My son and I got a used 2005 C6 Base model with the manual transmission two months ago with the idea that we will fix it up so he can have a nice sports car, and I can get in some bonding time with a kid who is growing up too fast. We discovered quickly that the car’s battery will drain overnight from fully charged to 10.5 volts, despite the car being placed in reverse and shut down completely.

This drain is causing the battery to discharge about 0.1 volts every two hours, giving us roughly a twenty hour window from fully charged to “dead” at 11.5 V.

We use a Battery Tender to keep the battery charged when not driving it, so this isn’t a situation where the car is broken down and useless. It fires up with no hesitation as long as the battery is charged, and seems to perform just fine in all respects when it is running. We just want to fix the battery drain in case he ever wants to go somewhere and park it overnight.

Here are the steps we’ve taken to resolve this problem:

1) We tested the battery for a baseline voltage after disconnecting from the trickle charger and turning on the headlights for 90 seconds to remove surplus voltage. The battery checked in at 12.55V, but immediately started losing the 0.05 V every hour.

2) Checked the battery when running to make sure the alternator was functioning. Charging voltage of 14.3 V @ 1,500 RPM.

3) Checked the draw by running the DMM in series with the negative terminal and negative cable. We have done this test four times, with two different DMMs, and we are looking at 20-25 mA of draw after 20 minutes when the car goes to sleep.







4) Performed a voltage drop test across each fuse in both fuse boxes. Every fuse registered 0.0 on the 200 mV setting, no fuses were blown, they all matched the amp ratings called for on the inside of the fuse box.

Still losing 0.1 V per 2 hours.

5) Pulled every fuse and relay in both fuse boxes. Still seeing the drain at the battery with all fuses and relays removed.

6) Completely disconnected the alternator, both from B+ and at the voltage regulator. Still seeing the drain at the battery.

7) Disconnected the wires on the solenoid, mounted on top of the starter. Still saw the battery drain voltage.

8) Checked the resistance of the ground wire at the battery, from the terminal down to the ground where it bolts to the frame. There is no short in the wire, and the connection is solid.

9) Got a new battery, despite the old one testing as good and showing 440 CCA out of the original 660. New battery immediately began draining like the previous battery.

10) Disconnected everything from the battery, disconnected the starter at the solenoid, the hot wire going into the fuse box that’s mounted to the passenger side, and the alternator at both B+ and the regulator. No drain on the battery when it was all disconnected. Began adding circuits back, one at a time. Began with the starter, connected it at the battery, no drain, so cables are good. Connected the starter at the solenoid, no drain. Doesn’t look like a short in the ignition system.

Reconnected the fuse box’s hot wire. We did the voltage drop test, pulled the relays, and measured drain at the battery again for giggles. Still showing no hot fuses, and a 20-25 mA drain when the DMM is in series with the circuits. No drain on the battery.

Reconnected the alternator.

Battery began to drain once everything was attached, but we have tested each component separately, and I am unaware of any drain that might result from two different circuits working in tandem off the battery.

So I’m out of ideas here. I’d like to think that there is a bad wire somewhere under the hood, but we need to know which fused circuit to check to begin to find it, and all of the fuses are showing 0.0 mV when I use the DMM to measure post to post on the back of the fuse while it’s in its socket.

Anyone want to help a dad not look like an idiot in front of his son? I have a tone generator, so I can trace bad wires if that’s really the issue, but I’d think I’d see the drain in the Amp test at the battery.

 

My first thought is: Do you have a different DMM to test power draw? I've had DMMs that have failed with the current measurements. For the voltage to drop that consistently, 20mA is NOT enough to cause those issues. Secondly, IIRC the 2005 C6 had to be parked with the shifter in R or it had bad battery draw issues...

 

Yes, I have used two DMMs to test the draw. The second DMM is in the third picture.

Yes, it is parked with the shifter in reverse. The car does actually go to sleep after 20 minutes.

