# amp voltage drop when car is on



## 14alpinestang (Jan 14, 2014)

hey guys, new to the forum and i just finished installing my system in my 14 Mustang GT. The problem im having is that when i check the voltage when the car is turned off i get normal 12-14 volts to the amps (they arent turned on) but when i turn the car on and check the voltage im getting 2.4 volts to both the amps. i checked and double checked my wiring. The ground for the HU seems fine and the ground for the amps seems fine as well (could be wrong). was wondering if anyone had any ideas of what it could be. The fuses are fine on the power wires as well. Any ideas or thought would be appreciated. Thanks.


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## ATOMICTECH62 (Jan 24, 2009)

That cant be right.The engine would not run with 2.4 volts.Check for 12 volts between the remote wire and ground.If there is 12-14 volts then its the power wire.If not,its the ground.


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## 14alpinestang (Jan 14, 2014)

thats what the amps are reading when i turn the car on.


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## spl152db (Dec 20, 2007)

your power wire is bad. loose connection at battery, fuse or distrobution terminal. check the voltage at EVERY "break" in the power wire. most likely loose fuse.


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## Chaos (Oct 27, 2005)

^Yeah. If the amps are "on" but the voltage is that low, then you almost certainly have at least one bad connection. Probably the chassis ground to the amp.


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## minbari (Mar 3, 2011)

There is no way the amplifiers are on with only 2.4 volts. most amplifiers wont even come on with less than 9 volts.


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## 14alpinestang (Jan 14, 2014)

minbari said:


> There is no way the amplifiers are on with only 2.4 volts. most amplifiers wont even come on with less than 9 volts.


they arent coming on. thats the issue here.


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## minbari (Mar 3, 2011)

ok, then as others have said, you either have a bad ground connection/wire or you have a bad power wire/connection/fuse.

with a resistance in the wire/connection/fuse/etc of only 2 ohms and you will drop 10 volts.(assuming an idle current about 5 amps) That would only leave 2-3volts for the device.


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## humandrummachine (May 17, 2010)

What kind of amps?


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## MetricMuscle (Sep 16, 2013)

Where are you sticking the multimeter probes?
This may sound crazy but technically you can't check battery voltage just between + and - terminals, you have to check across a load. Stick the probes on the + and - lugs going into the amp.
The reason we are able to read battery voltage by sticking the probes on the + and - terminals is because there is slight resistance or load within the battery itself when it is not connected to any other load.


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## Jepalan (Jun 27, 2013)

MetricMuscle said:


> This may sound crazy but technically you can't check battery voltage just between + and - terminals, you have to check across a load. Stick the probes on the + and - lugs going into the amp.
> The reason we are able to read battery voltage by sticking the probes on the + and - terminals is because there is slight resistance or load within the battery itself when it is not connected to any other load.


This makes no sense to me at all, and even less sense relative to the OPs problem. Care to elaborate?

I am guessing it is much more likely that OP is: 
1) using the wrong setting on his DMM, or has the probes plugged into in the wrong jacks of the DMM
2) has a bad fuse or power/ground connection as already discussed.


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## MetricMuscle (Sep 16, 2013)

Maybe I'm figuring this wrong.
Have you tried different spots for the - probe on the multimeter?

Amp off there is no connection from + to - thru the power supply. Should read ~12VDC.

Amp on you may just be reading the voltage drop across the power supply. Which should actually be ~12VDC.

I agree, if the amp is not coming on then you probably have other issues.


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## MetricMuscle (Sep 16, 2013)

Jepalan said:


> This makes no sense to me at all, and even less sense relative to the OPs problem. Care to elaborate?
> 
> I am guessing it is much more likely that OP is:
> 1) using the wrong setting on his DMM, or has the probes plugged into in the wrong jacks of the DMM
> 2) has a bad fuse or power/ground connection as already discussed.


How is he reading 12VDC with the key off then?

