1997 Q45 BATTERY DRAIN - BCM ?

[joes92m30]

Hey guys, been a while since i've hung out here, i'm tearing my hair out on this one. Just background, my first car was a J30, which i miss like hell, but now i have a 92 M30 and a 99 Q45. so here's the question: (and yeah i read 104 results on searches for this one!)

My folks are retired--and have the proverbial Sunday driver Q45. In december 1997, i drove my dad to the dealership to buy it--he brought 42k with him in cash in a briefcase, ha. It now has about 68,000 miles on it.

For the past few years it's been going dead when left for more than a day. I did a load test, it was drawing .49 amps instead of the .035 I've read it should be drawing when at rest. I narrowed it down to the interior lights, when I pull the fuse on them it drops to .18 amps. (i might have my decimal wrong here, not sure how to read the meter.) No lights stay on, and even when the interior switch is set to off it still draws that much.

So all i can come up with is the BCM. I think it runs the timers and sleep functions / etc on the interior lighting. Any help here? I just replaced the alternator after reading that was the most likely culprit and the dmaned thing was still dead a few days later. I don't want to tear into the car too much, it's so stock...it's like a time capsule. I looked at the BCM under the dash and no one's ever touched a thing under there, all factory fresh still! Unlike my cars that I tear apart and rebuild constantly.

Any help is much appreciated.

[Q451990]

Sounds like you have a real gem of a Q there!

I would probably try unplugging the BCM and see what happens. I know you don't want to tear into the dash, but you have to get to the parts somehow...

My only other thought would be the lamp in the trunk. If the switch for it was faulty - you'd never know it since you can't see in there.

Heath

[joes92m30]

Thanks for the advice Heath, I gave it a shot.

Unplugging just the BCM: dropped from .49 to .35.

Pulling just the 10A fuse for the interior lights: dropped from .49 to .18

Unplugging the BCM AND pulling the fuse: dropped to .03!

I'm a little slow, any ideas what this means?

[Q451990]

Check out the diagram on EL-310 of the service manual - availble at www.nicoclub.com/FSM/ if you don't have it.

It appears that the BCM gets voltage directly from the battery through one fused connection on pin 105 - but it also sees voltage through other various inputs, like when the ignition switch is on, various lamps, etc.

Everything in the car is fused, so I would try pulling each fuse and noting the current drop (if any) to see if you can narrow it down that way.

Heath

[joes92m30]

I think I need an exorcism at this point. I switched out the BCM and the draw dropped to .18, which i thought was decent. And three days later, battery dead again. So...new BCM, new alternator. I'm at a loss here, I thought I'd have this fixed and given back to my folks...but at this point they are thinking of trading it in on a PRIUS. In other words, I gotta figure it out! Any other suggestions? I'm willing to try anything at this point!@ thanks JM

[Lokim]

Try unplugging the trunk light switch and checking your draw. If no result, disconnect the glovebox light switch and check your draw.

[Skibane]

I think I need an exorcism at this point. I switched out the BCM and the draw dropped to .18, which i thought was decent. And three days later, battery dead again.

That doesn't make much sense...

0.18 amps over 3 days is just 13 amp-hours of battery drain:

0.18 amps X 24 hours X 3 days = 12.96 amp-hours

The Q45's Size 24 battery has a typical capacity of around 70-85 amp-hours - which means that 13 amp-hours of drain shouldn't have come anywhere close to killing it.

Are you sure you're using your meter correctly?

BTW, I've noticed that on my 2000, the current drain is somewhat higher with the hood open.

[Lokim]

Any results?