Blown fuse puts circuit on ground?

 I installed a 12V plug last night and it took a lot longer than it
should have.  I blew a fuse near the start of the job (by leaving my
voltmeter set to resistance when I wanted to measure voltage) but didn't
notice, and once it occurred to me, thought what I was seeing ruled that
out.

With fuse in replaced and ignition on, it is now 0V from positive
battery terminal to the lead, and 12V between the lead and ground.  With
ignition off, 0V between lead and ground.  All just what you would expect.

But with the blown fuse, it was 12V between the positive battery
terminal and the lead, and 0V between the lead and ground, with or
without the ignition on.  This was with the other end of the lead
disconnected from everything (heated grips in this case).  I kept
looking for a short but never found it.  Finding and replacing the blown
fuse appears to have fixed it.

What gives?  It's as if the blown fuse somehow connected the lead to ground.

 
Sounds like it's just telling you it's not seeing voltage, same
as it would read if it were not connected to anything at all.

Check for continuity or ohms and I'd bet you see infinity.

 Rob Kleinschmidt wrote:

No, that's just it.  With the fuse blown, I would have expected to see
0V between the positive battery terminal and the lead, as if it were
connected to nothing.  Instead I saw 12V, as if the lead were connected
to ground.


Are there any special considerations checking continuity between
different voltages?  If I measured resistance on a pair of battery
terminals, would that a) measure internal resistance of the battery; b)
give a nonsense result, since the multimeter applies a small voltage to
measure resistance but the battery already has a much larger voltage; or
c) effectively short-circuit the battery destroying both it and my
multimeter :)

 


And that IS the point.  It IS still connected to ground......THROUGH some
device.....light bulb, fan motor, fuel pump, circuit board, etc. that was
actively carrying current (or available to) up until the time the fuse blew.
Something is still connected to that lead, regardless of what you may think.

And no, Rob, it's not a joke.  It is pretty basic electronics.

 

Missed that part earlier. So try two things:

1) Pull the good fuse and measure voltage from +12 to the terminal.

2) Put the blown fuse back in and repeat.

I think you should see 0v in both cases.

Could be an intermittant ground or an activated
relay I suppose but the fact that it appeared and
then disappeared is pretty amazing.