MHarrisEP

Okay, I will try that.

Assuming it's a short, what should I check? Or would my amp just be bad?

zoomer

iTrader: (3)

No one seems to understand the basics of ohms law.. V=IxR
When the amp is off, it draws little or no current.. That means there is no IxR voltage drop in the wires and you read 12 volts. When the amp is on, it draws current and there is a 7 volt drop because the current it draws times the resistanc in either your power or ground wires, in other workds IxR=7v 12v-7v=5v
Now.. where is the voltage drop?
Easy to find out! Look at the series circuit: Sorry I dont want to draw picitures.

(chassis ground)(ground wire with resistance)(amp)(power wire with resistance)(fuse)(battery)

To find out where the high resistance is that is causing the IxR voltage drop, turn the amp on. Put the negative clip from your voltmeter to independant chassis ground. Then measure the following:
1: ground where your ground wire is connected: should read zero volts:
2: Ground terminal of your ground wire. should read zero
3: copper wire just comming out of ground terminal: should be zero volts
4: copper wire connected to amp terminal ground: should be zero volts.
5: amp ground terminal Should be zero

If any of the above voltages is not zero, then you have found the problem to be between chasssis ground and the point you are measuring

If all are zero, then test the positive side of the circuit:


6: amp positive terminal: should be 12 volts but you say it is 5 volts...
7: power wire at amp pos terminal.. should be 12 votls.. what is it?
8: Power wire before and after fuse should be 12 volts.. what is it?
9: Power wire at batterey where it goes into connector: should be 12 volts.. what is it?
10: powerwire connector at battery: should be 12 volts. .what is it?
11: Battery terminal: should be 12 volts.. what is it?

basically you try to measure at every point where there is a transition from one conductore to another.. that way you can find the weak point.

Remember, even if you measure voltage with a 100 ohm resistor between the battery and the voltmeter, you will measure 12 volts because you are not drawing any current, ( IxR=0 volts drop)... If you draw 100miliamps which could be your amp idle current, you will have you a 10 volt drop and only mesure 2 volts.!

LEt me know what you find!

PS: it is probably not a short. If all your ground and power wire is correct and the amp is shorted, that means it is drawing enough current thru the very low resistance power and ground wires to cause a 7 volt drop! do the math.. for example 10 feet of 2 gauge iwire has a resistance of 1.5 miliohms... V=IR and I=V/R=7/0.0015 over 4000 amps! NOT. In other words if your battery and alternator are good, and your amp was shorted. you would have smelled smoke, or blown a fuse! Remember Cold cranking amps of a battery is what 500amps at a battery voltage of 7-8 volts? You would notice somethign if you would be drawing 500 amps!

Father Yuli

iTrader: (17)

does the amp actually work? if it works, my guess is your multimeter is bad.

if it doesnt actually work, no sound is produced, then you might have a poor connection somewhere. just like Albert Einstain (the guy above this reply) said, draw will cause voltage drop, and a drop of 7volts is ubnormal for any car electronic unless something is wrong somewhere on the wires.

makesure the ground at the amp, the frame and at the battery is tightly secured. also check the fuse, might be a loose connection.

zoomer's reply was bang on with the physics aspect of it, he just forgot to put it in simpler terms.

what he was saying is this - voltage can go thru any type and quality of a connection when the draw is low (or none), what is limited by a poor connection is actually the draw (not the voltage) BUT when you attempt to pull more draw than a poor connection allows, you will experince the drop in voltage. Hence why he's saying to check all those spots.

also, i believe that the only way to achieve a reading of anything else but zero when you connect both leads of the multemeter to two different ground locations is by having a short somewhere.

MHarrisEP

Thanks a lot for the input. I ran through your steps, Zoomer, and found the problem.

I pulled my whole ground wire (painful process) and found that somewhere I had tapped into it (and admittedly not connected correctly) had heated up and melted.

So, that inspired me to redo all my wiring the correct way.

Thanks a lot, guys!

zoomer

iTrader: (3)

Tapped into the ground wire? this should have been pretty short from amp to chassis....WHat did you tap into it with? You mean you cut it and added some sort of connection to it and the result was a poor connection?

MHarrisEP

It's fairly short, maybe 4 feet. That's the closest ground I could find that was easily accessible. And yeah, I cut a portion of the shield off the wire and connected maybe an 8ga wire to run for a ground for my crossover. At the time, I just twisted and electrical taped it, and forgot about it. Big mistake.

So, I went through and made terminals for all the connections I had.

zoomer

iTrader: (3)

Not sure what you mean here.. If you measure with an ohmeter between two ground points you should get very low reading in the miliohms or lower. But the problem is that it takes very special instruments to be able to accurately read millihoms accurately. Not somethign you get off the shelf at most electronics stores.. Certainlynot Canadian Tire.

If you measure voltage between two ground points you should also get zero IF there is no current flowing between those two points. If there is current then the voltage you measure will be the current times the resistance.

But thanks Father Y for telling everyone I was a nurdgeek in your own very special kind way! You sure are a diplomat!

zoomer

iTrader: (3)

if you just cut the plastic insulation off it should not have affected the resistance of the 4 feet of ground UNLESS you also cut the wire itself. BUt the 8gauge would have been a crappy connection if just twisted. But great that you found it...

MHarrisEP

Right, the wire was in 2 pieces. I'm not exactly explaining it right, it was really sketchy.

But, yeah, it was in 2 pieces and they no longer had a strong connection.

JohnVroom

He pooched a splice in the power supply wiring

Sorry zoomer it was too drastic a voltage drop to be a load issue without (as Yuli put it the car would be unable to start) or even run the car... the amp would have turned on and then gone into protect as the amp and the battery get real hot real fast.

The symptoms did not indicate a shorted output or fault in the amp