A weak ground could be the issue, I do agree with that but the other options would not read correctly even with the amp off.

Electricity was so much easier to understand before I took so many classes for it.


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## humandrummachine (May 17, 2010)

MetricMuscle said:


> Where are you sticking the multimeter probes?
> This may sound crazy but technically you can't check battery voltage just between + and - terminals, you have to check across a load. Stick the probes on the + and - lugs going into the amp.
> The reason we are able to read battery voltage by sticking the probes on the + and - terminals is because there is slight resistance or load within the battery itself when it is not connected to any other load.


The meter has a lot of resistance in voltage mode, that's why you can check it across the battery terminals. Someone correct me if I'm wrong


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## spl152db (Dec 20, 2007)

humandrummachine said:


> The meter has a lot of resistance in voltage mode, that's why you can check it across the battery terminals. Someone correct me if I'm wrong


you're wrong.


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## minbari (Mar 3, 2011)

humandrummachine said:


> The meter has a lot of resistance in voltage mode, that's why you can check it across the battery terminals. Someone correct me if I'm wrong


second that, your are wrong. most meters have at least 1meg ohm resistance, some are in the gig ohm. they present no resistance in the circuit what so ever.

the reason you should measure voltage at the amplifier and not the batter is because of the resistance in the power and ground wire. If you measure it at the battery you will be measuring the battery. measuring it at the amplifier also measures the voltage drop in the power wire. That voltage drop is caused by the resistance in the wire when current flows in the wire. This is why people always say to make sure you are measuring voltage drop with the system nearly maxed out. You could have 100 ohms in the wire, but if you have no current flowing, you will measure battery voltage with no loss at all. draw even 1/2 an amp and you will drop nearly all the voltage in the wire.


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## humandrummachine (May 17, 2010)

Alright my bad if I'm still wrong, but can't you still check the battery voltage drop across the battery terminals with a voltmeter connected to them in parallel? That's what I was trying to say.

I understand what you mean about the amplifier voltage drop. Good call on that.


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## spl152db (Dec 20, 2007)

humandrummachine said:


> Alright my bad if I'm still wrong, but can't you still check the battery voltage drop across the battery terminals with a voltmeter connected to them in parallel? That's what I was trying to say.
> 
> I understand what you mean about the amplifier voltage drop. Good call on that.


no. you're still measuring the battery.


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## humandrummachine (May 17, 2010)

But if you are trying to measure the battery itself, that works right?


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## spl152db (Dec 20, 2007)

humandrummachine said:


> But if you are trying to measure the battery itself, that works right?


yes.


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## minbari (Mar 3, 2011)

humandrummachine said:


> But if you are trying to measure the battery itself, that works right?


yes, but you will only measure systemic voltage losses and not voltage loss at the amplifier.


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## Edward Conley (Mar 27, 2014)

I do not understand rightly that where are you getting low voltage ? You don't read DC voltage at the speakers or the amp speaker terminals, audio voltage is AC..


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## spl152db (Dec 20, 2007)

Edward Conley said:


> I do not understand rightly that where are you getting low voltage ? You don't read DC voltage at the speakers or the amp speaker terminals, audio voltage is AC..


comprehend your thoughts no?


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## liask3606 (3 mo ago)

Did you ever figure out the issue cuz I'm currently having the same one myself, I have 12.8v at my amp until I start my car then it drops down to under 3v... I know this is an old thread and probably won't get a response but what the hell....


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## MetricMuscle (Sep 16, 2013)

liask3606 said:


> Did you ever figure out the issue cuz I'm currently having the same one myself, I have 12.8v at my amp until I start my car then it drops down to under 3v... I know this is an old thread and probably won't get a response but what the hell....


Voltage drops when engine is running and amp is on or off or both? A poor connection can show full voltage with no load or very small load and then drop or read open as the load increases.

Where do you have the ground cable from the amp connected to the body or battery?


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