Old 02-14-2008, 05:44 PM   #1
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Default How do you check ground resistance

Can anyone explain how to check ground resistance? Would appreciate it.
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Old 02-15-2008, 01:43 AM   #2
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Never mind, I found it. This will help everyone who has noise in their system.

This is an old GM Technical Service Bulletin, but it should help anyone wanting to verify that their amp has a good ground:


When diagnosing electronic systems for incorrect operation, it is often necessary to verify that ground circuits are good. This article is intended to clarify what is meant by the term "good ground" and the preferred tools and methods for verifying it.

A "good ground" is a ground circuit that has a resistance of zero OHMS.

Ground circuit resistance can be measured in OHMS using a digital volt OHM meter (DVOM). When using a DVOM, it must be set on the 200 OHM scale to obtain an accurate measure of the circuit resistance. Many meters have both a 200 OHM scale and a 200 K scale. The 200 K scale will not measure zero OHMS accurately. If you are not sure how the meter is to be set for the 200 OHM scale, refer to the meter operating instructions for proper settings. If the meter is an autoranging or self-scaling meter, read the meter carefully to be sure which scale it is setting itself to.

Before measuring resistance in any circuit, the resistance of the meter should be measured by touching the leads together. A meter with a good battery and leads in good condition will read less than .2 OHMS usually zero. If the leads measure anything more, an accurate measure of the circuit resistance may not be possible.

Always remember - resistance cannot be measured accurately on a "live" circuit, All current flow through a circuit must be stopped by disconnecting its power source before measuring resistance.

Ground circuit resistance can also be checked by measuring the voltage drop across the circuit with a DVOM set on, the 2 volt scale. The voltage drop will be zero across a "good ground" circuit.

Remember, fully understand a meter's functions before using it!

To add to this, a good ground for car audio applications will have a return resistance reading of 1/2 ohm or less. I have yet to have a return reading of 0 ohms. If a ground return reading cannot be made to get below 1/2 ohm by means of the "BIG 3", then it is adviseable to ground direct to the battery. Electricity is an algebra equation, what you do to one side you must do to the other. Pay as much attention to the ground wire as you do the power wire.
The BIG 3 is a great place to start for a good ground, however it is the assumed proper method of grounding. What we are talking about here is the older and wiser 4th brother to the BIG 3 (the BIG 4).
So a proper ground wire will be as follows.
- clean of residue and paint.
- secure.
- have a resistance return of 1/2 ohm or less.
- be of adequate guage to carry the return as compared to the power wire.
To simplify the measuring of the return, use your meter as described. Disconnect the - battery terminal and disconnect the ground wire from you amp. If your dmm probes are not long enough, you will need to create a jumper extension out of some primary wire or whatever wire you have handy. Measure this wire for any resistance reading and subtract it from the total.
Many installers are not aware of this nor practice this method. It takes time and time = $ so don't get all ****y if you had a professional install done and this was not checked. A poor ground connection or high resistance reading may seem trivial under no load, but once you are pounding your nice new amp and it is drawing large amounts of current, this little reading has become a monster reading that has caused many an amp to fail for no apparent reason. It may be noticeable as a extremely hot running amplifier in a short time period, poor output levels or diminishing levels and of course a blown power supply or output section in the amplifier.
While the original article was written for the years gone by, it still is applicable to the newer generation of vehicles. A good ground is not about the amount or size of the metal in the return to the battery but about the resistance through it. Todays vehicles are a combination of metals, spot welds, glued together unibody panels and isolated chassis components. The return through these components is where the resistance reading comes into question and this is what we need people to understand, why the BIG 4 needs to be done if the BIG 3 does not solve the problem.
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Old 02-15-2008, 08:03 PM   #3
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What do you define as a Good ground?

Oh, and try that test with the engine running and tell me what you find.
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Old 02-16-2008, 12:30 PM   #4
Rob M
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The best way to check ground resistance is with a load tester, an ammeter and a voltmeter.

Resistance, in ohms, is the same as volts per amp. This means if you draw 20A from your ground point and your B+, and you measure 2V between your ground point and the battery - post, then your ground resistance is 2V/20A = 0.1 ohms.

Now, to put the theory into practice:

Get either a load tester or something that will draw a fairly large constant load (a powerful 12V portable flood light might work well), an ammeter that will read in the range you want (a clamp-on DC ammeter is probably easiest, but a regular multimeter with a 20A setting can work if you're careful not to draw more than 20A), and a voltmeter to measure the voltage drop from the negative post on the battery to your ground point.

Get a wire handy from the - post of the battery back to where your ground point is; you'll need it for testing the voltage drop with the voltmeter (can be a thin wire, there's not going to be any significant current going over this wire). Connect a voltmeter between this wire and your ground point and get ready to read it.

Connect the load where you'd connect your amplifier, with the ammeter (if not a clamp-on) in -series- with the load. Read the voltage and current (amps), and note what they are. It doesn't matter if any of the numbers are negative, just ignore it and treat it as positive. After you have done this, then disconnect the load.

Take the voltage reading you got (if it's in millivolts (mV) divide by 1000) and divide it by the current reading you got. That's your ground resistance in ohms.
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Old 02-19-2008, 12:01 AM   #5
David Navone
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Location: Stockton, CA USA
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If the problem is noise... then it will most likely have something to do with the engine / charging system. So checking for the DCR will be of little value.
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