I have an 89 Cherokee Limited 4.0 auto with a few modifications. I have a massive power drain that will kill a battery overnight. I have started the testing process but haven't had any success and the huge amp drain reading has me confused. Here is what I have done.

I am using a cheap multimeter and I am no pro but it seems to be working fine.
Brand new battery - 12.8 volts with engine off, 14.2 with engine running
Disconnect the negative cable from battery and jump with my multimeter, 12.8 amp draw reading.
Pulled all fuses one at a time from the fuse panel, absolutely no change in reading. Actually, I couldn't get any change in amp reading by opening doors, hatch, glove box, lights, etc.
Disconnected alternator, no change.
Pulled relays in engine compartment, no change.

Any ideas appreciated.

Clint

 

I think you might be doing something wrong, and that 12.8 is volts? You may need to change to a different socket on the meter to measure current, and change the setting to amps.

 

12.8 amps is something like having your headlights on. That's not a parasitic draw. I think you're doing something wrong. Post some pics of your setup.

 

I think you're just reading the voltage again. a 12.8A draw would blow the fuse in any normal multimeter as they are 10A.

 

What you need to do is move the red wire from the V/Ohm marked socket and onto the 10A socket, then turn your selection to 10A. It may be marked slightly differently on yours. When you first connect the meter in, you may get a spiked reading as capacitors charge etc but after that should be your constant parasitic draw.

 

A decent video on how to diagnose battery drain using a meter.
Often it's something simple like the glovebox light staying on.

 

If we do find there is a 12 amp drain, I vote for the alternator being energized at full tilt even when it shouldn't be. Nearly nothing else in a car can draw that much.

 

Even if the alternator field coil were somehow getting powered, that's maybe 2-amps, and it would likely being overcharging the battery when the engine was running. Maybe shorted diodes, but then he'd probably not be seeing the good charging voltage.

12-amps would completely drain the battery within 1-2 hour.

 

I agree that over night is not correct for 12 amps and 1+ hour is much closer. I didn't see a place where they said it is charging correctly when running. I'll admit I have not measured the field coil current on a non-running engine, but that is the time when the current would be highest and would reduce as soon as rpm came up. I agree that the charging system would be completely borked if it was doing this to a level that would be hard to believe and it is much more likely that we are getting bad data from using the meter incorrectly, but as I said, 'if...'

 

First post

 

PCM controls the ground side of the field coil, so it won't be drawing anything until the PCM thinks the engine is running.

 

Thanks so much for the replies. I am no electrician so thanks for the help with my multimeter. I was able to check my battery voltage and I had the battery checked. All good. I just remembered that when I put on the battery charger the first time, it couldn't keep up with the drain. Meaning, with the battery connected to the jeep overnight, the charger could only charge the battery to 70%. When I disconnected the battery from the jeep I could charge it to 100%. It could be draining in one hour. I think that means the jeep is really draining the battery.

Here is a pic of what I'm using.

I set it to DC volts/amps and dialed it to the 20 scale. That is where I got my 12.8 amp draw.

Any ideas what to do next. I plan on getting a better multimeter:-)

 

How are you connecting that thing? Does it have cables or what?

 

Yep, I see that it is charging. I don't see that it is charging correctly. If the PCM MOSFET is blown closed it could be connecting the field coil to ground all the time at full current, even when everything is in the off position. If the field control wire is rubbed through and shorted to ground, again, energized at full current all the time. Unfortunately, getting from questionable measurements and wild speculation to fact seems like it will be a journey in this case. I expect we will get there after a while, but first comes making sure we are using the DMM correctly.

 

That meter is only rated for 200ma, 0.2A. If you just turned the dial to 20 I suspect you were getting an Ohms reading.

This is the meter I use at work with an added external fuse. The external fuse is there to save blowing the internal fuse which isn't replaceable. Ask me how I know 3 times.