w126 ground loop noise through speakers
 

i am installing a new stereo in a 1991 350sdl. it originally had the "premium" sound with 2 amps behind the rear seat. the new equipment is: mb quart 5ch amp, kenwood head unit, polk db speakers for rear deck and doors, mb quart speakers for dash, polk 8in subwoofer, and rockford fosgate 4awg wiring and rca cables. i extended the speaker wiring from each original amp harness to the trunk where the amp is. everything sounded great until i shorted one or both of the speaker terminals to the aluminum dash board while installing the driver's side front speaker. :( it mad a pop noise and now ALL electrical noise comes over the speakers. the clock, the seatbelt light flashing on the overhead dome light, very loud when you turn on a larger load like the hazards or turning the key to "run", and of course alternator whine when running. all speakers sound perfect though when you play music. in my troubleshooting attempts, i have tried:
-using 2 other known good amps
-using 2 other known good head units
-wiring head unit directly to battery using no original wiring
-cleaned amp ground at rear seat belt bolt again
-cleaned chasis grounds in battery area
-cleaned all positive wires near battery
-checked and cleaned all fuses

the only thing that seemed to help was making a cable go from the stereo's body to the outer part of the rca plugs. if i unplug the rca's from the stereo, all electrical noise goes away and i can plug them right into my phone and listen without any issues or noises. any ideas what could cause this??? everything was fine until that speaker grounded out. i'm guessing something on the car is causing all this because i have tried using all new equipment. i really need to figure this out because the stereo is pretty much unusable now. :(

no ideas? :(

What happens when you ground the radio to negative or the chassis? Have you swapped the RCA cables.

Ground the stereo's body? I think I tried that and it popped but don't remember it helping. Rca's are new but will test with others in case I fried them. What about the capacitor on the back of the alternator? Maybe? It's so wired everything was perfect until the speaker grounded out.

The cap on the alt failing could cause the noise as well. If you shorted the amp output it could have put out a hard load and killed that cap. Longshot though.

try here.

Car Alarm, Car Stereo, Mobile Video, and Cruise Control Info for Installers

i'm going to try another one i have. what's weird is if i unplug the rca cables from the head unit, the noises completely go away. i can even plug them into my phone through an adapter and listen to music still without noises so the problem must my at the front of the car? i am going to try some different rca cables but i don't see that helping. i also don't understand why grounding the rca cable's shield makes most of this noise go away. does that help determine anything? i know this isn't "just the way it is" because it was perfect before. so strange!

The fact that you heard a pop when the grounding incident happened is what worries me. While I never recommend working on anything electrical when hot, usually what happens with a speaker wire shorts to ground is that the associated amp goes into thermal protection and shuts down until it cools off. You'll get no sound out of that channel for a bit, but usually nothing permanent happens.

I'm likewise skeptical that the RCAs are the cause here, but it wouldn't hurt to definitively eliminate them as a suspect.

If I had to guess, based on the fact that connecting the HU to the RCA shield eliminates the problem, I'd suspect that something in the headunit's ground path is munched, either within the headunit itself, or the actual ground wire in the HU harness. Since you've tried wiring the HU direct to the battery, and you're tried replacing the HU, I might see what happens when you do both, just on the off chance you have a compound failure.

Well, the pop was through the speakers, not a capacitor blowing up. I know it was a big mistake working on this while on, I didn't think of the dash being aluminum underneath and the chance of touching anything. It only stayed touched for a second. I immediately pulled it back out and the amp never went into protection mode which is odd. And I've tried multiple head units and amps with different power cable configurations with the same issue so I don't think the audio equipment is the problem unless like you said the rca cables went south. It's like something on the car is dumping all this noise in and the head unit is picking it up? But all audio equipment have filters on power inputs.
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ok, so i have installed 3 PAC brand ground loop isolators (front, rear, sub) and grounded them to the head unit's frame. it took care of 98% of the noises! replacing the alternator capacitor and testing another set of nice rca cables laying over the seats did nothing. this whole thing really bothers me though because i know it was working perfectly and these isolators are a band aid. i would love to solve the issue and remove the isolators at some time. any more thoughts would be much appreciated but i'm glad this sounds good now!

glad you made so much progress, that sounds really frustrating.

would an optical audio link would avoid the ground loop entirely?

i never thought of that! didn't know they were out there for car audio. you'd only need 1 for a system with amps for all channels i assume because for home audio is supports at least 6 channels. there is still a bit of alternator whine and i have turned the amp gain down as much as possible.

there is stereo optical audio, i've never seen it in a car but anything is possible. maybe just this little guy?
Electronic Goldmine - Audio Opto Coupler

i'm not sure what the "whine" is, but if it's continual slight voltage variations, you can try a capacitor in parallel with the load to smooth them out. i guess the size needed would depend on the size of the variations, and that you would want to measure with an oscilloscope. nonetheless just installing one of a largeish size could possibly help.