Periodically people post looking for troubleshooting help.
Recently I lost all sound from all my speakers except the sub,
so I thought I'd document here what I did to troubleshoot, for
those whose knowledge of electronics is limited (to the old pros
here this is very basic).
so...
I had parked the car with everything working, and an hour later
the main speakers didn't work, but the sub did. I figured I'd focus
on the common point, the amp that powers all but the sub. I was
a little worried because the amp failed while off and parked,
rather than on (maybe a blown fuse?) or driving (bump loosened
something up?). My hope was it was something simple like a
loose remote turn on wire.
Inspecting the amp found no loose wires.
Turning on the headunit, I noted that the amp "safe" LED was lit,
indicating that it was getting power, was trying to turn on, but was
detecting a fault such as a short in the speaker wire.
I used the ohm setting on my multimeter to check the resistance of
each pair of speaker wires at the amp end, and they all came out
around 4 ohms, so no shorts there (a short would produce a reading
near 0 ohms, while a broken/disconnected wire would show infinite
ohms).
I then switched to DC volts to check the input voltage at the amp,
to the amp's ground point.
hmmm, less than 1 volt, which would certainly generate an
undervoltage fault! Is it a problem with the ground? No, same
result no matter what ground I test (+ probe touching the amp's +
input, - probe touching various ground points).
Pop open the distribution block, inspect the fuses (they look fine),
and check the voltage into the block - 12v. then check the voltage
on the other side of the fuses - 12v to the sub amp, 0.5v to the main
amp. Pull out the main amp fuse, check its resistance: infinite.
It's amazing, the fuse looks indistinguishable from new, but clearly
failed internally, initially passing some current but after a little
handling passing none.
Put in a new fuse, good as new. It only took a few minutes to find
the fault, by carefully testing one thing at a time, including a part
that *looked* like it couldn't possibly have a problem.

 

Good job, glass agu fuses have a tendency to do that.

 

Thanks. Mine is still having issues, but your post helped me get more info.