How to measure a proper ground using a DMM

[lotusguy]

HI,

This is one for all you Double E's out there. I am seeking information to confirm my understanding of how to measure a proper ground.

As I understand it, I can use a digital multi-meter set to measure resistance (Ohms) and would be looking for infinite resistance is this correct? Hopefully someone alot smarter than I am can confirm this. TIA!
Happy Motoring! ...Jim'85TE

[Dr.Hess]

Hey Jim,
No, you want zero resistance for a ground. Of course, zero resistance would be super conducting so a small resistance is the best you will get without dropping the car's temp to near zero K.
With your meter set to the lowest Ohm (resistance) scale, hook one lead (aligator clip, etc.) on a known good ground such as the negative terminal of the battery, which is my gold standard or the motor or whatever. Use the other lead to probe around for the ground. When you get very low resistance, you are at a ground. You may need to extend your leads depending on where you are looking and just how good you want to be.
Be careful to not touch anything with +12v while you are probing or you will potentially blow the meter.

Dr.Hess
Radio Electronics Officer, USMM (Ret.)

[lotusguy]

Dr.Hess said:

Hey Jim, No, you want zero resistance for a ground. Of course, zero resistance would be super conducting so a small resistance is the best you will get without dropping the car's temp to near zero K. With your meter set to the lowest Ohm (resistance) scale, hook one lead (aligator clip, etc.) on a known good ground such as the negative terminal of the battery, which is my gold standard or the motor or whatever. Use the other lead to probe around for the ground. When you get very low resistance, you are at a ground. You may need to extend your leads depending on where you are looking and just how good you want to be. Be careful to not touch anything with +12v while you are probing or you will potentially blow the meter. Dr.Hess Radio Electronics Officer, USMM (Ret.)

Hey Doc,

Thanks for the info. and clearing up my question. I had infinite resistance written in some notes which seemed counter-intuitive to me which is why I asked.

Interesting point about Superconductivity. Imagine the spider-cracking which would occur at 0k ! Yeooow!
Happy Motoring! ...Jim'85TE

[rlearp]

Or, turn your meter to continuity and place the probes where you want to measure. Beep means you've got a connection and are good to go. Either way works and both use the same ciruit on the MM, just some MMs don't have this feature.

[lotusguy]

rlearp said:

Or, turn your meter to continuity and place the probes where you want to measure. Beep means you've got a connection and are good to go. Either way works and both use the same ciruit on the MM, just some MMs don't have this feature.

Ron,

Good point, my MM has both functions, but I'm concerned that I could get continuity and still have a ground with higher than optimum resistance, the ohm meter is the way to go.
Happy Motoring! ...Jim'85TE

[maigret]

You can also use the DMM to detect resistance through voltage drop across part of the circuit.

Set DMM to DC voltage and probe the sections you wish to check. This technique has the advantage of checking the circuit under load and you won't blow your meter. (provided it is set to more than 12V scale )

Indicated 12V will show a break in the circuit. 0V if the connection good. Somewhere in between indicates a poor connection.

[rlearp]

lotusguy said:

Ron, Good point, my MM has both functions, but I'm concerned that I could get continuity and still have a ground with higher than optimum resistance, the ohm meter is the way to go. Happy Motoring! ...Jim'85TE

Not likely. The threshold on those is set very low so this sort of thing doesn't happen. They're thinking ahead, at least on the quality meters from Fluke and others.

Ron