Inconsistent Resistance Measurements

[v10climber]

Have a question for the GJ hive mind that has been bugging me for years. I work on a number of cars/trailers/boats where I'm trying to check continuity to ground. I've always struggled with getting a consistent reading. Used cheap multimeters for years. A few years back I bought a "nicer" Klein MM400 and it's still the same problem. One probe in the wiring harness pin or fuse holder or whatever. Stick the other probe on a bolt head on the chassis or something and the multimeter shows it's an open circuit. Even though I know it's not. Hell probe 2 points 5 ft apart on my aluminum boat or car trailer and it says it's an open circuit even though it's a big gigantic continuous chunk of aluminum. I always assumed there's just enough paint/oxidized aluminum/whatever between my probe points that the reading goes to open. With enough poking I can usually get it working enough to verify what I am checking. But it's a real pain. What am I doing wrong? How can I be sure when I probe 2 points and the DMM shows an open circuit that there's ACTUALLY no continuity between them?

 

[MBfreak]

Resistance measurements in the below 0,5 Ohm range can be tricky if different metals comprise the path you measure.
Values are likely to be much influenced by the current level in the path.
When working on high current switchgear with alu, copper, steel with correct bimetal washers in the 10 mOhm range we always use 100A AC test current.
One easy way for the home setup is to use a 18 W 12 V filament lamp and run it off of a 12 V car battery. Measure the current in the circuit and the voltage over the test path, and calculate the resistance from that ( Test R = Voltahe over path / current in the circuit)

Ola

 

[Wrench97]

You are wasting your time measuring resistance to ground on cars/trucks with a ohm meter with no load on the circuit you can get a good reading but the circuit may be able to operate a light bulb, use a high amp test light as a load to test the circuit.

 

[American Locomotive]

Both above points are good - for things like trailer or boat wiring - a test light is good enough and perhaps even more appropriate.

But to actually answer your question: You either need to clean the metal before probing, or invest in some probes with ultra-sharp gold plated tips that can punch through the outer oxide layer. Probemaster I believe makes a very nice set of extremely sharp probes.

 

[bwringer]

With enough poking I can usually get it working enough to verify what I am checking....

That's the issue right there; until you punch through the oxides, paint, etc. you're not going to get anything useful. And even then, it's not all that useful sometimes. The oxides may not be visible; aluminum sprouts an invisible later of oxide pretty much instantly.

A resistance measurement is just the meter running some very small amount of current at a low voltage and trying to measure the loss. At lower values, this can be very inaccurate, and highly subject to variables like how hard you poke, sharpness of probe tips, etc.

A test of function, ability to carry current, as outlined above (like a test light or light bulb) is far more useful.

 

[tdkkart]

 

[uncwstudent]

OP, are you using the Klein meter leads? Ive bought two Klein meters where the measurements jumped around when measuring resistance. Switched to my Fluke leads and had no problems after that! Might be worth trying.

 

[RTM]

I use an idiot light for anything on a vehicle looking for ground. A meter is only a guess, as others have noted. Don't use an LED bulb to build one, unless it has a big resistor in it.

 

[mrjaw14]

your specific issue may be caused by a layer of corrosion on top of the aluminum that impedes the flow of electricity. I've had to "stab" a meter lead through paint or corrosion many times if I couldn't get a reading initially. If your meter has 1.5v batteries vs 9v batteries, this may be more pronounced.

As others have mentioned, resistance checks will get you into trouble when chasing electrical issues because if even a single strand of wire is intact it'll pass a continuity check. remember: there is no voltage drop without current flow. I've got in trouble with simple voltage checks on trailer lights only to find that as soon as any kind of load is put on it the voltage drops to where it won't light a bulb.

If possible I run a known good jumper from the true negative on a battery or where ever the ground is and whatever i'm testing.

 

[v10climber]

That's the issue right there; until you punch through the oxides, paint, etc. you're not going to get anything useful. And even then, it's not all that useful sometimes. The oxides may not be visible; aluminum sprouts an invisible later of oxide pretty much instantly.

A resistance measurement is just the meter running some very small amount of current at a low voltage and trying to measure the loss. At lower values, this can be very inaccurate, and highly subject to variables like how hard you poke, sharpness of probe tips, etc.

A test of function, ability to carry current, as outlined above (like a test light or light bulb) is far more useful.

Thanks everyone. Pretty much confirms what I was already thinking and added some helpful info. I'm using the standard Klein probes that came with the multimeter. I'll pick up a test light and use that more regularly since mine seems to have wandered off.

 

[dnschmidt]

The test light tells you more. Specifically whether the circuit can handle any current or not. Also, aluminum oxide is an excellent insulator. You've got to scratch your way though it to get to the conductor.