Fuse #21 traing help needed.

[gerlando]

I recently acquired an E30 that I drive once a week. Sitting in my garage caused the battery to drain to the point where it won't start. I think I've isolated the problem to Fuse #21. My problem now is that I'm an electrical noob and don't know what to do next to find the true culprit. It sounds like this circuit controls a lot of different things. Any advice on what to do?

Thanks.

[wycoco]

Buy a new battery and replace the fuse?

[gerlando]

The fuse isn't blown, but something on it's circuit is drawing power.

[trent]

Tell us what is on Fuse 21. Do you have an ETM?

[trent]

To answer your question, on fuse #21 is:
Auto Charge FLash Light
Ignition Warning/Seat Belt Warning
Interior Lights
Radio / Antenna
Trunk Light
Multi Function Clock
Service Inverval Indicatior

So you need to check these components for a draw

[e9nine]

Trunk Light

Service Inverval Indicatior

Make sure your trunk light is working. They are know to cause battery draw. This happened to me personally about 2 years ago.

SI indicator. Does your cluster work 100%?

There's an easy way to find out if these are to blame. Take the trunk light out or simply take a look at the SI indicators on the cluster.

[DaN]

check amp draw at the battery with everything off, and unplug everything on the circuit until it goes to normal.

[gerlando]

Thanks guys. I can start pulling and checking the obvious ones, but the bently manual also lists "active check control" and "fuel delivery" on 21. How do I check those?

[AndrewBird]

Disconnect the positive battery cable hook a multimeter up between the battery and the positive battery cable. Measure the amperage and start unhooking things on your list. Once the amperage draw goes away, you know what is causing the battery to go dead.

[gerlando]

A small update. I tested the draw on fuse 21 with everything off and it's showing a little over 60mA. This is after pulling the headunit, sub, and trunk light. Before I go further I thought I'd ask if anyone had any thoughts as to what might be drawing that level of power. I'm planning on getting to the bottom of this over the weekend and I'll update this thread with my findings since it seems like there's a lot of these types of threads without any resolution.

[netcsk]

60mA. 0.06 amps. Lets say it's a 100 Ampere-hour (100 amps in one hour @ 12v) battery (reasonable assumption).

100/Amperage = time to drain the battery
100/0.06 =~ 1600

This means it would take 1600 hours to drain the battery from full charge. Of course, this does not mean the battery would start after 1500 hours (the battery needs to have a certain voltage/amperage to turn the starter). Look elsewhere unless your talking about a dead battery after a month and a half of time ;).

Be extremely careful while doing this (try to work with only one hand while keeping your other hand in your pocket)
Remove the positive battery cable, hook one lead of the meter up to the battery terminal and the other lead up to the positive battery cable (the meter in series). Read what the total draw of the system is. Make sure this value is high enough to show that you're looking for an excess draw that exists.

If you remove a fuse and hook the ammeter on one side of the fuse and the other lead on the other side, you can get a reading that way on each circuit. I'd look for a draw of over 0.3 amps or 300mA before I looked into it. Of course, many 0.05 draws could cause the same thing. It's tedious, but you'll find the problem if you step through each circuit slowly.

Once you find the culprit fuse, start removing components on that fuse one at a time and watch if the draw drops.

[gerlando]

Thanks for the informative post, debugging electrical systems is completely new to me so the knowledge is certainly appreciated.

I forgot to state that previous to getting a multimeter, I did the spark test to figure out fuse 21 was the culprit. I confirmed this was true by pulling the fuse and letting the car sit for a week. It started up just fine after that so I pulled the headunit and sub and waited another week, this time keeping the fuse in. After a week the car failed to start again. The only other thing I did between that and testing with a multimeter was to pull the trunk light.

Anyway, now that I have proper test equipment (and some good advice) my plan is to:

1. Put the head unit, sub, & trunk light back on.
2. Test the draw at the battery.
3. Test the draw from each fuse.
4. Pull stuff like mad until I see it drop to 50mA or less.

[netcsk]

sounds good. Let us know how it all goes.

What you can do is have someone pull a fuse and replace it (if it is found to not lower the draw) while leaving your meter connected to the battery. This would be quicker than having to go through every fuse with the meter. Either way will work though. Just be careful.

But it does sound like you know it's on that fuse due to your round-about way of testing.

[gerlando]

Ok, over the past week I had the amp hooked back up to the battery but the head unit was still out. The car started up just fine yesterday so it's not the amp.

I did some digging today and I'm more confused and frustrated then ever. I tested the draw with/without the trunk light and that checked out ok. I also checked the battery at idle and it showed 14v so my alternator is doing it's job.

This leaves me with only the headunit as the culprit. I figured the constant and switched power were reversed but I checked the wiring with a multimeter and against the wiring listed here and everything was correct, yet the draw on fuse 21 goes off the chart (over 2 amps) on my meter when the head unit is connected.

I'm kind of at a loss as to what to do next, I suppose I could take the constant wire off but I'd rather get everything running 100%.

[benDASHDASH]

i had the same problem about a year and a half ago.. it was the glove box flashlight draining the battery. needless to say i threw that sucker away.