Battery drain on E30 with aftermarket oil gauge

[Waffleman]

So I recently had to recharge my 3 month old interstate battery (the last one in this car only lasted a year before it was totally kaput).

I hadn't driven it in about 10 days, its been very cold out but the guy at the tire shop who recharged my battery said that it shouldn't have drained it that dry that fast.

Now im not very good with electrical systems, but after reading a few DIY I bought my $5 harbor freight multimeter and got to testing.
When its on the 10A scale it with the #28 fuse removed I read .01 draw. When the fuse is in it reads .07.
The previous owner who rebuilt the engine installed a oil pressure gauge which uses the power from the cigarette lighter, there is a black with yellow stripe wire (ground?) that is clipped and there is no lightbulb plugged in the cig lighter light wire. Is there a problem here?

From what I read that shouldn't be enough to kill the battery like that, although some people were using different scales and different Amperage ratings, this is were my knowledge ends.

[anokES]

If the meter is pulling the power out of the cigarette liter it could drain easy if you're not driving and charging the battery through the alternator.

[reelizmpro]

The cigarette lighter is not switched, it is always ON. You need to rewire that positive lead to a switched 12v source.

[anokES]

^ :)

[Waffleman]

Ok good to know reelizmpro, thank you, i'll just live without the oil pressure gauge for now until I can get that taken care of.

[jperris]

A current draw of .07A (70 milliamps) is high for an e30, but in the normal range for many newer cars. Assuming you have a normal-sized battery with a capacity around 70Ah (amp-hours), that kind of draw should only have drained your battery by about 25% in ten days.

With that said, it sounds like you have a little too much draw on the fuse 28 circuit, which should be isolated. The power wire to the cigarette lighter should be a Red wire/Yellow stripe, the oil pressure gauge should not be wired into this circuit. The cigarette lighter ground wire is solid Brown.

The ashtray light is powered by a Gray wire/Red stripe from the dash lights circuit, which is fuse 23. The ashtray light ground is Brown wire/White stripe.

As for the aftermarket oil pressure gauge, it should have:
(1) a wire that gets 12v with the key on, but not with the key off,
(2) a wire that gets 12v from the dash lights circuit,
(3) a ground wire, and
(4) a wire going to the oil pressure sensor.

I dont' know what to tell you about the Black wire/Yellow stripe, may be the start input wire from the instrument cluster to the OBC. Shouldn't be causing a current draw if it's just cut and laying there.

Also keep in mind that if you have a power antenna, it is also powered by fuse 28. If present, try unplugging that and see what happens to your current draw.

[lentildude]

I just measured how much current my standard 85 323i e30 uses.

removed positive on car battery and with meter in current/shunt position and on 20ma range I measured 7ma draw. This is probably the trickle charge on SI board battery and the ECU.

[navytapaul]

Check your ground straps. I left one off after doing a valve adjustment and motor would hardly start.

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[mitchlikesbikes]

like everyone else said, you need a switched power source.