How Does That Work Series: Testing A Battery Drain Problem

A common complaint is a battery going dead after a car has been parked for a day or so. I just happen to have a 2013 GMC Sierra in the shop with that problem. I'll walk through the steps to diagnose this particular truck. First, we want to be sure the battery is fully charged and is a good battery. The electrical system tests aren't reliable unless we are using a good, fully charged battery. The battery had a 90% charge when the truck came in. A slow charge is needed to bring it up to 100% and that can take a few hours. Sometimes I switch batteries to let the original charge while I use a known good battery to move ahead with the tests. This truck was staying overnight so I just left the charger on the truck until it reached 100%.

After the battery is charged, it should be tested. Cranking the truck a few times with lights and accessories on will get rid of the surface charge so the test will be accurate.

Once the battery has been verified good and has a full charge it is time to measure the actual current draw. I like to use an amp clamp along with the meter so that I don't have to disturb a circuit.

You have to let enough time pass that the computers should go to sleep mode. If you start testing before that time you will be chasing a draw that isn't a problem. After waiting an hour I put the clamp on the negative battery cable and I can see excessive current draw. 

At this point the job is to find the circuit the problem is on. I had a small ground wire from the battery post with no draw and the large one with 377mA. The positive post had three cables from the positive post. I found the draw to be on the cable from battery to underhood fuse block. 

Next step is to start removing fuses from the block one at a time and watch the meter. When you see the draw fall to normal you have found the circuit and are a step closer. I like to take a picture of the fuses while all in place in case I have to refer to it when re-installing the fuses. As I remove each fuse I lay them out in a similar pattern but the backup of a picture is a good idea. If you remove a fuse that has no affect on the draw don't put it back in immediately. Just keep removing fuses and laying them aside until you find the one that affects the problem. Putting a fuse back in before you have found the problem can sometimes wake a computer that was in sleep mode and you will have to allow the timeout all over again. 

Eventually I did find the fuse for the problem circuit. I like to put the other fuses back after I know where the problem is and just leave that one out. 

Next up is a wiring diagram. It is sort of a roadmap for tracing further. We want to see what this fuse feeds. This fuse turns out to be the battery feed to the instrument panel fuse block. So I put the fuse back in place. We'll have to wait for the timeout again but I also had to leave the driver door open to access the instrument panel fuse block. The door is open but tripping the latch closed is enough to make the computer believe the door is closed. 

Just like the procedure for the underhood block, we'll pull fuses in the instrument panel block until we find the circuit with the draw. I found the problem circuit and left that fuse out, replacing the rest. 

Then it is back to the wiring diagram. The fuse powers the Vehicle Communication Interface Module. (OnStar Module). 

Having gotten this far I was able to find a related service bulletin. 

You will find different paths on different automobiles but the procedure for tracing battery drain is going to be basically the same. Thanks for reading!

Kenneth Hayes

G&G Auto Repair