Checking resistance

[88911coupe]

Ok, I will show my profound ignorance here but I can't figure out how to check resistance for a wire, i.e. a plug wire. I have a digital multimeter but don't know what to set it for or what to look for. There are 4 sections, DCV, ACV, DCmA, and an "Ohm" character. I assume it's the ohm one but what setting do I use? I have the black lead inserted into "COM" and the red lead into "V-ohmsign" with an arrow sign to it's right. There is also a "mA" jack to the left of the COM jack. It's a Sperry DM-350A if that matters.
Let me know if there's something I can tear up using this as well.
TIA and sorry for the dumb question.
Maybe I should just put this thing away before I really mess something up...

[911pcars]

Buck,
You're about 95% there. Yes, use the ohmmeter setting. I assume the meter is autoranging - the display shows the resistance directly (rather than presetting the expected resistance scale). With the meter ON, connect both leads together. The display should show almost zero (0) ohms indicating low resistance. Hold a test lead tightly in each hand. The display should show very high resistance (thousands of ohms).

This is the same method to test the resistance of any circuit (no source voltage applied, ever). Connect the leads to each end of the object to test. Read the display. If you have a spec to compare to, that's good (otherwise, it's "so-what?"). If you see the infinity sign, that means there is no continuity/open circuit, infinite resistance - usually bad in an electrical circuit unless you're checking a switch in the open position.

Check the resistance from the bottom of the distributor cap to the end of the spark plug cable. I think you should read about 6000 (6K) ohms resistance. Others can chime in with more accurate numbers. Excessive resistance might mean a bad wire, poor wire-to-distributor cap connection and/or faulty spark plug end connector.

Sorry for the lengthy, yes.

Sherwood Lee
http://members.rennlist.org/911pcars

[88911coupe]

Sherwood,
Thanks for the reply. I guess I should have said the meter in the Ohm "section" has several levels/settings.
It goes from 200 to 2k, 20k, 200k, 2m, 20m. Should I set it to be above the resistance I expect, say use 20k if I am expecting to see 6k on, say, a plug wire?
FYI, I put it on 200 and touched the leads together and it displayed around 25.2 but it is jumping up and down a point or two, is this normal? It just displays 1 if I don't have the leads touching.
Thanks,

[911pcars]

Buck,
Start with a higher display range than anticipated. This way you don't "peg" the meter (probably not possible with a digital display). Then switch to a lower range to more accurately read the resistance.

Not sure what's happening with your meter. With the leads in direct contact, you should have almost no resistance (0 ohms). Try cleaning the leads with some steel wool and see if that makes a difference. 25 ohms is close to zero but not close enough for an ohmmeter.

Conversely, 1 ohm is light years away from infinity to be accurate. How about taking your meter to a local Radio Shack, Sears or equivalent store and comparing with theirs? An inexpensive meter is fine for most purposes and is better than none.

Sherwood

[john70t]

The meter works by sending out (.5?) amps out the positive lead and calculating how much is returned through the negative.
(Ohms Law is: Voltage= Amps x Resistance. 12.6V= ? x 500Ohms)
Because the "sniffer" circuitry in the meter itself has low resistance and acts like a wire, it can be damaged by connecting it to current flowing through it in a "live" circuit.

Also it can be fooled by a circuit that's not completely isolated, show good result for a battery cable thats half cut through, or a coil that's not hot and vibrating.
Real world conditions using a voltage drop across the component would find the problem there (i.e. hooking the volt meter to both ends of the positive battery cable and seeing 3 volts while the engine is cranking).