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I was trying to measure some voltage on cables going to ECU (car computer) and I had my multimeter in wrong configuration. It was set for 10 sec max 10A fused. When I touched the cables there was a little spark. I hope that I didn't kill my car ECU. And this was on a running car.

So I was wondering: can you break anything when using multimeter? I guess I can break the multimeter itself but that's ok. I'm more worried about car.

What would happen if I used the black lead on positive and red on negative?

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    \$\begingroup\$ The max amperage refers to using the meter in current mode. In voltage mode your meter should present a high impedance (in the tens of megohms), so its probes should not short circuit anything in a car (unless you used the metal tip of a single probe to create a short). Make sure your meter is in voltage mode, not current mode: in current mode, there is a low impedance between the two probes, which is bad news if you connect it to a voltage source. \$\endgroup\$
    – Kaz
    Jun 10, 2014 at 0:51
  • \$\begingroup\$ have a look here \$\endgroup\$
    – Vladimir Cravero
    Jun 10, 2014 at 7:16

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The first thing you should do is read the manual so you will understand the significance of 10 sec Max 10A fused and all the other features of your meter. To save time right now, the usual meaning of such a statement is that the range referred to can handle a maximum of 10 amperes for up to 10 seconds and there is a fuse to prevent damage from currents greater than 10 amperes. Unfortunately for you, this implies that your meter was set to measure current when you wanted to measure voltage. Thus you apparently put close to a dead short across the points that you attached your probes to. That is what caused the spark. Since you were shorting the voltage going to the ECU, you probably did not damage it. Did you check that your car starts and operates properly? If not, you may have blown the fuse that feeds the ECU. In the future, check very carefully the setting on your meter before connecting it to anything. If you want to be very safe, try measuring the voltage on an ordinary alkaline battery to make sure the meter is doing what you want it to. If you reverse the leads what measuring a voltage, the meter will simply display a negative voltage. This will not harm either the meter or the device.

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  • \$\begingroup\$ Hello, Thank you, Car starts and operates okay, I'm just paranoid about some hidden damage, It was actually 10sec Max 20A fused I will read manual for sure. Thank you \$\endgroup\$
    – user45317
    Jun 10, 2014 at 1:07
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Normally a multimeter will have three or four jacks on it. One is for measuring voltage (red), one or two are for measuring current (red), and one is for your ground (black). The ground jack is typically called common. To measure voltage attach your probes to the voltage measurement jack and common. In the picture they would be the two right jacks. To measure current attach your probes to the current measurement jack and common. In the picture the current measurement terminals are the two on the left. One is for measuring larger currents, the other for smaller currents.

Reversing the leads in either setup will not cause any problems, except your measurement will have a sign opposite the one you expected (1 would be -1). In the voltage measurement configuration you typically cannot break anything unless the voltage is too high for the multimeter. If you use the current measurement terminals when you intended to measure voltage you will probably blow the fuse in your multimeter, and possible damage what you were testing.

http://upload.wikimedia.org/wikipedia/commons/thumb/f/f1/Fluke87-V_Multimeter.jpg/640px-Fluke87-V_Multimeter.jpg

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