Blown fuses with a multimeter, WTH???
 

I was putting my instrument cluster back into my truck after installing a Rocketman's Volt meter and my temp gauge with added LEDs.

The volt meter needs a key on power source so it doesn't run down the battery. I figured the old radio lead would be perfect for this application. So I grabbed my multimeter and hooked my common lead to where I intended to run the ground and grabbed the radio connector to find the hot wire. I hit all the pins with key off and verified there were no always hot wires in the connector then turned the key over to run and ran it around the connector again. I found the pin that was hot this time, just under 12v, and traced back the lead to a good piont to clip it. So far so good right?

So I clipped the lead and put a bullet connector on it and went to verify the circuit was still correct and NOTHING. WTH? :-huh I couldn't get a reading anywhere I could think of along the wires up to the connector I just put on. I moved the ground around to check that but still got nothing.

I decided to see if I could get a reading somewhere so put my probe on the positive connection of the cigar lighter and it read the same voltage that originally got on the radio lead. OK, so now I know not completely crazy, maybe just a bit stupid.

Now that I know that I still have current in the cab I decide to see if my original ground point is still good. Nothing at the radio lead again so I go back to the cigar lighter, NOTHING! WTF!!!! :-arrgh

So I dinked around a bit more then it occurred to me to check if I was getting anything at the fuse box. I hit the one side of the radio fuse and I got the same reading I got when I started then went to the other side and it was dead. I popped the fuse out and sure enough it was blown. I went back and checked the cigar lighter and again it was dead. I then went back and checked the fuse for the lighter and got the same result. Of course I didn't have any spare fuses but I was able to verify that I had a good ground so I hooked up everything as I had planned and just set the cluster into the dash just in case I had to pull it again.

I took the truck down to Autozone and they either don't have the right size or right rating of fuse I need. I then go across the street to the Parts Plus affiliate and he gives me pretty much all he has for each "since they are hard to find these days". I went out the truck and popped the fuses in and everything works as expected. I told the guy what happened and he was at a loss to explain why a multimeter would blow a fuse.

Anyone have any ideas why a multimeter would blow a fuse? I can't for life of me figure it out. The radio fuse is a 7.5 amp and the lighter fuse is a 15 amp and it blew both of them. Could it be that they were just old and tired or is there something else I should be worried about?

Only way I could see blowing a fuse with a multimeter is if your checking 12v leads with the continuity setting on the meter. Ive never had that issue before, but thats the only way I could see that happening.

I was actually using voltage setting.

Yeah thats weird that it would blow fuses if your using the correct setting. Are you running through the settings on the meter while your leads are clipped to your ground and 12v your testing? Other than that Im not sure what else would cause it.

I had that issue once, that's why I always disconect the battery cable whenever I do electrical work.
I never was sure why it happened. I figured that either I created some kind of voltage spike or else accidently grounded the hot wire without realizing it. Either way, lesson learned. I disconect the battery cable.
fmc400 can probably explain why.

If you disconnect the battery you can't get a voltage reading. You could check continuity, which is probably the important part but why not check voltage while you're at it?

Not that I'm aware of. I just clipped a test lead to the common and used the probe for hot.

A voltmeter is very high impedance and draws negligible current. If connecting a DMM blows a fuse, then the DMM is accidently being used as an ammeter (very low impedance). You mentioned that you had it set to volts, but were the leads in the right place on the meter? Most meters require you to change lead positions between amps and volts. If you're confident that the DMM is configured properly and it's blowing fuses, then the meter is faulty.

The only other explanation is that you shorted the signals you're reading to ground with the probes of your meter while taking the measurement. This is very easy to do. Or, you let an exposed connector touch sheetmetal. It doesn't take much to pop a fuse back there with all of that grounded metal. I would put my money on one of those two.

Again, it's possible but I'm not aware of doing anything like that. I don't think I'll let that particular meter near my truck again and just use it on my bench. I'll pick up a better one to work on it in the future.

I found a Vichy VC 97 digital auto ranging multimeter on ebay. I did some research and it's supposed to be as close as you can get to a Fluke for under $100. The suggested retail is about $45 but it's listed by a bunch of sellers in the $25-$35 range.