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I am currently installing a dash cam (Thinkware F770) on my car (2017 Ford Mustang EcoBoost). The camera comes with an optional hardwiring kit so it will stay on even if the car has been parked and the ignition is off.

The kit has three cables I need to connect. B+ (gets power no matter if ignition is on or off), ACC (gets power only if ignition is on) and ground. Those cables have to be connected to suitable fuses. I chose the B+ cable to the "Demand Light" (dome light) circuit fuse and the ACC cable to the "power window" circuit fuse.

In order to check which fuse has power wether ignition is on and off I used my multi meter. Ignition was off. I connected the black multi meter cable to a metal part on the car and touched the "Demand Lights" circuit fuse (10A). My multi meter was set to 12V DC. When I touched the fuse with the red cable, it threw lots of sparks that I feel was unusual for 12 volts. But the multi meter showed 11.x volts. After a while and with lots of sparks when touching the fuse with the red cable, it did not show anything at all anymore.

I checked the fuse and it was now blown. The dome light ("demand light") was not working anymore and worked again after I replaced the fuse. Is there anything in the process I did wrong? Why would a fuse blow by using a multi meter?

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  • explain again the part where there were sparks flying and you kept poking at it? Your meter is either broken or not set up with the leads in the correct ports for voltage measurement. – agentp Oct 6 '17 at 0:44
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You did a very common mistake. Your mistake was that you short circuited the wire using your multimeter. The 10A port has a very low resistance shunt from the 10A port to the COM port. It's essentially a short circuit that can be used to measure strong currents.

The low resistance shunt is wired directly between the ports and it's not disabled by the multimeter setting wheel. So no matter which mode the multimeter is in, it's always a short circuit.

When working with a car to measure if there is any voltage in some point, connect the COM port to the car chassis and the V/mA/Ohm port to the pins whose voltage you want to measure. Don't use the mA settings, measure only volts.

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    if this is the case, how would the meter have "showed 11.x volts" ? – dandavis Oct 5 '17 at 6:41
  • Good point - my meters don't show volts when using the current connections... – Solar Mike Oct 5 '17 at 8:01
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    it showed 11.x volts because 1. the first time u touched on the opposite side of fuse, thus there was short circuit but no current was flowing through the fuse. or 2. as it is a 10amps fuse it might needed to heat up a bit before it could blow – Nilabja Oct 6 '17 at 10:46
  • All you comments make good sense, since I pretty much did trial and error. I think what happened is that I switched to the 10A port in between because it was not reading anything, which probably was for a different reason. I tested everything again with the V/mA/Ohm port and now it works. Thanks! – mitchkman Oct 6 '17 at 14:11
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The other possibility is that with the + probe you touched earth or ground at the same time as the live terminal or wire - sometimes making a small probe extension is good. I have used a paper clip before...

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