Impossible problem?

I've got a customer vehicle (96 grand-am)  that came in today for a troubleshoot for no audio from subwoofers. So i checked the subs...they're OK. I benchtested the amp... it's OK  then i hooked up a CD walkman as an alternative signal source and Ta-da! the subs fire up. Ok, so its RCA's forward.... RCA's test ok, so i benchtest the head unit and the pre-outs work fine....  He had tried to install the unit himself and he blew out the car's 12+ constant (seen it a million times before...) so his "buddy" t-tapped onto the ignition column wires to achieve a 12 +, a switched 12, and a ground (to the dash subframe) which worked, but i redid all of this to alleviate any possibility that poor connections could cause such a bizarre problem, but Lo! no fix.  SOOOO.... how can a good head unit, good rca's, a good amp, and good subs produce no output? (especially when the subs work when hooked to an external signal?!?)   .............Anybody?.............Mitchell Whitney, Installer/technician

Mainstreet A/V

remote wire? which deck outputs did you test? are the testing circumstances the same for both units?Someone pass me that bottle.

its not the remote. the amp turns on, it just acts as though its not getting a signal from the headunit. i tried two different headunits in the vehicle (both his. one sony and one JVC). on the bench ALL preouts ( on both units ) test fine. and with the radio on in the vehicle, if you unhook the Headunit from the amp and replace it witch an external source (such as a CD walkman) the amp and subs operate fine. its the damn weirdest thing i've seen in 8 years.Mitchell Whitney, Installer/technician

Mainstreet A/V

maybe the the rca's aren't passing enough current.  Did you try new rca's from the deck to the amp?? sounds like the only thing you haven't tryed.   Low level rca signals are so low in current its hard to determine if rca's are good by useing a walkman like you said.   Try that and  hope for the best.

Already did it. with the head wired in but removed from the dash i just hooked up a new set of RCA's between the head and the amp with the same result. Messed up, huh?Mitchell Whitney, Installer/technician

Mainstreet A/V

Nope. Eliminated that chance earlier. This is actually day 2 of this ordeal. On  Tuesday he came in and said his subs "werent hitting as hard as they used to".  he had installed the whole system himself and it showed.  First up, he had 4 holes (not ports, just holes) on the enclosure....Anyway, The amp was lit but i could see that the ground wire was just wrapped and TAPED to a chasis bolt and sure enough a gentle tug and away it came. Also he had 4ga up to within 4 inches of the amp and then a distribution block with 8ga the rest of the way to the amp. Circuit City told him to do that one....  So after redoing the ground and putting a proper terminal on the 4ga primary, and also putting a female disconnect on the remote wire (its a Rockford Fosgate with the male spade for turn on, which he had wrapped the wire through...), then adjusting the gain (it was completely minimized which is a first) everything was Operational and away he went. So today begins day 2. He came in and said that the setup worked all day Tuesday, but today when he started his car he said " the subs just wouldnt work". and then from there is where this post topic starts. Anybody got ideas? xx(Mitchell Whitney, Installer/technician

Mainstreet A/V

I'm definately not an expert, but the only thing I can think of is maybe the HU has a Sub control in the menus that may be turned off, or just very very low.

nope. tried all outputs, checked all menus, plus they work fine on the bench.  Good thought, though.Mitchell Whitney, Installer/technician

Mainstreet A/V

put in a test tone cd and check rca voltage at the deck, then the rca ends at the amp. Or run an external set of rca's to the amp from the head unit. Its all about basics, if you are thorough, you will find the problem.Want to know about Audi, I thought you did.