Measuring resistance?

[z0n3d0u7 10+ year member]

Im trying to troubleshoot a charging problem that popped up all of the sudden, So i started trying to figure out how to measure the resistance in my big 3 wires, to make sure they are all good.

I flipped my DMM to the 200 ohm's setting, And put it on each end of all cables.. the Cables came out to 0.4 when it finally settled. Is this a good sign?

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[helotaxi 5,000+ posts]

Touch the leads of the multimeter together first. That is your "zero" reading. Your big 3 wires should be low enough resistance that they show basically no change from the "zero."

 

[z0n3d0u7 10+ year member]

Good deal. My DMM leads float around the same.

Took the alt and batt to be tested. Batts good low charge but alt was bad. I decided to bring the batt and new alt home to test, versus leave it there for 8 hours. Installed everything, Turned the car over and it turned over solid but i let out instead of giving it a lil gas. Next turn over, Click. So i took it back to autozone to have it charged over night, And we shall see the outcome tomorrow.

I shoulda done a cs144 swap, [Plus one eventually where the smog pump used to be.] However i dont really have the cash for it at the moment.

 

[smoka 10+ year member]

You can't really measure how 'good' your ground connection is. You could have a hairline wire connecting ground and your meter will read 0 resistance. You really won't be able to tell if your grounds are bad until you start moving current through them.

 

[keep_hope_alive Premium Member]

you need a real good meter to be accurate at low resistances, and you seem to only have one decimal point on that setting. you don't have a 20 ohm setting? i'll usually insert a measured ~10 ohm resistor in series so the meter is a bit more accurate. start by measuring the resistor (includes leads), take note of that value - then just take the difference between that value and what you measure. i have a special probe just for low resistance measurements. also, touching leads has contact resistance + meter lead resistance you are measuring. contact resistance can vary with oils and other film. keep probes clean for these measurements.

resistance is always present. the best thing to do is properly size and terminate the wiring, properly prep the connecting surfaces, and keep connections tight and free of corrosion.

 

[helotaxi 5,000+ posts]

You can't really measure how 'good' your ground connection is. You could have a hairline wire connecting ground and your meter will read 0 resistance. You really won't be able to tell if your grounds are bad until you start moving current through them.

Actually it's a fairly simple process to measure ground resistance. The problem is getting a good enough meter to measure it with the required level of precision.

 

[smoka 10+ year member]

Actually it's a fairly simple process to measure ground resistance. The problem is getting a good enough meter to measure it with the required level of precision.

Not if you are troubleshooting a bad ground. I could get a wire the size of a human hair and it will read 0 resistance. But when you run a few amps through it the resistance will increase alot. An ohm meter will tell you the resistance measurement, but it will not tell you the quality or capacity of that wire.

When dealing with charging problems, just remember that electrons flow from neg to pos so you want to make sure you don't have any bottlenecks in your grounds.

If you want to properly check your grounds, here is what you do.

Play a sine wave or other constant sound at a medium level so you get some current flowing.

Take one lead from your meter to the pos of the batt. Take the neg lead (you will prob have to use an ext wire) and put the neg lead at several areas where you suspect there is a bad ground. Look for any voltage difference between the locations that you test.

Measuring voltage drop instead of resistance is the proper way to test for bad grounds.

 

[helotaxi 5,000+ posts]

There are definitional resistances for wire. Unless you heat the wire up a lot, those resistances don't change when you run current through the wire. That human hair thickness wire will have a very high resistance if it's more than an inch or so long.

The reason that measuring voltage drop is what most people do is that they don't have a meter sensitive enough to measure ground resistance directly. The leads usually have a much higher resistance than the ground but the resistance doesn't magically jump when you apply voltage.