PoLaK

iTrader: (1)

I'm not to talented with a multimeter, but I was poking around the engine bay the other day checking out how many ohms of resistance i had between the negative terminal on the battery and various ground points.

I was hard pressed to find something that didn't have a -.000 reading, but it doesn't seem that all those ground points would work as well (least in my head). Is there something else i should be looking at perhaps, how long it takes to get a reading of .000? (that seems like i would be dependent on how good your meter was tho...

TownDrunk

The fact that you're reading 0 when you check from the battery terminal to the various ground points is a good thing. No impedence = good ground.

paulmasoner

iTrader: (1)

^^^agreed, but in my experience if your using the proper scale you'll never get a perfect reading of 0 Ohms.

Polak, its something i can understand and do when i put my hands on a meter, but i cant explain... just make sure your using the smallest scale the meter has. as you switch to the smaller scale you should be able to register minor amounts of resistance. as far as what numbers are good/acceptable to see.. i dunno?

perhaps someone smarter can chime in here.. i know a few who are good with this kind of thing.. Team, RR, Lurch..

TownDrunk

True. The meter is using a very small current to make the calculation. If you're attempting to see how much current a particular ground can handle, or how well it can handle a paricular amount of current, then you're measuring it incorrectly.

Easy_E1

iTrader: (2)

"0" is no resistance. Resistance on a ground is bad. If you had resistance you would'nt have a good ground.

TownDrunk

Yeah, I think we all agree there. I think it really depends on what he's trying to measure. Resistance is one thing, but load capacity is a completely different thing.

PoLaK

iTrader: (1)

Yes I know this, and I'm using the 2M OHM setting.
But is there anything i can do beside buy a more expensive meter? I'm using your standard $30 Digital Multimeter from Radio Shack.

How about how long it takes for the MM to get to the .000 reading is that a function of the ground point or just a function of the randomness of processing time taken by the MMs circuitry.

paulmasoner

iTrader: (1)

uhhh, you guys are gonna make me interested and go read up on this stuff again... and i wont need to know it for a couple months.. at which point i'll have to read up again.... ughhh

TownDrunk

The time it takes the meter to get the reading is irrelevent in this case. You'll want to put the meter on the lowest setting as Paul said. This will give you the lowest resolution. The "2M" setting is giving you a reading max of 2 MegaOhms, but gives you a lowest reading of 1 KiloOhm (when it reads 0.001), or 1000 Ohms.

For instance, my meter's lowest setting is "3" for 3 Ohms. When I set it there, and I get a reading of 0.001, this means it's 1 MicroOhm.

So, try setting your meter to the lowest setting and see if you see a difference. But again, it really depends on what you're trying to measure.

PoLaK

iTrader: (1)

ah okay gotcha it makes sense to me why my MMs "200" is better than 2M.

But why does it sometimes read negative values?

And is a good ground considered less than .5? (i've heard this before)

TownDrunk

I'm not sure why it would read negative values (unless it's -0 which is the same as 0)... You can't have a negative resistance in an electronic circuit, so if you can find one with a negative value, then you've got something really special there

Anyhow, if you're reading half an ohm, that's pretty good. But, if it's a 0.5 when you have the scale set to 200 Ohms, that then that should be 50 Ohms, not half an Ohm (assuming your meter uses an X.XXX readout). I wouldn't say 50 Ohms is all that great. If your readout is something like XXX.XXX, then a 0.5 is half an Ohm and you're looking good.

Hope that helps. I gotta go hang out with the wife so she quits giving me "the eye" for being on here while she wants snuggle time.

PoLaK

iTrader: (1)

The first picture is the resistance of my upper lip. Maybe i'm a freak of nature, or maybe I just need to get a better MM.

