Car Stereos: GMC Stereo speakers, internal amplifier, digital multimeter

Question
I bought a 2002 GMC Sierra 1500, with an aftermarket (Pioneer) stereo in it.  Stereo was working great when "suddenly" all the speakers quit working at once - no sound at all - stereo appears to be working fine.  Replaced Pioneer with an original GMC stereo, which was bench-checked good - but still no sound out of speakers, except a crack in the speakers when the wiring harness is intially plugged into stereo.  All dash wiring is original.  Can you provide a fix or troubleshooting path?  Is there a fuse/circuit breaker in the speaker harness?  Everything appears to be working, but no sound.

Answer
Hi Tom,

There certainly won't be any kind of fuse or breaker in the speaker wiring.  A factory amplifier is a possibility, but not likely unless your truck is equipped with the factory Bose audio system.

Usually, when I run into a problem like yours, I suspect a shorted speaker wire, or a defective speaker.  A problem with one speaker can kill the sound to all the speakers, as the head unit's internal amplifier shuts down for protection.

Troubleshooting is easiest with a digital multimeter equipped with a "continuity" or "resistance" function.  The first thing I'll do is test each speaker wire for continuity with chassis ground.  You can do this by setting the meter for "continuity", then connect the black probe to a ground point like a metal dash component, or the outer ring of the cigarette lighter.  Use the red probe to test each speaker wire.  If you find a speaker wire that shows a connection to chassis ground, it's the likely source of your problem.  

If you don't find anything with the chassis ground test, then you should test the resistance through each speaker pair.  Each speaker has a positive and negative wire at the head unit harness; setting the meter for "resistance" and touching one probe to each wire will tell you the resistance through the wire and the speaker.  You should not get a reading below about 4 ohms for each speaker.  If you find a much lower reading, it might mean that you have a speaker with a shorted voice coil; or perhaps the speaker wires have broken in a door-to-body conduit and are touching each other.

If you find a problem on a speaker line, you can temporarily disconnect both wires for that speaker; the head unit should play normally through the three remaining speakers.   Often the easiest way to fix the problem completely is to run new wiring from the head unit to the speaker location; this is quicker and easier than trying to trace the factory wire, looking for the damaged point.  If the problem is a speaker with a shorted voice coil, you'll need to replace the speaker.

If you have after-market speakers installed in your truck, it's common for one of the connections to come off the speaker terminal and touch chassis metal in the door or through the speaker basket.  In this case, the fix is as easy as re-connecting the speaker more securely.

Hope this helps!

Brian