2016 GMC Yukon parasitic draw

This truck belongs to my boss. He said the battery dies after a few days of not being used. He sent his son th her the battery exchanged because it was less than a year old. The battery shop said the battery is perfect and the truck has a huge draw. The sales guy went out to the truck, he removed the negative cable and put a test light in between the ground cable and negative side of the battery and the light lit up bright. I put a dvom in 10amp mode and instantly see 5.5 amps. Within about 3 minutes that amperage drops down to 46 milliamps and every 10 seconds exactly it bounces up to 221ma.
If I put a test light in between the cable and battery, the test light never goes out. So at 46 milliamps with a 221mh spike every 10 seconds, does this car really have a bad draw. The guy next door told me the onstar system is causing the spikes and they are normal but this is the same guy telling me it has a bad draw.

Within about 3 minutes that amperage drops down to 46 milliamps and every 10 seconds exactly it bounces up to 221ma.

46mA by itself isn't terrible, but 221mA is definitely excessive. My general rule of thumb is 50mA or less. I wouldn't classify this as a BAD draw, but that's probably a matter of perspective.

I do have some questions about your measurement. How long did you test for draw? Were all doors shut/locked, all windows rolled up, hood latch tripped, and the keys removed?

I put a dvom in 10amp mode and instantly see 5.5 amps.

That's a pretty standard reaction when reconnecting a battery cable - especially on a vehicle like this one with dozens of modules.

The sales guy went out to the truck, he removed the negative cable and put a test light in between the ground cable and negative side of the battery and the light lit up bright.

That is NOT a good test for draw. On older vehicles without networked modules, maybe, but not this '16 GMC. :silly:

The problem is that 5.5A reading that you saw. It's not an arbitrary number, it's most/all of the modules in the vehicle waking up because they've just been reconnected to the battery via the hot-at-all-times fuses. Those same modules need the chance to go through their internal routines for powering down, and it takes more than the 200mA that test light carries to do that. You end up with dozens of modules that can't power down, because they never started up correctly in the first place.

ISTR Paul has a Youtube video on parasitic drain.
Parasitic drains are a pita to measure the easy way is a DC clamp meter or better an current clamp and an oscilloscope but even that can be misleading where you are looking for a sub 50ma draw.
Working with a normal multimeter is tricker --- one thing to watch out for is most multimeters are fused at 10 amps and it is very easy to blow the fuse which can cause much confusion.
A couple of years back I hacked together this for finding a parasitic draw on a BMW --- just a cheap master swtich and a couple of battery cables to allow an easy disconnect. It is connected to replace the main battery ground cable you can then then connect the multimeter to measure the parasitic drain.



One of the problems is to get a true picture the vehicle has to be fully locked down in a state that puts all the modules into sleep mode often having the hood open can prevent this. One of the tricks I use is to put a heavy weight (2 pound hammer) on the hood open detect switch plunger to fool the alarm and BCU.

Where a parasitic drain is fairly large the main suspects are (1) Anything plugged into the diagnostic socket or cigar lighter socket. (2) Anything aftermarket, ie. Radio head units, cameras, GPS trackers (3) Water intrusion especially if car has modules in trunk --- for example CD changers in spare wheel well. (4) Alternator (5) Screen wiper motor.