Question about measuring ground resistance. Proper Method?

[gamehawk55 10+ year member]

Hey guys,

I had a question about what is the "proper" method of measuring your amps ground resistance and other ground points resistances. Am I correct in taking the measurement with the car on/amp powered? Because I always see "make sure your grounds measure 0-0.4ohms"....which they do with the car OFF. With everything powered however is a different story.

Recently I was having bad voltage drop issues on my T2500-1BDCP. Measuring the resistance with car off measured 0.2ohms. But with amp ON was 13-15ohms with no load, and could spike to 30ohms+ with heavy bass. This was a couple weeks ago I finally noticed this. After messing with grounds for a while and today doing more ground cable work I now have 1.0-1.1ohms at the ring terminal with the T2500 powered on at idle. And 0.6ohms on my AQ 200.2. Both amps grounded to same spot on top of each other.

Anyways. Long story short...Is that the correct way to measure ground resistance? With the amps/car on? If so, is 1ohms resistance really that bad to still have? This is the best I've been able to get to this amp without a dedicated ground run from the front to back.

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[gamehawk55 10+ year member]

Anybody?

 

[Jepalan]

You will not get an accurate measurement with most DMMs when there is any amount of current flowing through the wire. You want everything off when trying to measure resistance of a wire. You also may need to subtract ou thte effects of the DMM's lead resistance. Touch the leads together first and subtract the reading from whatever you read when you measure the ground resistance.

What points are you measuring between when the car/amp are off?

 

[gamehawk55 10+ year member]

You will not get an accurate measurement with most DMMs when there is any amount of current flowing through the wire. You want everything off when trying to measure resistance of a wire. You also may need to subtract ou thte effects of the DMM's lead resistance. Touch the leads together first and subtract the reading from whatever you read when you measure the ground resistance.
What points are you measuring between when the car/amp are off?

Measuring between each end of each ground. So from -ve terminal of amp to ring terminal at grounding spot, etc.

And like I said...With everything off all grounds measured 0.2ohms. But then when I was having massive voltage drop issues my T2500 Amps ground would measure 15ohms after it turned on. And now that it measures 1ohm once it turns on the voltage is holding MUCH stronger at the amp than before. Hence why it made more sense to me to measure the resistance with current trying to flow through the wires, because obviously the 0.2ohm reading with the car off didn't mean sh*t once everything was powered.

 

[maylar 10+ year member]

Measuring between each end of each ground. So from -ve terminal of amp to ring terminal at grounding spot, etc.
And like I said...With everything off all grounds measured 0.2ohms. But then when I was having massive voltage drop issues my T2500 Amps ground would measure 15ohms after it turned on. And now that it measures 1ohm once it turns on the voltage is holding MUCH stronger at the amp than before. Hence why it made more sense to me to measure the resistance with current trying to flow through the wires, because obviously the 0.2ohm reading with the car off didn't mean sh*t once everything was powered.

An ohmmeter measures resistance by putting a controlled constant current through the circuit and measuring the voltage across it. Then the processor makes the calculation to resistance. This current is very small - milliamps - and any other current flowing through the wire will throw the measurement off. The circuit MUST be powered down.

Making low resistance measurements with a 2 wire DMM is inherently inaccurate anyway, because the resistance of the meter's leads is a couple tenths of an ohm and meters are usually +/- 1 digit and +/- some percentage so that last tenth of an ohm is meaningless. And unfortunately, a tenth of an ohm at 20 amps is 2 volts drop, which we care about.

There are fancy 4 wire ohmmeters that compensate for lead resistance, but for our purposes a voltage measurement under actual load is more useful than making resistance measurements of grounds. Pick a good ground point for the neg meter lead and touch the bolts and terminals along the ground wire connections. You should never see more than about a tenth of a volt difference if everything is tight.

 

[CrossoverOre 10+ year member]

any time i measure resistance for amps I will go from the amp terminal to the neg post of the battery. meter the wires you are using to reach that far and subtract from the total.