High Resistance Ground

I tested the ground resistance to my amp in my 98 ford escort and found that it was a rather large 3.2 ohms. It was measured from where the amp ground is bolted to the chassis under the front passenger seat. There is definately no paint where the connection is made. The amp (QTX 200) appears to work excellently, and when the really heavy bass notes hit the battery voltage doesnt drop below 13.75V (From 14.10V when the alternator is running but the amp is off). The amp is fused at 40A, and so is not particularly high power. My question is: do i really need to try and fix the high resistance ground? Upgrading the big 3 would b a pain in terms of both money and difficulty (getting to some of the connections would require me to remove half the stuff under the hood), but if it needs to be done it needs to be done. Thanks

you measured from the grounding point to where?

-------------

Did you use an existing seat bolt?  They are notorious for being bad grounding points.

-------------
Support the12volt.com

oooooo, bad ground that in time will lead to the failure of the amp.

-------------
Top Secret, I can tell you but then my wife will kill me.

It was measured between where the amp ground is connected under an existing seat bolt (which may be the problem thanks DYohn) and the battery negative, so that the reading showed the total resistance to the flow of current. Why are seat bolts bad grounding points? Also, why does restricting the current to the amp slightly cause it to fail? I can certainly understand that it could affect the performance, but as long as it has adequate voltage (min 13.75V as stated originaly) I dont really see how damage would be caused. Thanks for the help so far guys, I'm off to test some other grounding points and see if it is just the seat bolt that is the problem.

Seat bolts are bad grounds because the are hardened steel, and because the mounting hole is generally a welded-in nut.  Use a dedicated ground.  You are not "restricting the current" to the amp, you are causing it to draw more current than necessary, which causes internal components to heat up more than they should and to fail faster.

-------------
Support the12volt.com

You were definately right about the seat bolt being a bad ground. I just tested the resistance using the seat belt bolt instead (amoungst a number of other places) and it is now just under an ohm. Is that an acceptable resistance level? How does the increased resistance in the power supply to the amp cause it to draw more current? That seems counter-intuitive to me. The resistance in the power connection is in series with the amp itself, meaning that there is some voltage drop in the chassis of the car, and therefore less voltage across the amp power inputs. Using ohms law that would suggest that there will be a smaller current draw in total. thanks

The amplifier will draw whatever current it needs to produce the power being demanded from it.  If the supply system causes a voltage drop, then (using Ohm's Law) P=IE (power = current times voltage) when voltage goes down, current must increase to provide the same power.

-------------
Support the12volt.com

Remember as well that you are also measuring at rest, this little measurement becomes something far larger once the demand increases.

-------------
Top Secret, I can tell you but then my wife will kill me.

I agree that to recieve the same power the current must increase to make up for the drop in voltage, but i would argue that the amp is not recieving the same power. In a series circuit the current flow is directly controlled by the voltage supply (fixed in this case) and the total resistance in the circuit. As far as i am aware the amp cannot draw more current just because it wants to, its own impedance to the circuit must be reduced. What i also do not understand is how my amp can succesfully run with a ground resistance of 3.2 ohms. By ohms law the maximum current flow is (I = V/R) 14.1/3.2 = 4.4 amps. There is no way the amp would work with the current restricted that much, yet it seems to work fine. Confusing!