I've posted about this before in rec.audio.car almost a year ago. I was reminded of this today because I just got a letter from the dealership I had my car repaired at last year demanding their $2000. For what? An interior light control module, or something. It was supposed to be under warranty, but they're not covering it because they've determined that the audio system was at fault. Screw magnusson-moss.
So basically they spent over a month trying to figure out why my interior lights were flickering on and off after you shut off the car. They couldn't figure it out, so they assumed the audio system was at fault. They dropped it off at a car audio shop in Massachusetts (one I had dealt with in the past, actually, so I knew how clueless they were). The shop determined the audio system was "drawing 9 amps even when it was off". Nevermind the fact that they claimed it STILL drew 9 amps even when they unhooked the 4 ga. power wire to it. Of course, when I got the car back I checked it myself and found that they were lying through their teeth. And not very convincingly either.
So this is a $1600 bill already. Just "diagnostics." No big deal, they told me. They'd put it through the warranty. Finally, they bring it to a real repair shop, and they had it diagnosed and repaired in 6 hours - $400. Much more reasonable. But altogether, they had my car for two months and now they're trying to stick me with a $2000 bill. [Turns out they forgot to send the $50 owner transfer fee on the warranty they sold me when I bought the car, but illegally tried to pass it through anyway by saying the repair work was to be performed for the previous owner.]
Anyway, I've paraphrased the conversation I had with the audio shop below. The wild claims this guy made were pretty funny. But more importantly, I think it serves to emphasize that when you're faced with a car audio question it's best to ask people like npdang, chad, or whoever else that has a solid working knowledge of the electronics and physics behind these things, and practical experience diagnosing and fixing equipment, and NOT your average brick and mortar shop owner. That's not to say all owners are ignorant. That's certainly not the case. But be careful!
Here are some assertions made by the "professional" I spoke to over the phone:
Professional: The audio system was showing a 9A draw even when the car was off.
Me: But wasn't the 4ga. wire that was supplying it disconnected?
Professional: Yeah, but it can get its power from other sources.
You see, the way I wired it, only the 4ga wire supplied power to the entire audio system. No remote wires. No ignition wires. The car PC was turned on by a simple push button switch - the power to the car PC was from the 4ga. wire. The car PC then turned on the amplifiers through their remote inputs via the USB output and a couple of relays.
Me: But isn't it all supplied by that single 4 ga. wire?
Professional: Actually, there were two 4 ga. wires hooked up to it. And there are other sources powering the equipment anyway.
Two lies in a row. No, there weren't two 4ga. wires. There was only one. And no, there were no
Professional: When the car was dropped off to me, the car PC was on.
Not only was the 4ga. cable not attached, but the power plug was pulled from the car PC and the distribution block.
Professional: The amplifier was generating feedback in the electrical system. I know this because I disconnected the positive wire to the amplifier and left the ground wire attached and it was still delivering large amounts of current through the ground.
Me: But don't you need the positive wire to 'close the loop'?
He rewrote Kirchoff's Law in one fell swoop. Way to go.
Professional: Today's Cadillacs aren't designed to handle big 4 ga. wires. They create so much RF that it's like having a broadcast antenna in your car. It damages the sensitive electronics.
Ah, a transmitting power wire. I love it. Interestingly, the sensor that went bad was nowhere near the "broadcast antenna" that I call a wire. In fact, nothing was - not even the stock 4ga. wire running from the alternator.
Professional: Every time I would unhook a device from the distribution block, the current draw would decrease. It's my opinion that all of your amps and processors went bad at the same time and caused the car to malfunction.
Zebras or horses?