Definition of "infinite resistance"

[CROW]

Weekend only mechanic and am about to show my ignorance of things electrical. Am trying to locate an intermittent problem on a 1991 GMC Sonoma with a 4. 3 litre engine. Problem could be the coil, have a book that indicates one reading when using an ohmmeter's high scale should indicate a "very high, or infinite resistance value". Can someone please tell me what this means?

Thanks in advance for any help.

 

[bighammer]

Open circuit or no connection.



If you play with your ohm meter, when the probes are touching, resistance is zero. When they are appart the resistance is infinite.



Hope this helps. Good luck.

 

[BK]

bighammer is correct.





some more 411. .



there is a primary and secondary of the coil.

The ignition coil is a "step up" transformer.

It takes the 12Vdc and makes the 20kV or soo that lights your spark plugs.



You should find a good coil and measure it's primary and secondary resistance and compare them with that of your's.

The seconday, being the highvoltage output will have a much higher resistance than the primary.



But... these coils do funny things, since there is a very high number of turns on the highvoltage side, a few shorted turns can cause funny ringing of the highvoltage and cause a bad spark.

This short can be intermittent and you probably will not be able to determine a bad coil with an Ohm meter. A dead coil yes, intermittent - no.



A few shorted turns within thousands on the secondary= no. .

A few shorted turns effects the "Q" of the transformer, effecting the waveform, but since the change in resistance is minor. . take 3 away from 30,000... . to small of a difference to measure and well with in the measurement errors.



If you can borrow or layout the bucks for a new one, thats your best bet.

If you can duplicate the problem while idling at the curb, you'll need more than an Ohm meter to find a bad coil by actual trouble shooting you'll need and Oscilliscope. The Ohm meter and volt meter will let you shove wires around and look for connections making and breaking.



Bob

 

[QRTRHRS]

Tom,



The term "infinite" is used to describe anything so numerous as to be impossible to count. Look up in the sky and see an infinite number of stars. Now let's see how we can apply infinite to electricity.



To start with, "ohm's" are a measurement term for resistance. When you apply voltage to a circuit, you allow "current" to flow. For example, hook up a light bulb to a 12 volt battery. The filiment that glows is actually a resistor so to speak. The resistance to the flow of current generates heat and hence light. Higher current flow means more heat, more light. Less current flow, less heat less light. Did you ever notice some lights seem to become brighter just prior to blowing? The filiment is burning up, less resistance, more light. The filiment blows, no light, no measureable resistance.



Further, lets say you have two light bulb's. One 60 watt, one 100 watt. If you were to measure the resistance, the 60 watt bulb would actually measure higher in value. Now, you drop that bulb and the filiment breaks. You have no physical connection so you cannot measure resistance at all. It's there though.



Now, lets take your coil and put the ohmmeter probes on it. Let's say your meter has an analog guage (not digital) and the needle pegs to the right. This could mean a couple of things. One, you have a broken wire within the coil. No current flow is possible. You cannot measure the value of resistance. Two and less likely, your meter just does not measure that high.



Been a long time since I had my face in a basic electricity book but I hope this helps.