Tom 400 CFI

Why does it exist? Why would you want any resistance??

My prof in auto school years ago claimed that resistance in the secondary ignition components was necessary to force the coil to "build more voltage" before "forcing it's way through the resistance" thereby producing a better spark. He wasn't too bright though.

If you really need resistance for that reason, why not use zero resistance components and run a huge plug gap?

Reason I'm asking is because I just finished working on a car that had bad plug wires. I replaced them w/the proper wires for that application. When I was done, the car ran for 1 day, then started blowing spark out of the coil wire boot, at the coil, jumping to ground on the coil body. I removed the coil wire from the coil, cleaned the post on the coil, applied dielectric grease and reinstalled the wire/boot on the coil. That worked for about 5 minutes.

I ohm'ed the wires and the brand new coil wire was 1900 ohms. The plug wires were ~5000 ohms. In addition this thing has resistor plugs as well. The NGK "R" plugs ohm'ed 1900 ohms from the tip of the plug, to the tip of the electrode. By the time the voltage makes it to the spark plug, it has to pass through over 10,000 ohms of resistance, not including th resistance added by the gap in the distributor and the plug gap (under compression) at the plug as well. It's easier for the spark to squeak out from under the boot (at the coil post) and jump to ground, which is exactly what it did.

I pulled the caps/boots of the wires and ohmed the wire itself and the wires ohmed at about .5 ohm. All the resistance is in the caps/boots! Meaning it's an engineered in amount of resistance. I went on Alldatapro and checked the spec and all the waire assemblies were w/in spec for that car.

I eneded up making the thing work by replacing the boot on the coil end of the coil wire w/a different style that offered better cover, and it runs fine now...but why all the resistance???

kdrolt

One reason: reducing EMI with electronic components in the car. Another reason, less local radio frequency noise so you can hear the radio better.

http://www.magnecor.com/magnecor1/truth.htm

DENN_SHAH

iTrader: (1)

there is a good possibility it was leaking spark at the coil because of the old wires. there is probably a carbon track down the tower of the coil. odds are the leak will come back. the coil may also be damaged (carbon tracked internally) from firing through the bad wires, which i would say is a good probability because of the leak at the coil.

i would recommend replacing the old coil as a preventative measure.

Tom 400 CFI

I knew that....unfortuntaly, I forgot. Years ago. Thanks for the reminder, and the good link to read.

-Tom

Cflick

The short answer is radio interference.
Today, that slightly expands to interference with the ECM in computer controled cars.

Electro-magnetic radiation from a conductor is due to the *current* in that conductor, NOT the voltage. The voltage will go to whatever it needs to in order to cause current to flow.
( in an inductive discharge system ) (( or there's a total miss ))
Until current flows, determined by when the gap fires, the resistance doesn't matter a whit.
Once current does flow, that resistance limits it to a value that won't cause radio ( radiation ) interference from the magnetic field produced around the wire.
This, of course, greatly reduces spark energy.
There is a school of thought that once the spark occurs, and the gas is lit, additional spark energy is pointless. I agree.
Where I disagree, is in how much spark energy may be required to actually light a mixture in the gap that might be less than optimum, and to sustain the spark duration until such time as the gas is, in fact, burning.
Obviously I'm not alone in this belief, since coil-on-plug is becoming the norm.