From digest.v7.n402 Sun Oct 19 17:02:30 1997
From: Jim Cash <j.cash_at_sympatico.ca>
Date: Wed, 08 Oct 1997 09:55:46 -0400
Subject: Re: - Battery Discharge

Lloyd Barrera writes:
> I own a '86 BMW 528e. It has started to discharge the battery at RPM of
> between idle to about 1300 RPM. The Battery Indicator Light and the Anti-Lock
> Brake System Warning Indicator Light are on at these RPM. When I drive and
> the RPM goes above 1300 RPM these two indicator lights turn off. My battery is
> slowly being drained when the indicators are on. (At low RPM). Has anyone
> experienced a similar problem and what could be wrong? I suspect I have a bad
> alternator.

Lloyd

It might be a bad alternator, or the diode set in the alternator but first I would check the battery cables.

I had a problem with corrosion in the small +ve battery cable (leading to the fuse box) and it caused enough resistance so that the indicator lamp started to glow - very dimly - could only see it at night).

You might have corrosion in your cables, or a poor connection so that even if the alternator is putting out voltage it is not getting back to the battery.

You can test any cable or connection very easily with a small volt meter.

• - Put the meter on the DC volt scale of about 3 volts (or less).
• - put the 2 probes either side of the component you are testing (either end of the cable).
• - any voltage reading of greater than about .5v means there is too much resistance (in theory a power cable should have no voltage loss from one end to the other.

As I said, you may have an alternator problem, but it is cheaper to check your connections first.

Another thought - it is also possible that your alternator drive belt is slipping enough that it can not produce enough power if you have heavy drain (like when headlights, wipers, etc are on). If you can manually turn the alternator wheel (by holding on to the fins and twisting) then it is probably slipping. This test only requires a "moderate" amount of strength - if you apply gorillia strengh it will turn anyway.

Cheers

Jim Cash
London Ontario, Canada
E39 540ia