Diagnosing a parasitic drain

Hey, all --

I need your help in diagnosing what's causing a parasitic drain on my GRM-style '96 Tahoe.

Background & Symptoms - Everything was okay, then had it in to replace the intake manifold gasket last weekend. - After he put it back together (I didn't have time to do it myself), he had problems with the battery. - Replaced the battery last night, thinking the old one was just dead. - Today, the car won't start for my wife after she parked it for a few minutes while shopping.

What I've done so far: First, I should mention that I'm a bit of a noob when it comes to electrical issues...

I disconnected the negative terminal and connected one lead of my multimeter to the cable, one to the batter. Set on any of the Amps settings, the thing pegs the needle and will blow it's little fuse if I hold it for a second. Set on volts, it reads 12.x between the cable and the battery.

I went through an unplugged everything fuse by fuse and the volt rating never changed.

Again, I'm a noob here, but it appears that something is very wonky here. It must have a major drain on the battery to peg the needle like that... right? What do I do next?

Yow. If it pops the fuse in the DVOM, that's a serious draw, like over 10 amps. Can you say 'dead short to ground'? Pulling fuses doesn't kill it = something shorted between the battery and the fuseblock. I'm thinking a wire got pinched during the intake R&R.

You can also connect a test light instead of the DVOM (but in the same fashion) and start unplugging harnesses. If you disconnect a harness and the light goes out, you have just narrowed it way down. Start tracing that harness.

I'd get a good strong flashlight and a inspection mirror and start going around the edges of the intake, particularly around the back side. Look at bolts, brackets, etc. Try wiggling or tugging the harnesses (disconnect the battery first!), you are looking for one that doesn't move easily. It only takes a couple of strands to make a serious battery killin' draw.

I had a low current drain... that only happened at random :evil:

Could never track it down because I couldn't figure out when it was going to happen... ended up buying a $20 solar panel and putting it in the window. No problems since :nice:

Jensenman wrote: You can also connect a test light instead of the DVOM (but in the same fashion) and start unplugging harnesses. If you disconnect a harness and the light goes out, you have just narrowed it way down. Start tracing that harness. I'd get a good strong flashlight and a inspection mirror and start going around the edges of the intake, particularly around the back side. Look at bolts, brackets, etc. Try wiggling or tugging the harnesses (disconnect the battery first!), you are looking for one that doesn't move easily. It only takes a couple of strands to make a serious battery killin' draw.

Awesome -- I'll try both of those things. Oddly enough, I don't own a test light.

Yay! A reason to go to Harbor Freight!

What scale is your meter set at? Some meters have a separate place to plug the leads in for the 10 amp scale. Are you useing that? What is the rating of little fuse that's blowing? If you have a 10+ amp draw, somthing is going to be getting hot somewhere.

Meter: Set on 10 amp scale (or higher if it has it), black lead in commone, red lead in 10 amp hole, black lead to ground, red lead to negative of battery. What's the current drain? If all the fuses are out and you still have current drain, yeah, something between the battery and fuse box is shorting.

The multimiter is just a cheapie that I picked up at Wal-Mart last night for the task. My real tools are in storage a few hours away while I'm in temporary housing. It's this one.

It doesn't have any different plug-ins for the leads. Just the one set.

The fuse is a 0.5a 250v fuse. Now that I just typed that I realize that it's pretty puny. I'm going to get a 12v test light tonight.

It looks like the cables from the batter to the main under-hood fuse block run behind the motor -- so the dude probably did pinch the wires when reinstalling the manifold.

Next time remind me to go with an ASE certified mechanic.

Update:

I still need help. Badly :(

I picked up a 12v test light last night, and that also popped instantly. I unplugged all of the connectors going to the underhood fusebox and it's still reading off the chart on the multimeter.

I searched for any pinched wires or harnesses and found nothing out of the ordinary. Grounds are all solid (at least, all that I have seen/found).

I'm getting frantic... I need to move my things into storage this weekend so I can move out next weekend. This is a really bad time for this.

If I understand correctly, the most likely problem is that a 'live' cable somewhere is being grounded. Either it's pinched or maybe the insulation wore off. Correct?

Wow. :omg: If it popped a test light that's one helluva amp draw. Grounds won't do something like that.

You are correct, either the insulation's worn off or a wire is pinched between two metal parts. Since it started immediately after the intake was R&Red, to me that points to pinched wires. That doesn't mean that's the only possibility, unfortunately.

Try this: disconnect the battery and fusebox completely, then disconnect the main wire on the back of the alternator. Set the multimeter to amps (continuity), see if there is continuity between the alternator wire and the engine block. Continuity = short to ground. Do this on the other wires in the engine harness. Be aware this is a slow process, since some things (like temperature senders, etc) will have continuity. To do this quickly, you will most likely need a wiring diagram, so you can see which ones should have continuity to ground. For that matter, an alternator can have a diode go bad inside and cause a helluva amp draw. Usually you'll get a charge light on as well, though.

Also, GM generally runs the battery '+' cable to the starter solenoid terminal and there are fusible links on that same terminal which feed power to the rest of the car. Usually there's two and also the other wire which goes to the alternator, for a total of four (battery cable, fusible links, alternator wire), disconnect them from the terminal and check those for continuity to ground as well. Again, it really helps to have a wiring diagram so you don't go off on a wild goose chase.

EDIT: AutoZone has wiring diagrams on their Web site. They are Adobe PDF's so you can easily print them out. Good luck!

I think your "Mechanic" owes you some troubleshooting time. If he can change that gasket, he must have some skills, and it's pretty obvious he caused the problem.

Okay -- I think I narrowed it down a bit, but I'm still stuck.

I went and bought a ton of fuses so I could keep testing. I disconnected the fuse block again, and (using the right setting on my multimeter this time) saw that everything was okay.

Sweet.

Through the process of elimination, I found that the 50A BATT circuit is the one causing the problem. What does that do? Where do I go from here?

Also: the alternator was disconnected before running the test, so it rules that out.

Ahhh, it looks like that powers the in-car fuse block.

Back for more investigation!

The 50A 'BATT' fuse powers, among other things, the dome light circuit, radio/clock/PCM/BCM memory, stuff like that. Start yanking the inside fuses one at a time.

I did... no change in amp readings from the meter inline with the negative cable and the battery.

I yanked the fuse, and the car runs enough to get me through this weekend.

Although, interesting to note -- I charged it all night long and it still cranked slowly. So, I'm not ruling out a problem with the connection to the starter.

Couple of things to look at. Is the ignition off when you are testing? Are the doors closeed (interior lights off)? Any aftermarket accessories (cb, stereo amp, etc)?

When you disconnect the battery and reconnect it, lots of modules will "wake up," causing a big drain. What you need is a parasitic draw tester. It's basically a switch that attaches between the negative terminal and the negative cable that you can clip alligator leads on either side of. You attach the multimeter leads to it with the switch closed so power is flowing through the switch, not the meter. After a few minutes you open the switch and the battery power flows through the meter. Since there is no interruption of power the modules stay asleep.

Once you're able to get a reading on the meter start pulling and replacing fuses one at a time until most of the draw disappears. You should probably be around 15 mA or so when the vehicle is sitting. Once you figure out what circuit has the problem you can start correcting the problem.

Scott Sawyer, GM Master Tech