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Discussion Starter · #1 ·
Hi all,

So I've been getting a dead battery in the mornings and finally had a little time today to test for a current draw.

I have my cheapo harbor freight multimeter, and set it on 10A. disconnected the negative battery cable, touched the leads, and...the X starts making noise.

Almost like it's trying to set off the car alarm but there isn't enough current going through the multimeter to let it go right off.

I have noticed whenever I disconnect the negative cable and put it back on, it sets off the car alarm in full. Is that normal?

I do have the Nissan alarm (shock sensor or whatever they call it). I'm wondering if it could maybe be a relay related to that?

Have a look at this video. I had the GF film while I held the leads. I asked her to show the horn and she says "Where's the horn?!" - funny how I just assume she should know the X as well as me.


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The meter can pass 10A which is 120 watts worth of power - that's plenty for the biggest car alarm made. The stock horns are likely in the 20-40 watt range.

Alarms are picky things as the better ones have anti-tamper circuits that can sense attempts to disconnect the battery. Measuring a power draw is also a common way to detect doors opening.

I use to work for a shop that sold alarms and the top-of-the-line ones came with a second smaller battery to protect against someone cutting the first battery.

Once you get the amp meter in place have a look at the current draw and if it's more than a half an amp (6 watts) get concerned. Go to the fuse boxes (under the hood and passenger's side) and pluck a fuse one-at-a-time until you find the load.

I found a 25 watt draw from the control panel on my winch. I installed an on/off switch in the dash to control the load.
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