Battery Voltage on Ground Port of Amp

Phatboiondiet

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Hopefully I’m posting in the right spot been awhile since I’ve been here. I’m installing a system in my 2008 S550 Benz. I’m using a Lc2i Pro to planet audio bb2400.1 amp. I have a good ground and fused battery connection. I test my ground with a power probe it shows ground and I test my battery cable and also shows good 12+ volts. When I connect ground to amp, the ground reads as ground. When I connect the power(battery) cable to the amp the ground becomes a positive 7volts. I’ve tested my setup with another amp and same thing happened. So I put back my planet audio amp and it worked I was bumping but then the sub went boom boom and green light on amp was on but blue leds were not. I then supplied ground from the power probe to ground port with my ground wire on amp and everything worked and sub played again. I released ground with the power probe sub made sound and did a boom boom again and amp “ON” light was green and no blue leds again. I have no idea why battery voltage is showing on the amp ground side only when I connect the battery cable. But then I supply ground through power probe to my amp with the ground connected and it works like nothings wrong.
 
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Phatboiondiet

CarAudio.com Newbie
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My guess is bad connection at that amp board. Try pressing on that ground terminal with something other than DMM probe - ie screwdriver.
[/QUOTE

I tried another known good amp and same thing happened. When I supply ground from the power probe tool it functions normal.
 
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Phatboiondiet

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The ground spot is reading good ground with the probe. As soon as I connect the battery cable to the amp the ground shows 7volts. I’m probing the screws to hold the wires on the amp
And with everything connected if I supply ground to the screw holding the ground wire into the amp everything works. I even changed ground location but for some reason I’m still getting voltage on my ground. Makes no sense to me
 

1aespinoza

Junior Member
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And with everything connected if I supply ground to the screw holding the ground wire into the amp everything works. I even changed ground location but for some reason I’m still getting voltage on my ground. Makes no sense to me
It sounds as if your grounding locations are weak. It may show continuity on the multimeter but that does not guarantee ampacity. With the probes on the amp's power terminals, you should read 12+ volts with the amp on. You added a ground to the existing ground and the amp worked. That is why I suspect a weak ground.
 
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Water Bear

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Atlanta, GA
If you have a long run of 10 ga or 8 ga wire, you could do a temporary run from the amp ground to your battery negative outside the car. Hell you could even do that with speaker wire. Then see if you still read +7 after hooking up the positive. Obviously don't run your amp on music with it wired that way, but it would rule out a bad ground pretty quick (or confirm it).

Edit: The big concern here is some kind of short in the amp. It seems unusual to me that the ground should read anything either way after all you've done is plug in the positive.
 
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Phatboiondiet

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It sounds as if your grounding locations are weak. It may show continuity on the multimeter but that does not guarantee ampacity. With the probes on the amp's power terminals, you should read 12+ volts with the amp on. You added a ground to the existing ground and the amp worked. That is why I suspect a weak ground.

EXACTLY!!! I added ground to the existing ground and it worked. I suspected amp being the issue but I even tried a known good amp and same thing happening. I guess I’ll try finding another ground location again
 
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Phatboiondiet

CarAudio.com Newbie
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If you have a long run of 10 ga or 8 ga wire, you could do a temporary run from the amp ground to your battery negative outside the car. Hell you could even do that with speaker wire. Then see if you still read +7 after hooking up the positive. Obviously don't run your amp on music with it wired that way, but it would rule out a bad ground pretty quick (or confirm it).

Edit: The big concern here is some kind of short in the amp. It seems unusual to me that the ground should read anything either way after all you've done is plug in the positive.

i even tried another known good amp and same thing happened. Luckily my battery is in the trunk. So what you’re saying is remove existing ground and try ground directly from battery?
 
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Water Bear

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i even tried another known good amp and same thing happened. Luckily my battery is in the trunk. So what you’re saying is remove existing ground and try ground directly from battery?

Yep, right on. Your trunk battery is in parallel with your under hood battery I assume - positive to positive, negative to negative. If so, grounding on the battery negative should work. If you see a spark when you connect to the negative that probably means your battery negative and original ground were not at the same voltage.
 
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Phatboiondiet

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Yep, right on. Your trunk battery is in parallel with your under hood battery I assume - positive to positive, negative to negative. If so, grounding on the battery negative should work. If you see a spark when you connect to the negative that probably means your battery negative and original ground were not at the same voltage.

I do get a big spark on my negative when I connect the battery cable to the amp. As for the battery I didn’t originally put it in the trunk it’s just how the car comes. I do have have a second battery under the hood but I believe that’s for the starter. So I’ll run the my negative directly to the battery and see what happens. I appreciate the help from all of you
 

metalheadjoe

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The reason you have voltage on the ground terminal with the cable disconnected could be because it's a series circuit: Power goes from positive to amp to negative (according to conventional theory anyway). The difference in potential is what makes electricity flow. If you don't have a ground connected, power makes its way through the amp to the ground terminal but does not have anywhere to go from there, so the potential is between the amp's ground terminal and battery negative.

I've never tested this. I'm merely theorizing.
 
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Phatboiondiet

CarAudio.com Newbie
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The reason you have voltage on the ground terminal with the cable disconnected could be because it's a series circuit: Power goes from positive to amp to negative (according to conventional theory anyway). The difference in potential is what makes electricity flow. If you don't have a ground connected, power makes its way through the amp to the ground terminal but does not have anywhere to go from there, so the potential is between the amp's ground terminal and battery negative.

I've never tested this. I'm merely theorizing.

Thanks for the reply! I went ahead and did what Water Bear recommended just now and the amp powers on and ground is ground showing -63V and my power probe lights green! Amp is on and all leds!! Super happy but my question now is could I run my negative from the battery permanently?
 

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