Audio Systems: Car Subwoofer Troubles, car stereo system, little pioneer

Question
Hey i got a quick question and hopefully you can give me an answer. recently my car stereo system has been goin on the fritz. i have 2 12" MTX 7500's in my trunk with a thunder 5601 amp and i've had them in my car for about 3 months, i bought them off a friend. they never gave me problems until about 2 weeks ago when i got in my car nd the subs didnt turn on. ever since then my subs only come on when they feel like it. Originally i thought i overheated the amp but now it seems like when its hot out they'll turn on. its weird because there is still power running to my amp but just not to the subs. all the lights on my amp are on and i've checked every fuse. i took it to a custom audio shop nd they thought it was a bad ground so they moved my ground nd after that my subs worked fine... for about 5 hours, then it started happening again. ive tried wigglin wires nd even taking wires out nd then putting them back in but nothing seems to work nd to keep the on. normally if i play around with my gain dial on the amp the subs will turn on but its a hassle to keep doing that, nd it doesn't always work. the guy at the custom audio shop also thought it may have something to do with my radio deck. i have a little pioneer deck... after reading all that what do you think it could be?

Answer
Isn't that annoying when you have a phantom problem like that?  

Here is my thought.

First we must determine if the problem is internal to the amp or external to it.  By internal I mean something wrong inside the amp.  Of course you can replace the amp with another one just like it and see if that solves the problem but that is not always an easy or viable solution. Or, we can check the voltages feeding the amp by monitoring a voltmeter connected directly to the 12v terminals on the amplifier and when the subs cut off see if the power line goes off (or reduces to some degree.)   Or we can take a separate known good speaker and connect it to the amp output (with the subs disconnected) and see if it is working properly.  In other words, we can do a few things that will confirm the internal workings of the amp is okey and that it is not suffering from some kind of intermittent condition.  (Power supply components are famous for doing such things, for example.)

By external, I mean things that can go wrong when there is absolutely nothing defective about the amplifier itself.  This means either on the power or signal input side of the amp OR on the speaker output side of the amp.   Firs thing to check - as above - is the power line to make sure it is always getting power and that the alternator, battery, bad fuse or something is not arbitrarily dropping voltage to the amp.  Even though you said the lights are on it is possible that a reduction in voltage for along enough period will execute the power manager, shut down the amplifier but the voltages are high enough to keep the lamps working.   But, the most common cause would be (other than a bad ground which seems to ground - pardon the pun - you have gone over) bad speaker wires or a bad speaker voice coil.  Speaker wires can be shorted down some where along the way that will cause the overload protector to cut in and keep it off for a while.  Shorts to ground and shorts between the wires ( a strand of the twisted pair touching or some such thing).  The best way to check that is to run another set of wires temporarily and see what happens.  If you have a VOM you can measure the dc resistance of the subs and see if one of the coils has a shorted condition - which sometimes is music/bass sensitive.  The dc resistance of a voice coil should be just a little lower than the impedance rating of the speaker driver. For example, if the impedance is rated at 4 ohms you might find the resistance to be around 3 to 3.8 ohms, depending on the makers design.  But, if you measured 1 or 2 ohms or less that could be the problem.

Finally, if the subs are not impedance matched to the amplifier it is possible that over time the mismatch has placed undue stress on the amp and it is failing simply because the overloading of the output has made it sensitive to music and temperature changes.  This condition will eventually cause the amp to fail.  The impedance of the load on the amp should not be lower than the rating of the amp.  

Hope these ideas help.  If you need any clarification or other ideas please let me know. Hope you get to the bottom of it.

C