Spark Plug Wire Resistance Problem

[Martin Benade]

But the spring is self-adjusting for length. A clever solution.

[Mike Wilson]

Got it. Thanks for that info, Martin.

Mike

[Thomas Sottile]

What about the one in the plugs there is a resister in the new Now Bosch plugs

[Martin Benade]

That will cause no problem, it’s ok if Bosch chose to add that.

[Thomas Sottile]

Talk to Jeff Adams about that he has had many problems with the plugs

[Fred Winterburn]

-As a note, The plug voltage with a 32 thou gap on a medium compression engine under heavy load is more like 16 thousand volts. There is also a voltage overshoot that adds considerably more voltage, so the ignition system must compensate for that as well. The plug wires and all secondary side insulation, are subjected to the gap voltage plus the overshoot which can easily add another 8 thousand volts. Counter-intuitively, the weaker the ignition system, the higher the voltage overshoot and the more voltage stress on secondary side insulation. The increased voltage overshoot with a weak ignition is due to the need to properly pre-ionize the gap which takes some current flow in the form of a corona discharge between the spark plug electrodes. A weaker system needs more voltage to produce the required current for that corona discharge. This is as it applies to inductive ignitions.

-With points switching current, the voltage overshoot is lower than with the same coil switched by a transistor even when the voltage drop of the transistor is taken into account. This is because the condenser slows the voltage rise and the peak available voltage compared to a transistor switch. In doing so, the plug gap has more time to pre-ionize as the voltage rises which reduces the overshoot. The condenser gives other advantages over a transistor switching the current and despite the slower voltage rise is no worse than a transistor switch ignition to overcome fouling (actually slightly better, but still poor).

-Also, adding more secondary side resistance increases the voltage overshoot very slightly, but measurable. More importantly, increased secondary side resistance reduces spark energy and duration. Fred

The spark plug connector has a resistor that usually measures 0.5 K Ohms to 1K Ohm. The distributor rotor also has a resistor under the epoxy and from the center of the rotor to the tip of the rotor should measure about 5K ohms. There have been frequent reports about new high tension leads (formerly called spark plug wires", that are bad from the manufacturer. If your voltmeter setup is good and you measure 5K ohms on the rotor and zero ohms measuring from the distributor end of the wire to the connector that slips over the end of the spark plug - change the leads or the connectors.

If the resistors in the spark plug connectors are broken, have a bad connection, etc and the car still runs it's because the voltage that the plugs fire is usually between 8,000 volts and 10,000 volts. As a friend said when I explained the ignition system: "That's a lot of volts".

[Leo Dreisilker]

Paul, thanks for posting this issue. I have a hesitation that develops around 3,000 rpm through apprx. 3,500 rpm. My shop had checked carbs and everything and said all devices are good. Recently hesitation has gotten worse. How long should plug wires last? Mine have about 38,000 miles on them.

[jay harris]

I had the same problem with my '65 SC. I traced it to the thick film, laser trimmed resistors in the Beru wire set I got from Stoddard. I happened to have a set of old VW leads that I broke apart and found a real wire wound resistor inside that fit perfectly in the wire cap. Problem solved.