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Michael, Auto Service Technician
Category: Car
Satisfied Customers: 919
Experience:  21yrs.Foreign and domestic driveability diagnosis and repair/ ASE Cert./Fuel Injection Specialist.
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2001 740il battery drain.

Customer Question

I have a 2001 BMW 740il. It has been relatively trouble free for the 1st 26000 miles but suddenly I'm having battery drain. The battery was replaced a little over a year ago under warrantee and all systems were checked OK. Now yesterday I was unloading groceries from the trunk and I was hearing a whirring noise which was intermittant. I pulled open the cover over the GPS CD drive and CD Changer. About every 5 to ten seconds that GPS would reject the NAV CD and then reload it. I finally took the CD out but noticed the Power light remained on. What could cause this?
Submitted: 11 years ago.
Category: Car
Customer: replied 11 years ago.
Relist: No answer yet.
I already listed everything I know about this problem. I went away for a week and when I returned my battery was DOA. That has happened twice this past two months. I could not find a problem until I heard the GPS CD loading and unloading. I removed the CD and at least the whirring noise stopped. Now it looks like my NAV System will be DOA until I find a fix. If you don't have any answers just say so and refund my money!
Expert:  Michael replied 11 years ago.

What you are experiencing is known as a parasitic drain.


With the introduction of on-board computers,
memory radios, memory seats,
and other accessories, current is required to
maintain memories when the ignition switch is
in the off position. As a result of the required
continuous energy supply, testing methods have
changed. If we use the conventional means,
inaccurate conclusions can result in condemning
good parts and circuits, and wasting a lot of
diagnostic time.

The desired test instrument is a Digital Ammeter
capable of reading as low as one milliamp
and up to 20 amps. Meters of a lower amp range
can be used by forming a shunt using two
jumper wires connected by alligator clips. Reason:
the wake-up call necessary to charge the
capacitors in a full option state-of-the-art automotive
electrical system can require as much as
6 amps for one minute. Many digital meters are
fused at 2 amps and therefore have withdrawal
symptoms when subjected to this much current.
The shunt allows the system to properly power
up the capacitors while protecting the meter
from excessive current loads. Once the capacitors
are charged, disconnect the shunt and read
the ammeter for actual drain.

Some tech manuals which include a Battery
Testing Section reflect acceptable current drains.
Listed is a typical example of a parasitic drain
(load) for a General Motors Vehicle.
Device---Milliamp Draw
Voltage Regulator 1 - 2
Digital Clock 3 - 4.5
Quartz Clock 7
ECM 6.5 - 8
ETR Radio & Clock 7
Load Leveling 4
Memory Seat 3
Twilight Sentinel 3

Conventional electrical drain test procedures
may not identify the cause of the condition.
Vehicles equipped with Electronic Control
Modules can exhibit a failure mode within the
ECM which can result in a high parasitic current
drain on the battery. Conventional testing methods
may not detect the cause. The reason for that is, when the battery cable is disconnected, the solid state
circuit which caused the excessive drain may
not re-occur once the electrical circuit continuity
is restored. Cycling the ignition switch to the
run position, then back to the off position may
cause the electrical drain to re-occur. Caution:
the ignition switch should not be turned to the
accessory, run, or start position with the ammeter
installed in series with the battery terminal
as damage to the meter may result. Only turn the switch to the run position after first installing a shunt across the ammeter.

A full option vehicle can experience a
parasitic load of 25-35 milliamps. Remember, a
milliamp is one thousandth of an amp. Twenty-
five thousandths of an amp is not much current.
Should you determine the current draw exceeds
the allowable limit as specified by the manufacturer,
it will be necessary to identify the circuit
which is creating the draw. It’s best to use the
process of elimination. Look for the obvious.
Check the lamp circuits (courtesy, glove box,
trunk lamp, etc.) for bulbs illuminated when
they shouldn’t be. For example, a glove box or
trunk lamp may remain illuminated when the lid
or door is closed. To check the trunk lamp it will
be necessary to get in there with it. Caution:
make sure you have a helper, and most importantly,
make sure he is your friend,hahaha
If the visual inspection fails to identify the drain,
pull fuses, one at a time, to find the source.
Remember, if the fuse box is in the glove box,
the bulb will be illuminated when the door is
open. Either record its draw or disconnect the
bulb. Once you identify the circuit creating the
draw, refer to the service manual or a wiring
diagram to determine what devices are on that
circuit. Disconnecting these devices one at a
time should reveal the problem circuit. If pulling
fuses fails to identify the drain, it may be
necessary to disconnect relays one at a time. It
is simply a process of elimination, isolating
circuits to identify the problem circuit. Remember,
anything operated by a switch is a likely
candidate in causing current drain.

1. Make certain the ignition switch is in the Lock position and all electrical accessories are off and
doors closed. If equipped with a hood lamp, remove the bulb or disconnect its electrical connector.
2. Fabricate two 12 gauge jumper wires equipped with alligator clips on each end. Install the jumper
wires in series with the negative battery post and cable.
Next, attach the recommended ammeter test leads in parallel with the jumper wires.
3. Rotate the ignition switch to the Run position. Caution: Do not rotate the ignition switch to the Start
position as the test leads and ammeter will get very hot, and damage to the vehicle and wiring
harness may result.
4. Rotate the ignition switch back to the Lock position. Make certain that electrical continuity between
the negative battery post and cable is not interrupted. Allow a one minute wait after placing the
switch in the Lock position. The wait allows the capacitors time to charge (failure to do so may result
in misdiagnosis).
5. Disconnect the alligator clips.
The ammeter is now in series with the battery post and cable. Read the amp draw on the ammeter and follow the recommended test procedure if the drain exceeds the allowable parasitic load.


In 1970, Delco produced a service
bulletin which addressed misdiagnosing
by using a voltmeter to test
for electrical drains. With a 63 ampere-
hour battery: (1) use a typical
1000 ohms-per-volt voltmeter (2) read
the 20 volt scale on the voltmeter.
If the battery open circuit voltage
is 12.6 volts, a reading of 6.3 volts on
a voltmeter connected in series indicates
current leakage that would require
12 years for the battery to become discharged! Even a
reading of 12 volts would require over 200 days for the battery
to become discharged!
It’s perfectly normal for a voltmeter connected in series to
read over 6 volts. In fact, many vehicles will have readings
over 12 volts and this reading DOES NOT indicate a defect.

Customer: replied 11 years ago.
Reply to Michael's Post: The answer is probably a good one but does not address my problem. GPS CD is loading and unloading while igition switch is in off position and car is garaged for the night. Now I have removed the CD "Calif/Nevada" 2001 and I no longer have an operable Nav system.
Expert:  Michael replied 11 years ago.
Sorry about that.I thought your concern was a battery drain issue. The question will remain open for other experts.Remember...You do not pay until you are happy with and accept an answer on JA.Good Luck.