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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

David
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4 Answers4

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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.

Pᴀᴜʟsᴛᴇʀ2
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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.

Jupiter
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No parasitic drain, your alternator is charging your battery, everything is working. You are good. You need a new battery.

MJH
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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.

fraxinus
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