Chapter A: General Diagnosis
25
ABNORMAL OR PHANTOM BATTERY
DRAIN PROBLEMS
Phantom battery drain, or parasitic draw, is caused by
something in the vehicle constantly drawing current
from the battery when the ignition is off. Although a cer-
tain amount of discharge is normal and can be expect-
ed, a fully charged battery in good condition should not
lose its charge when left idle for a few days or even
weeks. This loss of voltage can cause driveability prob-
lems if the draw is enough to cause the PCM to lose
stored operational information. The loss of learned val-
ues for any of the following systems may cause unde-
sirable performance issues:
• Fuel trim values
• Ignition timing advance information
• Transmission shift points
Typical causes of battery drain include:
• Acid, moisture, dirt, or corrosion on top of the bat-
tery case
• An accessory, such as a trunk light, glove box light,
underhood light, or cigarette lighter, remaining on
when the vehicle is not in use
• Parasitic drains required to operate systems that
continue to work even when the vehicle is parked
and the ignition is off
The most serious of these are the parasitic losses re-
sulting from the advent of computer controls. Virtually all
late-model vehicles are equipped with computers that
control such things as engine operation, radio tuning,
suspension leveling, steering assist, antilock brakes,
and more. Each of these microprocessors contain ran-
dom access memory (RAM) that stores information rel-
evant to its job. Remember: RAM requires a constant
power supply that puts a continuous drain on the vehi-
cle electrical system. In addition, many electronic con-
trol systems conduct a self-diagnostic test after the en-
gine is switched off. The combined drain of several
computer memories, or diagnostic test routines, can
discharge a battery to the point where there is insuffi-
cient cranking power after only a few weeks.
Fig. 1-29.
Typical connector end view chart.
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