8/6/2019 Motor Secondary Ignition

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In the May edition of this series, we discussed spark plug and ig-nition wire maintenance. Thistime, we’ll look at the other im-portant parts of the secondary ignition supply chain: the coil,

distributor cap and rotor. As distributorcaps and rotors are rapidly disappearingfrom the underhood landscape, we’llstart with them first.

Ignition points (another ignitioncomponent that has already disap-peared) served a very important pur-pose, besides the obvious one of switch-ing the current to the ignition coil off and on. Because the points were locat-ed inside the distributor, under the dis-tributor cap and rotor, it was necessary to remove the cap (and sometimes therotor) to gain access to the points whenthey required replacement or adjust-ment. Removing the cap for this main-tenance chore provided a perfect op-portunity to inspect the cap and rotorfor wear. If the cap or rotor lookedfunky, it was a simple matter to recom-mend replacement to the customer. Formany years, cap and rotor inspectionand replacement were considered apart of what was once called a “tuneup.”

Nearly all vehicles in regular servicetoday are equipped with electronic igni-tion, which means no more points. Un-like the points and condenser, the elec-tronic ignition components inside thedistributor normally require no inspec-tion, adjustment or replacement, unlessthey should happen to fail unexpectedly.This means that many months or even years may pass before the inside of thedistributor cap and the rotor receive a visual inspection.

The cap and rotor are wear items, which means they do not have an un-limited life span. As the coil feeds cur-rent to the spark plugs on distributor ig-nition vehicles, it must pass through therotor and cap before it reaches the plug wires. The current arcs as it makes the jump across the rotor gap, which gradu-ally erodes the metal rotor tip and cap

electrodes. As more wear accumulates,the gap grows larger and the coil mustproduce more voltage to compensate.  When demand exceeds supply, enginemisses and loss of performance may re-sult, which is why the cap and rotorshould be inspected (and replaced, if necessary) according to the manufactur-er’s PM recommendations.

Perhaps you remember the ’70s, when auto manufacturers first toutedthe “high-energy” ignition systems ontheir vehicles. Coils in these systems  were capable of producing approxi-mately 40kV or more—about twice what the old points ignition systemscould put out. This higher voltage capa-bility is now standard on all vehicles.Most of the time, the coil needs to pro-duce only between 10 and 20kV—notmuch different than in the old days.This is because the actual coil output isalways just what’s needed to fire thespark plug gaps, no more and no less.

So why is the extra reserve needed?Under normal conditions, the engine will ask the coil for higher-than-normalkV output during heavy acceleration orheavy loading. The engine will also re-quire higher kV if the fuel mixture is ei-ther too rich or too lean. A completelack of fuel really forces the coil topump out extra-high kV, as it attemptsto ignite fuel that isn’t there. The extrastress of cranking the engine at cold

temperatures when there’s no fuel inthe cylinder may send an otherwise per-fectly good coil to an early grave.

A worn-out distributor cap, rotor,spark plugs or plug wires also force thecoil to put out higher kV, to compensatefor the losses as the spark tries to makeits way to the end of the line. Plugs,plug wires, caps and rotors increase inresistance as they age. If these parts arenot replaced when they become worn,the increased resistance cuts spark du-ration and spark energy delivered to themixture. In addition to reducing engineperformance, the coil’s useful life may also be cut short by worn ignition parts.Coil life span is directly related to theamount of time it spends producinghigh voltage (.20 kV).

Many late-model vehicles have dis-pensed with distributor ignition systemsand are equipped with coil-on-plug ig-nition systems, similar to the one in thephoto on this page. While these systemshave cut down on the number of mov-ing parts and “wear items” have beenreduced, problems can still occur. When installing replacement boots oncoil packs, don’t forget to use bootgrease. The grease improves the sealbetween the boot and the tower on theignition coil, which keeps the coil’s high voltage headed in the right direction (tothe spark plug).

Dried out or cracked boots must bereplaced, as they can no longer hold thisseal. Once an arc has gone through aboot or spark plug wire, a burn path iscreated. After that, arcing is most likely to reoccur when maximum coil kV is re-quired (during high load or accelera-tion). A snap acceleration test in a darkroom often will reveal intermittentfaults. Internal failure had caused thecoil pack in the photograph to burnthrough completely, producing anythingbut an intermittent problem!

70 August 2003

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SECONDARY IGNITION

BY KARL SEYFERT