Car Alternator / battery problem

[BruceP]

One method is to measure voltage across the fuses. There is a 'lookup chart' which equates these voltage-readings to current based on the fuse-rating.

With machine turned OFF, if you have ANY voltage across a fuse, there is current flowing thru that fuse.

The youTube channel "South Main Auto" (https://www.youtube.com/channel/UCtAGzm9e_liY7ko1PBhzTHA) has some VERY good detailed videos how to troubleshoot, isolate and repair parasitic draws.

 

[Daren Todd]

I'm assuming by disconnected, the cables are removed from your battery?

If so, check your battery again. If it's continuing to drop then the battery has an internal short and won't hold a charge.

Replace the battery and your good to go.

If the battery is still holding your original reading, then the battery is good and the issue is somewhere else.

 

[Fordtech86]

Yes battery is bad if its measuring 6 volts after a couple days unhooked (thats 3 bad cells, likely plates split in the middle). Can not check for draw by using ohms between the cables because there is no current flowing if battery is disconnected (and if it was connected your still not checking anything that way). You do need to measure amps, Bruce’s way seemed interesting (I didn’t follow his link) but not how Ive been trained to find a draw on a vehicle. Its an older car so not as complex to check but its different then a tractor (aside from modern computer controlled tractors to an extent). I would replace battery and re evaluate. Modules will start shutting down around 10 volts so if you lose that between your plates in the battery while driving it explains why it just shuts down. Also when checking alternator output with voltmeter while running, turn as much stuff on as you can (blower motor, lights, etc) to get as much load on it. Also let engine/alternator get up to temp.

But I wouldn’t go any farther until battery is replaced.

If still an issue after that I can get you some more vehicle specific info if needed.

 

[random]

Thanks for all the input. I'll be checking the battery this afternoon when I have a break. I've left it disconnected, so if it's down, then we know that's at least a major culprit.

 

[chim]

I didn't see anyone mention that a vehicle with an alternator won't run without a battery. They need the power from the battery to supply power to the alternator to create a magnetic field so it can produce voltage.

Older vehicles with generators could run without a battery because the generators had enough residual magnetism in them to start the generation process.

 

[Tooljunkie]

Doing a resistance test means nothing. If battery dropped to 6.7 volts while disconnected, you may want to consider a 24 hour trickle charge and retest disconnected battery for another 24 hours.
amp draw will determine if there is a parasitic drain.
typically i use a disconnect device, and after a drive cycle. I install disconnect inline on pos battery post, road test. Depending on vehicle, i open both doors and trip the latches to fool car into thinking doors are closed and lock it. Wait 30 minutes and then connect ammeter in line,and open the installed disconnect.
set meter on 10 amp test. .03 amp or less is acceptable.

 

[Tooljunkie]

And disconnecting battery while car is running is taboo. You will release the magic smoke in the most expensive electronic component in the car.
Anything computer equipped is just a puff of smoke away from an expensive repair.

 

[random]

So checking the battery yesterday yielded 9.5 - so it's lost 2.5 volts overnight.
Battery is pretty new, still under warranty, so that's getting replaced before we do anything else. Thanks all for the help!

 

[Bountyhunter]

W

So checking the battery yesterday yielded 9.5 - so it's lost 2.5 volts overnight.
Battery is pretty new, still under warranty, so that's getting replaced before we do anything else. Thanks all for the help!

What I have done in the past is connect a taillight socket with an 1157 bulb in series between the negative terminal and cable with the positive connected. With everything off, if the bulb lights up you have a short. If the bulb stays off, you are good to go. There are many small draws when the car is shut off ,such as the clock, pcm etc. If the bulb lights up, you have a big draw. The battery should not loose any voltage with it disconnected

H

 

[GreensvilleJay]

re: 9.5 - so it's lost 2.5 volts overnight.

yup, 'bad battery , it's lost' a cell...

 

[lugbolt]

gotta remember that basically everything is ecu controlled now

the ecu uses power even if the engine is off and key removed from ignition (assuming it even has a key) it monitors EVERYTHING

so when u are doing a "drain" test, the ecu is going to show a drain, as does the radio, airbag control unit(s), and of course the pesky ccrm (if it has one-a lot of cars do). So those need to be isolated.

In this case the battery is junk and I bet that will solve most if not all of the problem.

removing batt cable from batt with engine running can destroy the alternator and sometimes any one of the many confusers I mean computers on the car. Or since we are on a kubota site, the same applies to kubota. I had to replace a couple alternators this past summer because of this practice. One ECU but I am not sure what caused the ecu problem, and the customer was of no help whatsoever in diagnosing the root cause. "It just quit"--and that's the only information I could ever get out of him.

lot of times the alternator's output is controlled by the ecu. the alternator itself only charges what it is told to, e.g., there is usually a sensing wire (or set of wires) that feed current from the ecu to the alternator's regulator assembly. The regulator "senses" the voltage and adjusts output accordingly. IF the ecu "thinks" that the operator is running in such a way that output can be reduced-which reduces load on the engine and therefore improves fuel economy and reduces certain exhaust gas emissions, you can bet they have it programmed to do just that. Even my 92 Mustang has an ecu controlled alternator. That was 1980's technology as the same ecu was used from 89 on up, and similar strategies used even before that on some of the t-birds, Merkurs, etc.

we gotta remember that mfg's are doing all things possible to reduce emissions, improve nvh, improve mpg, and a slew of other things--because for one they have some eligibilty for certain credits if they can meet some criteria. Getting those credits means they can sell the car cheaper, which means they have a competitive advantage which in the car market is a huge deal! Even have variable displacement oil pumps (old tech, but recently used on cars), and variable displacement a/c compressors...just to name 2 of the many strategies.

 

[random]

Ok, everyone, I got it: don't disconnect the battery while it's running!

New battery installed yesterday, voltage checked out 14.5 while running, 12.4 while off. Just checked, 12.4 now, so looks like this one is holding. Couldn't detect any current drain with car turned off either. Thanks for the help!