The second picture is the 200 readout when I had the two prongs touching, and why I said .5

Brettus

iTrader: (2)

my multimeter has an adjustment so you can set it to zero with the prongs touching eachother .

paulmasoner

iTrader: (1)

show off!!

nice sig quote too, i saw that post and was like WTF - serisously!?!?

secret_squirrel

A little off topic, but make sure you don't try and "ohm out" a powered circuit. Just a fyi. I'm sure you know this, but for anyone reading this thread its good info. Personal experience; Ohm meter + 110/208v powered circuit = Exploding MultiMeter!

PoLaK

iTrader: (1)

Well i played around a little more and got it down. So heres my problem, I made a battery change and slight relocation of the terminals + a master switch you can see the thread here:

https://www.rx8club.com/series-i-tech-garage-22/where-did-you-mount-your-battery-148448/

Everything is a-okay except my idle is low @500rpm and rough. Now:

I measured the resistance in all the lines that I fabricated and they all registered 00.1-00.3

Except for the wire going directly from the positive terminal on the battery to the master disconnect switch (you'll need to look at the other thread to see what I mean). This wire has a ANL fuse on it, its 250amp, and the resistance in the line measures 00.5-00.6. I would think that the higher the amperage on the fuse the less resistance it would cause but am I wrong here?

Would possible going down to a 150amp fuse help fix my rougher idle ?

Or should I start looking somewhere else for my problem, maybe at the battery itself.

DarkBrew

Ohm meters all work by injecting current and measuring voltage
Your negative ohm readings indicate the presence of an existing current.
Disconnect the battery and drain down any charges left in the system
Use a four wire meter on the lowest setting possible. Four wire meters separate the current force and the voltage measure into separate wires in order to measure very low resistances (in the milli Ohm range). Note: When using a four wire meter the two probes for each end of the measurement must touch the desired measure point independently.
Cheap approach: Start the car with everything connected and measure voltage across various points. Any significant voltage drop indicate a 'high' resistance point.

Jon316G

iTrader: (1)

You should expect to see a little more resistance with a fuse (more material to pass through). Don't even worry about what you are reading, its not important with that small of a measurement. What you learned is that you have continuity between two points (meaning, there is no short).

NO NO NO!!!! NEVER do that with a fuse... keep it at the current rating. A fuse has a metal conductor in it that is calibrated so that when a certain current rating is reached, the conductor is literally heated and melts, which is what breaks the circuit! Lowering the fuse amp will only cause it to blow! Fuses are installed based upon the wire its connected to... its there (at that rating) for a reason!!!

PoLaK

iTrader: (1)

I mean the only reason that its currently at 250 is because I chose 250 based upon what other people had done when they relocated batteries over much longer lengths i.e. to the trunk.

For a less than 24inch cable maybe the 250amp fuse is a bit much no?

So you saying that my rougher idle probably has nothing to do with any of my cables as the impedance in the cables are minuet?

How bout the batter itself how might one test a battery?

Jon316G

iTrader: (1)

^^ I'll have to admit that I didn't read that other thread... so let me do that now. I thought that these were wires/fuses that came with the car... my bad

Jon316G

iTrader: (1)

OK... I read the whole thing (sorry about that)!
VERY NICE... I'll give you props for that idea!
Your rough idle should have nothing to do with what you modified because when the car is running, its using electricity generated by the alternator.....
still thinking..............

Jon316G

iTrader: (1)

Everything appears fine... I would say that your low idle is because of having the battery disconnected for so long that it reset the PCM (NVRAM) and now has to relearn the fuel trim.
Everytime I do a reset, I get low RPM idles for a couple drive cycles... sometimes it feels like its gonna stall.
I would run the car for a few cycles and see if it improves!

PoLaK

iTrader: (1)

heh alright we'll see. Thanks for the help.

Jon316G

iTrader: (1)

Keep us posted... I'm curious!

secret_squirrel

+1. I've disconnected mine a few times for various reasons, and I experienced similar RPM fluxes at intial startup. By the way, great idea with the master switch and mounting bracket. No more fiddling with the terminal clamp when you need to work on electrical. Plus, any unwitting thieves will be like, "Wtf? This mazda won't start!?"