 

Did you do a clamp on meter at battery negative cable for real loss? Did you infared each fuse for temperture change? After it goes to sleep you should test each fuse, how with all doors closed hood down key out of ignition and if you have to leave something open latch it manualy over ride the door latch. Then wait an hour and go across each fuse checking for on the scale of meter milyvolts not amps. Going across the negative side of your meter can create more problems.on your battery. Another way is to pull a fuse each night before and check voltage in morning, might take some time but will eliminate fuses. then you can start on all your grounds! Another quicky do a 200 amp load check on battery and this will tell you if you have a good battery. I use a 100 amp load tester and that works to but a deuce will tell you condition real quickly. Also under your battery there was a question with regard to corossion because of over charging, loose battery fill caps etc

 

I had an electrical parasitic draw on my 2011 GS ... a massive .463 amp draw from the battery all the time the car was completely shut down.

By pulling the 'BATT5' 30 amp fuse in the engine compartment fuse box, my amp draw went down to .007

Well, the 'BATT5' fuse is the main 30 amp fuse for:

1. Body Control Module (BCM)
2. Tele Sw / Mem Seat Module
3. Driv Dr Sw
4. Ign Sw / Intr Sens
5. OnStar
6. TPA Fuses

However, these 6 items also have there own separate fuse.

I narrowed it down to just the OnStar fuse, and by pulling just this OnStar fuse (located in the passenger foot well fuse box), the electrical load again went down to .007 amps

But, it looks like you have already pulled all your fuses and checked the draw ... good luck
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This is how I hooked up my multimeter to test the draw:

 

EP1429, something doesn't jive here. The pictures are kind of dark, but I assume you have your ammeter connected correctly. If so, 20mA is a very good reading for a car whose electrical system is functioning normally. No excessive battery drain will occur at that level. Therefore, it is not clear why you are experiencing a battery drain problem.

 

I’ve done the voltage drop test, and after I couldn’t find anything pulling power across the fuses when the car was asleep, I pulled the fuses and relays. And yes, I used the 200 mV setting on my DMM.

 

We set ours up in the same manner, just using the negative terminal. We zip tie one lead to the post and the other to the cable to make sure they are secure and have a good connection. We can try the positive terminal tomorrow though.

 

I 100% agree with you. Something doesn’t jive here. That’s why I’m asking. I’ve probably watched 100 hours of YouTube videos about C6 electrical systems, and I still can’t come up with anything.

Maybe the starter is melted from being next to the exhaust pipes? I’ve seen a couple of videos where people mention that, but there doesn’t appear to be any issue with the starter cranking, as long as the battery isn’t drained.

We took everything off the battery again tonight, trying to run it down. Put all of the circuits back on, one at a time, testing the wires for shorts before actually hooking up the starter’s hot wire, the B+ wire and then the regulator snap link on the alternator, and finally the hot wire running to the fuse box.

None of them showed a drain once we hooked them up to juice. Did the voltage drop test across each fuse again, pulled all of the relays again. Even when we got all three main circuits hooked up, the battery was experiencing no drain. Sat at 12.77 V for three hours and didn’t budge.

So the boy fired it up and took it for a quick spin, he was gone a little more than half an hour, and since he parked it an hour ago, it has bled off 0.2 V of charge. I’m going to have to hook it up to the trickle charger, or it’ll be dead by 8 AM.

 

All I can come up with is that there is a huge battery drain at some time when you are not measuring it. At 20mA, the car can sit for over a month and still start. If the battery is dead after sitting overnight, the drain is in the neighborhood of 500mA or more.

 

We can see the drain happening. You can watch the DMM’s voltage reading drop in real time when the battery is bleeding charge. Let’s say the initial reading is 12.6V. If you stand there for about ten minutes, it’ll begin to flicker on the display between 12.6 and 12.59.

After a minute of oscillation between the two, it’ll settle on 12.59, and ten minutes later, it’ll begin to flicker again, between 12.59 and 12.58. Eventually it will settle on 12.58, and the cycle repeats itself until the battery is drained.

What we can’t see is the short which is causing it. We can fix whatever is wrong, if we can find it. But we can’t locate even the first breadcrumb that leads us back home.

