How to Test Impedance of Ignition Wires
 

I have a 72T 2.4, MFI T

Can anyone explain the process to test impedance of ignition wires?

My understanding is that there are also resistors on the ends that should be tested.

Thanks.

Get a multimeter. Set it to the Ohm setting. Disconnect a wire from the spark plug and the distributor (keep track of which wire goes where). Put one multimeter probe on each end of the ignition wire and read the resistance. The resistance will vary a bit for the different wires. If a particular wire has a problem, the resistance will either be very low (near zero if the resistor is shorted out) or very high. Most of the resistance is from the resistor at the end, rather than the wire itself, so you don't really need to test the resistor separately.

above is correct for checking the resistance, but will not check the impedance

impedance is an "AC concept"; resistance is a "DC concept"

why do you want to test the impedance??

Impedance would not be a part of the wires, as the car runs on DC. However, many people confuse the two and refer to all resistance as impedance.

Some high voltage wires are wound like coils around some sort of carbon based core.
I guess the impedance would be of concern if the coil were broken, but the carbon core would still measure resistance ( higher than the coil).
Bob

I assumed that IAN was concerned about a potential problem with one of his ignition wires and therefore wanted to test the resistance of the wires. This is a very easy thing to check, so he might as well do it. If the total resistance of a given wire is not to spec (I think around 3K Ohms), he can unscrew the end to check the resistor on its own, or just replace that wire (or the whole set if they are old).

I agree Pazzo.

I just don't want more confusion floating around in cyberspace than we already have...

Ummmmm, No.

In fact, no no no no NO.

When a signal changes in voltage there will always be an AC component of some magnitude.

Induction can create an AC component.
The "alternator" produces AC.
The spark system, creates a voltage pulse - see above.

If he is really after figuring out his spark wires, he will be dealing with RF anyway.

I bet he just wants to know the resistance, and not design or troubleshoot some racing magic, leading edge of technology spark ignition systems, but ya never know...


PS RF = radio freq.

This is not true, the coil is a transformer which is an AC device. An ignition system runs on a square wave, which can be reduced to a series of AC waves.

The best way I've found to check the distributor, cable and plug is to simply use a timing light on the cylinder you wish to check. If it flashes it's firing. I discovered this when I had a bad plug (+100k ohm resistance rather than 6 - 10k) It's amazing how well it runs on 5 cylinders!

AC by electrical definitions changes polarity (Alternating Current [flow])... the spark is a dc pulse by electronics definitions because the polarity does not change.

The alternator does produce AC, which is why there are diodes... to change to a DC sine wave, and capacitors to level out the DC ripple.

afai know impedance changes with frequency

ron - yes; here is an intor wiki
http://en.wikipedia.org/wiki/Impedance

dog - no, we are talking about an AC component in the signal

I am not trying to argue, just get the physics here correct.

There should be no ac component to the signal in the wires, the coil is used only to increase the voltage of the dc supplied, this is what a coil does, nothing to do with ac or dc, any signal supplied would be changed by the coil. For it to be an AC signal the electron flow would have to reverse direction, which it does not. There is no ac signal imposed on the dc signal, just a varying dc signal. When 12V DC is supplied the coil "charges" via a magnetic field that stores this energy, when the 12V DC is removed (by the points or hall effect, etc) the magnetic field collapses, inducing a high voltage pulse in the wiring, this is referred to as Counter-EMF (Counter Voltage) and is the product of the formula V=LR R being the resistance in the circuit, L being the inductance in Henrys. The fact the DC is rising and falling is what can create radio interference, but it is not an AC signal, it is pulsating DC which is different, because the flow does not reverse.

Here are some info sources:

http://home.earthlink.net/~jimlux/hv/igncoil.htm

http://wiki.answers.com/Q/How_do_ignition_coils_work_in_cars

http://www.scribd.com/doc/7267325/Army-Refrigeration-and-Air-Conditioning-Vol-1 page 44

http://hyperphysics.phy-astr.gsu.edu/HBASE/magnetic/ignition.html#c1

The last is from a major university, I hope they have it right!

PS: I certainly hope calling me dog was a typo, as I do not think it is very nice, but I believe it was indeed a typo.

looks like I contracted your user name - sorry

plug wires are well known to produce RF signals -- so there is definitely an AC component

see also the "large voltage spike" portion of your URL

think we are splitting hairs...

Pretty much it is splitting hairs... which is much less messy than splitting Hares. I just have a tendency to use the precise definitions of accepted physics, etc. because my job is like that (use a similar but not accurate word is a no-no) and I spend a lot of time with other people that are serious engineers. The RF signals are very peaky pulses from the coil, and a bane on all radio systems, but can be repressed by use of a capacitor to filter the signal very easily as the signal only goes positive so a sink to ground works great with little I2R heating of the capacitor, prolonging it's life. As for shielded cables, they work to a degree but I have never found any that would take away all the noise if the condensor was bad or the system had other issues. Good and clean contact at the plugs on both ends and a good connection of the spark plugs with the engine case is helpful. As far as I know there is no way to test the suppression without destroying the cable. If really bad tho they could be wrapped in a braided metal sheath grounded to the block on each end and thus act as a shield much like in a cable-tv line.

Interesting contraction, glad it only comes out as dog, would have been much worse if my username was Fastrodg. :)

PS: being in Oregon must drive you nuts, having to trust some gas station attendant to not spill fuel on your baby. I know it gets me in trouble traveling there because I always go on autopilot and try to pump my own.