
• Inspect the solenoid—check for loose mounting,
loose connections, and damaged wiring
• Inspect the starter motor—check for loose mount-
ing, proper pinion adjustment, and loose or dam-
aged wiring
If the system passes a visual inspection, perform cur-
rent draw and cranking voltage tests to determine the
general condition of the starter motor. Begin by verify-
ing that the battery is fully charged and in good condi-
tion. While testing, the starter motor is cranking the en-
gine; however, the engine must not start and run. To
prevent starting, either bypass the ignition switch with a
remote starter switch or disable the ignition. Do not
crank the starter motor for more than 15 seconds at a
time while testing. Allow two minutes between tests for
the motor to cool and prevent damage.
Fig. 1-30.
Maintenance-free battery state-of-charge indicator.
Fig. 1-31.
Connection for testing for parasitic draw.
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