 

You need to use current (amps) to track down your problem (not volts). First of all, you must witness a huge current draw (like 500mA) on that battery. You have not reported that so far. Maybe you need to keep your ammeter connected for 24 hours and check it from time to time to actually witness the huge battery drain that you have, then begin use your ammeter connected at various locations away from the battery to track that drain to a specific cause.

 

If you perform the 24 hour drain test and find that the battery is dead without witnessing a huge current draw, then one must assume that the battery is draining internally, and a new battery is in order.

 

I am measuring the draw in amps. My meters read 20 mA of draw across the negative terminal when the car is asleep.

I am measuring the strength of the battery in Volts.

I am measuring across the fuses in the voltage drop tests in mV.

The purpose of disconnecting everything at the battery, and then reattaching it one circuit at a time, one wire at a time, is to try to isolate which circuit is drawing the current when it isn’t supposed to. The step after that would be going to everything on the hot circuit and measuring it, piece by piece. Right now, we don’t know which electrical component in the system to start with.

 

Yep. Bought a new battery 5 days ago. The same drain was witnessed immediately.

 

Here is the battery drain, time stamped, starting after the car was turned off and given 20 minutes to go to sleep.

 

Ok, we hooked up the DMM in series with the positive terminal and cable, as suggested above. It has doubled the amount of mA being pulled to 50 mA, and while high, that’s not enough to drain a fully charged battery in 10 hours.

We are going to try testing the fuses with the test light and then pulling fuses again if we can’t find a drain with it. Updates to follow.

 

Progress!!!

I have a friend who has a digital circuit reader that plugs into mini-blade fuses, and it discovered a big 1.8 A draw coming from the Batt 5 fuse in the under hood fuse box, along with a not-quite-as-big a draw from the Batt 3 J-style fuse. No clue why we wasn’t able to find it with our test light previously, I’m just glad it has been located.

We wrote down all the fuses that are powered off of those two circuits after using the car’s wiring diagram to discover that both circuits go to the BCM in the passenger side footwell. Got down and pulled every fuse from those circuits, but did not eliminate the draw from the upper fuse.

So if there is a hot fuse leading to another fuse box, and all of the fuses it is supposed to power have been pulled with no effect on measured draw, there must be a break between the underhood fuse box, and the interior BCM. But it can’t be a really bad break, since all of the things it is supposed to power, like the radio, still work. Power must be flowing through the circuit.

And since two circuits are being effected, and both run to the same place, it’s likely that there is a damaged wiring harness.

Now, here is where I could use some more help:

According to the wiring diagram, the circuits come directly off the under hood fuses, one of them is split, and the other pipes directly down to the BCM, which is only a couple of feet away. Does this mean that the wiring harness connecting the two fuses boxes is running down in between the fender and the chassis?

Do we need to remove the fender and wheel well to access the harness? How difficult is that?

It looks like there is a computer module in there, too. We can barely see the corner of it in a gap between the battery and the firewall. Is that the ECM? Would the ECM be the “S208” junction on the wiring diagram?

 

Re-read the post and found the answer to my question

 

Ok, I’ve played with this for a week, and here’s what I’ve discovered:

Took the under hood fuse box apart. No problems with the fuse block for either the Batt 3 or the Batt 5 circuits, and the connections were good.





So I took the fender off.



The wiring harness passed a visual check.

Hooked up a tone generator to the wires at the underhood fuse box and traced them to the BCM inside the car. We had a tone all the way to the run-crank fuse block on the BCM.

Not satisfied with just the tone, I also used my multimeter to check for continuity all the way from the under hood fuse block to the BCM, along all three wires, six inches at a time. There was no short nor break in the wires at all, and power is reaching the BCM.

To double check, I pulled all of the fuses in the BCM with my multimeter hooked up in series with the positive cable again.




The draw only disappeared once the run-crank wiring block (The red brick hanging there) was disconnected, which I did last. I checked the wires between that block and the one under the hood using my multimeter, and there is continuity all the way to the BCM’s run-crank block.

So if there is no draw down stream of the BCM’s fuses, no breaks or grounds in the wiring harness, and no draw once the BCM is completely disconnected, doesn’t that mean the BCM is the problem?