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Old 22nd July 2010, 06:17 AM   #1
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Question sudden voltage sag

Hi all, so i installed a decent sized system in my car (1000Wrms) a couple months back and it has worked flawless until today. I was on my way home and the sub amp started going into under voltage cut out. I was kind of shocked because this had never done this before without the battery being drained pretty low and i have had it cranked alot higher. At first i had thought i blew the rectifier or possibly the regulator in the alternator and the battery was just getting drained but when i put a meter on the amp input it read 14v. However as soon as i would turn the volume up the voltage would sag down to about 8.5v were it would normally stay up at around 11.5-12v. I have read that this could be caused by battery sulfation and admittantly the battery is about 3 years old and it did get highly discharged for a little while. I would like to know what your opinions are on this as for what could be causing this problem and why.

thanks.
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Old 22nd July 2010, 07:55 AM   #2
grjr is offline grjr  United States
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well check the condition of you electrical power connections and if they are ok then it may be that something is on its way out eg. battery,alternator
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Old 22nd July 2010, 08:00 AM   #3
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Upgrade your grounding.

you probably added an extra positive wire from battery to this amp.

Did you also add extra earth wiring?

Might be that current earth wiring is borderline.
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Old 22nd July 2010, 04:10 PM   #4
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Everything is wired with true 4AWG wire not the crappy stuff that is really not 4AWG. The battery is under the driver seat so it has about a 4-5 foot run. However i did not run a earth wire back to the battery itself. The earth is tied to the chassis. I can kind of see how that may be a problem but why would it happen suddenly?
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Old 22nd July 2010, 04:18 PM   #5
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YEs don't run seperate ground wire. This will crete ground loop. Ground to chassis.

I am not saying this is it. But srsly 99% of all car wiring problems are ground issues.

All i know is i added an amp to my system and all sorts of weird things happened. Since it's under the seat . Use the offcut of the wire you ran from the positive terminal to the amp. And crimp it onto the negative terminal en bolt it down somewhere close onto the body. Worst case scenario it won't solve your problem , but i would start there and then check up on the amp ground wire itself also.

People tend to neglect them. i am hoping your problem is as simple as this
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Old 22nd July 2010, 04:26 PM   #6
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Do you get the same voltage sag if you measure the voltage directly on the battery terminals?
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Old 22nd July 2010, 11:00 PM   #7
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The problem is defiantly bad ground connection. Why this happened so suddenly i am not sure. measuring at the battery i get 14.1v and with the amp just in idle mode the voltage on the input is 13.9v. When i crank it up like usual the voltage drops significantly down to about 8.5v were at the battery the voltage would stay solid at about 13.9v. My plan is so run a 4AWG cable back to the battery but in a fashion that will not create a ground loop (at least to my understanding.) I will still have it tied to the chassis were is currently is but run another cable off that same bolt directly to the battery creating a good low resistance path back to ground.

let me know your thoughts.
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Old 23rd July 2010, 12:34 AM   #8
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Check the various connection points and fuses. Measure the voltage of each connection point by placing one meter probe on each side of the connection and driving the amp until you get the drop in voltage at the amp. If the connection at that point is weak, the meter will read several volts. If the connection is good, the voltage drop will be in the millivolt range.
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Old 23rd July 2010, 12:36 AM   #9
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I dont think running a wire from the bolt to the battery is worth wasting your time.

First thing did you scrape the paint away and make sure you have clean bare metal where the amp is grounded?? If not do so and make sure your connections are tight.

Second thing ive seen alot of lately If you have an inline fuse from the battery and you have a glass fuse in the holder check that also ive seen alot of the ends run hot and break the connection causing voltage drop.

So if you have a fuse holder inline if you have a spare fuse replace it and see what happens first. Also check your ground wire from amp to the chasis.

Running a ground from the truck to your battery is a waste of time and money in my opion unless you are running multiple batteries and high output alternators.
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Old 23rd July 2010, 02:31 AM   #10
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Running a ground from the chassis tie point back to the battery ground terminal is really no big deal at all for me. I have extra 4AWG cable and the amp is under the driver seat so it's only maybe a 3-4' run. Would be pretty easy to do in my case. May be overkill yes but i think it would solve the problem. I tested for a voltage drop across the inline fuse and it was fine so i highly doubt that is the problem.

tangmonster said in an earlier post about this ground loop thing. I am not sure if he meant not to run a wire straight to the battery negative or not to tie it to chassis and still run a wire to the battery negative.
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