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I was debugging a battery. The resting voltage was 10.8, which means either the battery is bad or there is something draining it. So to check the drain I had my multimeter connected to measure current. There was no drain, so I assume it's the battery. I then boosted my car, and I wanted to measure the voltage on the battery coming from the alternator. I turned the dial to measure voltage, but I forgot to change the wire connection on the multimeter. There was a small spark, and the wires heated up, but the car kept running and I pulled my hands away in a second. Did I mess something up? How can I check to make sure everything is ok? After I realized my mess up I correctly measure the voltage on the battery with the car running at 14.1v

Edit: car is a Mazda 3 2005 Z6

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  • Welcome to Motor Vehicle Maintenance & Repair! Commented Apr 26, 2022 at 16:31
  • What year, make and model is your car? Newer cars have more electronics and sensitivity to a short. Does everything (e.g. lights, radio, etc.) seem to work? If everything is working then likely you did not cause any damage. You should still check the alternator (using the multimeter correctly of course) to make sure the alternator is charging the battery. Car running, battery voltage should be between ~ 13.8-14.8 volts.
    – MJH
    Commented Apr 26, 2022 at 16:32

4 Answers 4

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No, you most likely didn't mess anything on the car up, though you might have fried your multimeter. If the vehicle still runs and nothing seems messed up, more than likely nothing is.

Something you could do better. If you are going to measure voltage, you can do this directly without disconnecting the output lead on the alternator. You don't want to put your DMM inline in the circuit. You check this by putting the positive lead from the DMM to the output terminal and the ground lead to ground. This is basically the same as checking your battery at rest, but you're now checking it live.

Another thing to consider is you should never run your vehicle without the battery connected. While this used to be a troubleshooting technique back when we had generators instead of alternators, these days you run a considerable risk of frying both your alternator and/or any electronics tied to the system. The battery acts as a buffer and helps prevents surges/spikes which could wipe everything out if severe enough.

EDIT: 14.1vdc running voltage is right where it should be.

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  • I guess I wasn't clear in my question. I put the DMM inline with the battery to measure draw when the car was off to verify there is no parasitic draw. I confirmed there wasn't and reconnected the battery. After I boosted the car I tried to measure voltage across the battery terminals to check the alternator, and I realized I forgot to change the wire on the DMM
    – David
    Commented Apr 26, 2022 at 16:47
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    Hopefully your multimeter has a fuse on the current sense inputs.
    – jwh20
    Commented Apr 26, 2022 at 16:48
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    @David A cheap DMM is an expensive fuse. Commented Apr 27, 2022 at 6:49
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    @TooTea OTOH the leads on a cheap multimeter are pretty thin inside all the plastic, and the contact might not have been good, so some series resistance. It's quite possible the meter has survived, though possibly losing a little of whatever accuracy it had to start with
    – Chris H
    Commented Apr 27, 2022 at 9:28
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    @ChrisH Sure, what I meant is that if anything breaks, it will be the DMM, not the car.
    – TooTea
    Commented Apr 27, 2022 at 9:34
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You did no damage to the car with this. If your DMM does not have a fuse, you may have ruined the amp part of the DMM. If you measure a known amp source and get an accurate reading your good to go. If not you will need a new DMM if you want to measure current again.

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    There's a slim chance it's damaged other parts of the meter, so test before relying on it for anything too important
    – Chris H
    Commented Apr 27, 2022 at 9:29
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No parasitic drain, your alternator is charging your battery, everything is working. You are good. You need a new battery.

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10.8V is a clear indication of a dead battery. There is a specific failure mode of the lead-acid starter batteries that leads to one cell shorted or almost shorted. 10.8V are five good, charged cells and one dead.

In regard to the alternator - the garden variety of a car alternator has enough power to melt and burn the test leads of a multimeter without even feeling overloaded. These probably have a total resistance of ~0.2-0.5 ohm, meaning 25-60 ampere at 12V.

Cars with less than 60A alternator are not made since ~1990. And the great majority of car alternators are perfectly OK when shorted.

The battery (even with one dead cell) can do the same with ease.

Don't worry and be careful.

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