two40 Posted June 21, 2003 Share Posted June 21, 2003 “The amount of current flowing in a circuit made up of pure resistances is directly proportional to the electromotive forces impressed on the circuit and inversely proportional to the total resistance of the circuit.†So which one of you electrikiti minded people can simply explain ohms law to a simpleton (read: me)? What is the relationship of the resistance to the circuit? Reason I ask is my leads. I noticed last Tuesday night that they were arching and after having a chat to a mechanic today he told me to check them with a multimeter. Wanting to learn something instead of open my wallet for others I went to buy one and borrowed a newish set of leads off my friend to use as a comparison. From what the mechanic told me the leads should have 3k to 4k ohms per 11". My coil lead is about 10" and measures in at 1.59k ohms. I checked other leads taking into consideration the length and they were all 1/5 to 1/8 of what they should be when compared to the control set of leads. Anyhoo, what do the ohms tell you and can someone possibly compare the ohms in another application so I can understand a bit more about leads? Thanks guys Quote Link to comment Share on other sites More sharing options...
Guest greimann Posted June 21, 2003 Share Posted June 21, 2003 In general terms, the higher the resistance in a coil lead or plug wire, the less voltage reaches the other end. Your wires actually transmit more voltage to the plugs than your control wires. Comparing two different sets of spark plug wires is really difficult because of the way they are manufactured. I use MSD Super Conductors that are rated at 50 ohms per foot. Very low resistance. When checking a spark plug wire for resistance, you are really looking for a very high value or an open circuit. This would be trouble. A low reading is not cause for alarm. Ohms law is a fundamental relationship between resistance, voltage and current. V=IR is the basic formula and the arithemetic combinations thereof. Another formula, P=IV , is Power in watts is equal to amperage times voltage, is also very useful. Browse here for some further reading: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html#c1 BTW, the arcing of your lead is probably due to voltage leaking around the boots or moisture on the wires. Put some dielectric grease on the inside of the boots may help. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted June 21, 2003 Share Posted June 21, 2003 That is a pretty tough phrasing of Ohms law. Ohms law says that the voltage drop across a circuit is equal to the current through the circuit times the circuit's resistance. Or Volts = ( current flow ) X ( resistance ) If you know any two of the above (say voltage drop and current flow), then you can calculate the third (in this case resistance). So much for the hard way. The easy way to think of electricity is as water flowing through a valve or pipe. If you open a valve half way, then you will get a current flowing through the pipe. The "current" is measured in say litres per hour (down south anyway). If you open the valve further, then the valve has less "resistance" to flow and allows more current to pass. Similarly, increasing the water pressure will make the water come out faster also. So think of voltage as a measure of the pressure forcing current to flow through the circuit (pipe). Ohms law expresses this in units of "volts". Current is current (volume per second). Electrical current is generally expressed in amps, which is the number of electrons passing through the wire per second (close enough). Resistance is just some mathmatical set of units to make the equation work, hence Mr. Ohm used his own name to define resistance. In a car, the battery or generator is the voltage source (water pump). Everything else, every wire, connector, fuse, bulb etc has some resistance. If current flows through a circuit, than anything in the circuit with resistance will cause a voltage drop. Just like a kink in a garden hose, every little resistance causes a voltage drop that reduces current flow somewhat. And just like too many kinks in the hose will cause such a pressure loss that no water will flow, dirty or broken electrical connectors will have so much resistance that the voltage at the load (bulb, starter motor, etc.) will not have enough umph to light them up. Trying to translate, by leads I believe you mean plug wires. If you see arcs off your plug wires to say the valve covers, then we need to take the water analogy a bit further. Just like water, electricity will always take the path of least resistance. A cut in a garden hose near the spigot means most of the water will flow out the cut rather than down the hose. With a plug wire, the insulation is suppose to offer a super high resistance to electrical flow, typically millions of ohms (megaohms). Thus the spark goes down the wire to the plug. But, if the insulation gets old, it gets dirty or has small cracks which reduces the insulating capability. At the same time, if small breaks form in the center conductor or the connector at the plug gets corroded, then they offer an increased resistance to flow. Thus the spark now finds it is easier to jump through the plug wire to the valve cover then to flow down the wire. Ignition circuits are a little more complicated than say a turn signal circuit because of the high voltages needed to force a current through the large resistance of the plug gap, and because the current is not a steady, direct current (DC) but pulsed or alternating. The high voltage means even something like a little bit of dirt will cause an arc to ground, especially dirt in a small crack. The pulsed current means things can act like a transformer, causing current to flow in a wire completely isolated from the main circuit. This means a layer of dirt or grease on the outside of an otherwise perfect plug wire, or putting two wires too close together, will cause arcing. So when the mechanic told you to measure the resistance of the wire, he had you checking for breaks or corroded terminators. But, if you are seeing arcs, first trying cleaning the outside of the wires with hand cleaner and put in a fresh set of spark plugs. If you still see arcs, or if you find even hairline cracks in say the plastic insulator on the coil, then replace the plug wires and/or coil. Quote Link to comment Share on other sites More sharing options...
Xander Posted June 21, 2003 Share Posted June 21, 2003 To make things a little more difficult. You should also keep in mind that the voltage in the sparkplug leads is very high. like more than 30.000 Volts. This kind of voltage has very different properties then your basic 12 volt system. Coper core wires have very little resistance (near zero Ohms). The resistance of a supression lead is a lot higher. does this mean that the spark gets less voltage? http://www.magnecor.com/magnecor1/truth.htm This is ofcours one manufacturers opinion but it is good info none the less. Xander Quote Link to comment Share on other sites More sharing options...
two40 Posted June 22, 2003 Author Share Posted June 22, 2003 Thanks for your help guys. Very useful explanation there Jim Powers. I'm buying new leads tomorrow so hopefully it will eliminate the arcing problem. If it doesn't is it possible that I have an earthing or ignition problem? Quote Link to comment Share on other sites More sharing options...
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