Re: E30: Battery Drain

["Russ Maki" <rinard@xxxxxxxxxx>]

Hi Tony --

The standard means of tracing current drains on these cars is to use a
digital ammeter (multimeter) of the kind you can get for cheap from Home
Depot or the like. You detatch the battery negative post (everything
electrical on the car is OFF), attach the leads of the meter to the post and
battery clamp and read the current flow in milliamps. My E30 drains about
110 milliamps because I have some aftermarket sound equipment; at any rate I
can leave the car parked for weeks and it fires right up with no problem
with a drain at that rate. If you're draining, say, 210 milliamps, my
experience is that you're gonna start having some problems with dead
batteries.

About this time, it's probably a little late to tell you that you'll be be
shutting down power to your radio when you unhook the battery cable, so if
it has any anti-theft codes programmed into it you'd better  make note of
them beforehand. I should also mention that my experiences with this problem
involved a 1987 325i. So, YMMV (Your "mileage" may vary).

Assuming your battery is draining at the higher rate or worse, the next
logical thing is to go to the fuse box and start pulling fuses, one at a
time, with an eye to the multimeter to see if the drain decreases when a
specific circuit is taken off line. If a single fuse cuts the drain off,
that's where your problem lies. Look in the owner's manual or Bentley repair
manual to see what accessories that fuse serves. Voila! You've traced your
current leak.

If your experience is similar to mine, however, you'll find that you've
pulled all of the fuses one at a time and none of them eliminates the power
drain. If that's the case, consult your Bentley repair manual and look in
the fuse box for the relay that controls the auxiliary fan at low speed. The
Bentley shows that the autocheck system circuitry runs through the low-speed
aux fan relay. I discovered this after some serious head-scratching -- if
I'd asked someone in the know I might have saved myself a lot of angst. A
BMWCCA tech adviser comes to mind.

Again, this is my experience, and YMMV, but I found that the old 40-amp
relay for the low-speed aux fan circuit permitted some current drain from
the autocheck system. All my 40-amp relays are 14 years old, so perhaps a
new relay of that amperage rating will do the trick. I live in the frozen
wastes of Wisconsin, where the aux fan seldom runs for extended periods --
so I popped in a new,  more available, 30-amp relay. Problem solved.

Russ Maki
Ixonia, Wis.


> From: "Tony Lin" <tonyplin@domain.elided>
> Subject: E30: Battery Drain
>
> Hi Folks-
>
> Can anyone suggest some possible reasons for a slow drain of current from
my
> battery when the car is off?  Clock?  Those silly idiot lights and their
> rechargeable batteries?
>
> It's a brand new battery in the car- I just replaced a 1yr-old one which
was
> under warranty.  Apparently the old battery drained down to nothing after
> the car was parked without driving for periods of a few weeks.  The new
> one's  showing a 14hr overnight drop from 12.2v to 11.8v.  At this rate,
3-4
> days of not driving will drop the battery to below cranking voltage.  I
> can't think that it's just another bad battery.
>
> Tony
>
> 85 318i

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