Mazda 3 - Parasitic Drain
 

Hi all, this is my first time posting... never needed to before! I have been fighting with this car for weeks and I am ready to throw in the towel. I will try and summarize what I have done and experienced. I would greatly appreciate any support or suggestions... this thing is killing me.

I have a 2009 Mazda 3 Hatch. It is completely stock in every way. I stopped driving the car for the snowy month of December waiting for snow tires to come in. It was my wife's DD until then with no issues. Upon getting the tires, I went to move the car to install tires and battery was dead. I jumped it with my truck and carried on. I checked out the battery (which was old) and it was frozen solid. I swapped it with a battery from one of my summer cars and the Mazda started fine. Then within 24-48 hours it would be dead again. This carried on for a week or 2 while I tried to figure it out.

So, I performed a current draw test and with everything as normal with the key out, and all lights off; I was seeing about 150-220 mA draw. I opened up the main fuse box and pulled all the fuses with no real improvement. I also disconnected the alternator and there was no change. When I open the door, even with the lights off, there is a moment where it will jump up a bit, and then drops again once the door is closed, likely dash lights, etc. I then traced it back to the bolted down fuse labeled BTN. I reviewed the wiring schematics for the car to determine that the BTN fuse runs all the power locks, windows, marker lights, brake lights, horn, etc etc. The interior lights and radio are on a different fuse. The BTN fuse then feeds into the interior fuse box located in the pass. footwell. I pulled each fuse for the interior fuse box one by one and there was no change. I then pulled out every damn fuse at the same time, and still no change. So then I unplugged the entire interior fuse panel at the large connectors feeding into it, and finally the drain stopped.

What I have determined is that there is no fused electronic device on the entire car that is causing this drain. It is always possible that there is a worn or damaged wire that is rubbing somewhere between the BTN fuse and the interior panel, but i would expect if this were true, that the current would be higher, and that I would have popped the BTN fuse. What I was hoping to know is if there is any other possible source of power. For example, even with all the fuses out, I can still see the little blinky light for the ignition lock... so the security device must still have power directly from the battery. Just to be safe, I swapped the battery with one from my other other summer car, and there was no change.
Adding to this paradigm is that a (guessing) 40Ah battery with a 200mA drain should take about 8 days to drain. Assuming the car needs at least 50% capacity, it should be good for 4 days. Meanwhile, I worked on the car and changed the battery Saturday, and it started fine Sunday morning. On Monday morning, it was so dead that my wife said the key fob wouldnt unlock the door. So it seems to be intermittent as well.

If I just walk away for a few minutes, it appears that the current draw is settling down around 150 mA... still too high, but stable. That is with all of the interior fuses pulled, and all of the main panel fuses pulled... and 100% of that drain is coming from the BTN line.

Any thought, please let me know. Thanks for your time. It is greatly appreciated.

How are you measuring the drain?
Have you locked the car with the remote before testing?
How long are you measuring for?

Not locking your car will speed up depletion of the battery. Its a very big problem in the later Mazda's.

Which fuse box are you talking about draining the battery?

The interior fuse box is also the BCM - Body Control Module. This is basically a modern relay box.

Anything that had a relay in its circuit in older cars is now controlled thru the BCM.

That’s good to know. The interior fuse box located in the passenger footwell is fed by the main fuse called BTN, which is located in the fuse box under the hood, which basically comes directly from the battery. When I start disconnecting the main terminal blocks to the interior fuse box, the current draw will drop to 0. Like I mentioned, all the fuses are pulled out of it, and it is still drawing 150mA.So my opinion, is that there is either some electrical device pulling power that isn’t fused, OR, the fuse box itself has a short or fault internally.If it is indeed filled with electronic components and relays like you said, this makes sense. Im thinking my best bet might be to call the local wrecker and see if I can swap out the entire fuse box… this would at least rule that out. I will try and attached a picture of what the fuse box looks like so that we are all on the same page. As for keeping the car locked, I cannot argue that locking the car may change the power consumption, but we have always left the car unlocked while not in use, and never had an issue. So it would suggest that this isn’t the direct cause of my drain.Though I will try and test the drain again with the car locked, just to see what happens.Thanks a lot for your help.
https://cimg8.ibsrv.net/gimg/www.maz...d31f6d4fe9.jpg

That is the BCM.

Ive heard of other people having power drain issues from the BCM before.

If you do replace it, make sure its from exactly the same car as yours,
Same year,
Same engine
Same gearbox
Same seats
Same accessories / specs,
Same same shape
etc etc.

Yes, I did read that they had to be the same. Luckily I have no bells or whistles, and manual transmission. I'll call up the yard today and see what he has. Sounds like I may be on the right track though. Thanks for your help on this.
if I can get a replacement I'll let you know how it goes.
cheers.

So I dropped in at the yard and found the closest thing to my Mazda... it was an 06 Automatic, otherwise very close. After digging out the 3 foot of snow from beside the door, I pulled out the BCM unit.
I installed the new unit in the car, everything seemed to work. However.... still 140mA drain coming off the battery. When I unplug the green power block from the BCM, the drain goes away....
So either it is normal to have 140mA of draw all the time, or there is something else going on with an always live circuit.
I did take readings with the car locked and unlocked and there was no change at all.

Any other ideas? If not, Im thinking I may just wire a relay into the key so that the BCM loses power every time I take the key out... not sure what else to do...

Thanks.

You do need to wait 10 - 15 minutes after locking the vehicle for the whole system to go to sleep.

Drain should be less than 0.06amps

OK, so I took your advice. With the new BCM installed and everything as normal, fuses in, etc, I locked the car and walked away from it for 30 minutes or so with the ammeter left hooked up.
After the 30 minutes, the current reading was 0.01. Perfect! If I took the ammeter off, and put it back on, it would stay around 0.01. It wasnt until I opened the door that it woke the computer up again.
So the actual root cause of this issue could either be a door that wasnt closing before, not allowing the computer to sleep. (there was no door open light on the dash), or the BCM itself was faulty and wouldnt go to sleep.
I will continue to drive the car and hopefully everything is figured out.
The only issues I appear to have now, is the keyless entry doesnt work, but the power locks do. And there is a check engine light on the dash. I will run codes, but I suspect it has something to do with using the BCM out of a different car. I can live with that for now.
Anyways, fingers crossed that all is now well. grim_reaper, your help is much appreciated... I may otherwise still be chasing my tail without your guidance.

Cheers.