i replaced the battery in my 2005 MCS convertible about four months ago. Since I've been running it almost daily since, it is just now, while I am having issues with my t/stat replacement, that I have noticed that after sitting for 3 days, the battery is dead! I suspected this even before my t/stat issue, because about 2 weeks ago the same thing happened - but I thought maybe I left an interior light on or something. Now I know that something is very wrong, and it appears I have a short somewhere. After an hour or so on a trickle charger, I can restart the car - so I guess the battery is not completely drained.

Would the Peake tool diagnose a short in the electrical system? How else would I go about determining the source of the problem? I don't want to be one of those with a spontaneous engine bay fire!

 

No, a diagnostic tool won't tell you about a short or dark current. The only way I know of is to check the load on the battery, then start disconnecting things. When the load goes away, you've found your culprit.

 

It's possible the voltage regulator in your alternator kicked the bucket. I have also read other posts about the power steering fan continuing to run after their MINI was turned off. Try to do a visual inspection of the wiring under the bonnet. Look for frayed wiring grounding itself out.

 

I hate to sound dumb, but how would I check the load on the battery?

 

You have to a volt meter. I would ck with your local club to see if you can find help with this- you can chase your tail with this problem if you have never done this kind of job before.

 

try this with the car running use a volt meter with a digital readout across the negative and positive terminals if the voltage is not between 13.5-and 14.8 or so my guess will be your alternator needs replacing. if you have proper voltage then it could be a ground issue check your wires to make sure they are clean and connected

 

OK. So I did a bit of reading online, and got a multimeter.

After starting the battery with a jump box (it was dead overnight), I ran the car for several minutes and checked the voltage across the battery - with the car idling, it was in the 14 range. This means that my alternator is working and charging, right?

Next, I checked the amps by putting the multimeter in series with the battery (disconnected the positive cable, placed the multimeter in between the battery pole and the cable). The computer system briefly kicked on, then went to sleep after a few seconds, after which I got a "0.L" reading.
I tried the same thing with the negative side, and got the same reading. I had the multimeter set on the mA setting because I had accidentally blown the fuse to the 10A setting (we won't go there). Since most parasitic drains are supposed to be in the 25-plus mA range, I figured it was still OK to use the mA setting.

From what I read, this 0L reading means that I have an open circuit - something is causing resistance, either a broken wire, loose connection, etc.

If I start pulling fuses from the box, will I be able to isolate the circuit that is open? In other words, if I pull the fuses one by one until I get to one that does NOT give me the 0L reading, then I can I infer that that is the fuse to the open circuit?

 

I'm not an electrical guru, but I'd say the proper procedure would be to check the voltage with the car off - that's when it's pulling current. After the battery is charged, check the voltage across the battery, then start pulling fuses. When you see the voltage increase, you've found the source of the drain.
Is there a mechanic or EE in the house to correct me if I'm wrong?

Again, not sure, but I think the car would pull greater than mA when running - maybe that's why you're getting the OL?

 

Eric,

Many thanks for your help!
I checked the voltage and the current with everything in the car shut off - and after waiting a few minutes... So there shouldn't be an open circuit with everything turned off, I wouldn't think - unless there were some broken/disconnected wires somewhere. That's why I was thinking that if I started pulling fuses, whenever I got to the one that controlled the open circuit, the 0L would go away - but I don't know... I'd love to hear from an EE or mechanic in the house about this!

 

your meter will never handle the amps coming from the alt. mA is less then 1 amp OL stands for over load. When you are testing have the car off per Eric's post. With the meter on the battery car off how fast is the volts going down on the battery? when you pull main fuses look for the draining to stop/ slow down. But unless you are looking at wire drawings you are you shot gunning it. Good luck! have your battery tested to rule that out- even if it was just replaced!

 

As you were doing when you measure current you put the meter in series, so whatever current the thing is trying to pass through the meter. Anyway if the car isnt running and the key is out between whatever lights and junk come on when you open the door it shouldnt be drawing more than a few amps, certainly under 10.

14v is certainly in the butter zone for charging so your alt is probably good. A complete short to ground would either blow a fuse or go away after the wire shorting to ground melted. My geuss is that something that should be turning off is.

That's just a guess though.

 

UPDATE:

So, I checked all of the fuses both in the footwell area and the engine bay. None were blown. I have two "Add-A-Fuse" on the fuse block - one of them is attached to the convertible top circuit - that is the accessory lighter I installed a couple of years ago. I did notice that I had forgotten to replace a blown 5A fuse for the lighter - but I don't think that would cause a drain. The fuse blew a while ago, after I was trying to charge my Garmin Nuvi with a cord that I found out later was not the right one...

After I verified that all of the fuses were intact (and replaced the blown one) I decided to take it to Autozone, where I had gotten the battery in March.

They did a load test on it and it said the battery was OK, but low on charge. The guy asked me to turn on everything I could think of - lights, AC, radio, wipers - while I revved the engine to about 2500 RPM. He was in the back checking with my multimeter, and said that when I did that, the voltage began to drop from 14 to down below 12.6. His opinion was that the alternator is going bad. We checked under the bonnet for any burning smells with the engine running, but neither of us smelled anything.

Does this sound plausible as a reason for my battery going dead overnight? Keep in mind that on the days when it was going dead overnight, I had not driven the car for several days, and had only used a jumper pack to restart the battery...I would jump start it, maybe run the engine for 5 to 10 minutes - and it would be dead by morning.

Does this all fit with a bad alternator?

 

If the alternator is bad, you are running straight on battery and draining it. More than likely, it's not the entire alternator, but just the voltage regulator. You can either replace that, or get a rebuilt alternator altogether. Only way to know for sure is to take the alternator off and put it on the bench at the auto parts store.

 

Not always true. I just replaced the alternator on my 01 suburban after pulling it finding out it was good via autozone putting it back on and the banging my head for days trying to sort why it was not charging. I finally punted and got a reman unit and instant gratification!

You can go to any junkyard and find units for as little as 15 bucks that you could pick up and try rather then getting a remand or OEM. either way the first rule of DYI IMHO is try the simplest fix first. My guess is that you have a rough spot in the Alt and that it is working some of the time. You could unplug and spray the connections down with electrical cleaner and see if that helps. If the alt is working the car should pull at least 13.5v even with every electric on if its lower then that or its bouncing everywhere replace and see if that fixes the issue.

you can always just pay the core fee keep the old one and if its not it return the remand under its warranty.

 

Two other possibilities are a break in the stator wiring or the voltage regulator brushes were not installed properly.