Automotive History: Electronic Ignition – Losing the Points, Part 2

In the last installment I discussed some of the early transistor ignitions that were offered in the 1960s.  GM, Ford and Chrysler all used similar ignitions where more efficient and faster switching transistors supplemented or replaced the mechanical points.  However, adding transistors to a standard ignition wasn’t the only way to improve upon the weaknesses of the breaker-point ignition.  General Motors Delco-Remy division didn’t put all of its eggs in one basket, and actually made a second electronic ignition system during this time, but one that used a different concept all together.

Most automotive ignitions are inductive ignitions.  This means the ignition relies on inductance between the primary and secondary windings in the coil to create the high secondary voltage.  As discussed in part one, as RPM’s increase, there is less time for the coil to charge, and the result is the ignition energy is reduced at high RPM.  Nevertheless, this isn’t the only way to produce a high secondary voltage.  Another concept is capacitive discharge ignition, which has been around since the beginning of the century.  However, it was the advent of the transistor that made this ignition system much more palatable for use in production vehicles. Commonly called CDI or CD ignition, Delco-Remy released its transistorized CD ignition for 1967 Oldsmobile and Pontiacs.

Oldsmobile’s version of the Delco-Remy CD Ignition Control box. This box has been modified with a clear cover on the rear to see the internal components.

The key concept behind the CD ignition is that it uses a capacitor.  A capacitor is a component that is used to store an electrical charge.  Inside the CD ignition control box is an invertor and transformer which charge up the capacitor within. The invertor draws power from the battery and the transformer steps up the voltage to about 300-400 volts.  This is stored in the capacitor.  When the primary circuit is opened, the capacitor discharges its output to the ignition coil’s primary side very quickly.  This induces the secondary circuit, which further steps up the approximately 300-400 volts by a factor of approximately 100 times, and the spark plug fires.

CD Ignition showed significant advantages over conventional ignition, in particular at high RPM, cold starts or fuel soaked conditions.

A big advantage of the CD ignition is that the ignition energy is created by charging the capacitor rather than the coil.  A capacitor can be charged in a fraction of the time it takes to charge a coil, which means that a CD ignition will not lose energy as quickly at high RPM like an inductive system.  In addition, some CD ignition systems are powered directly from the battery by large heavy gauge wires for minimal resistance.  The higher voltage from the capacitor is put into the coil, which acts as a step up transformer, results in correspondingly higher secondary voltage, about 100 times the primary voltage (depending on the turns ratio of the coil). This means that the spark produced is significantly stronger than that of an inductive coil. As a result some CD ignitions, like Delco-Remy’s, used a specialized coil.

Delco-Remy aftermarket CD ignition was points triggered. Note specialized coil, similar to later “e-coils” used by newer ignitions.

Like a transistorized inductive ignition, CD ignition can be triggered by mechanical breaker-points or by a magnetic pickup coil assembly.  If triggered by a set of points, very little power actually goes through the points resulting in very minimal wear as they are simply used as a triggering mechanism.  The ignition’s electrical current bypasses the points, and is supplied from the battery to the ignition control box (which contains the capacitor) to the coil.  The end result is significantly reduced breaker point wear and maintenance. The regular production option version of the Delco-Remy CD ignition, however, used GM’s advanced magnetic pulse distributor.  With this, maintenance was reduced even further, and the ignition would stay in tune far longer. Delco-Remy also sold the CD ignition in the aftermarket as a retrofit kit and offered a version that allowed it to be triggered by a breaker-point distributor.

Typically a CD ignition has significantly higher secondary voltage than a standard breaker point ignition, and the end result is a far stronger spark.  In fact, it was so strong; GM and other aftermarket CD ignition manufacturers, boasted that it would fire a fouled or fuel soaked spark plug.  The high voltage and the ability to fire fouled plugs, meant that spark plugs would last longer and the car started better in cold conditions.  Further because the quick charging capacitor could fully charge in much shorter time, CD ignition was far better at high RPM.

The Delco-Remy Capactive Discharge ignition was available on Oldsmobile and Pontiacs as a regular production option.  In appearance it looked very similar to the Delco-Remy transistor ignition, consisting three specialized components, a control box, a special red coil and a magnetic pulse distributor.  However, it should be noted the more complex CD control box was considerably larger than the transistorized amplifier.

The 1963-66 Pontiac transistor ignition, option 671, was an inductive transistorized ignition as described in the previous article.  For 1967, despite the lack of name change, the Pontiac transistor ignition option was revised to be the Delco-Remy CD ignition. It was a $114 option and only available on premium fueled engines without an AM/FM radio.  Oldsmobile also added the same CD ignition to its option list for the 1967 model year, calling it the “Ultra-High Voltage” or UHV ignition.  It cost about $100 and was offered in 1967-68 under option code K-66 on Oldsmobile 400, 425, 455 engines.  Some sources claim it was also available in 1969, but I couldn’t find an option listing for 1969.


A modern MSD CD ignition wiring diagram. These systems remain popular with the muscle car and hot rod crowd, because of the aforementioned advantages of CD ignition. A modern MSD CD ignition has more output than the Delco-Remy CD ignition.

The Delco-Remy CD ignition was relatively short lived.  Like the Delco-Remy transistor ignition it reduced maintenance but it required more costly components, was relatively expensive and probably few outside of serious high performance drivers noticed any significant difference.  It could also be argued the added complexity of CD ignition made it less reliable than other choices.  This ignition offered much stronger spark with a higher voltage, but the trade-off was that it was very short in duration. So while this is a great ignition system for a high revving, rich running, muscle car engine, it is not the best choice for an engine that needs complete combustion to minimize emissions.  Longer spark duration became more important as emissions regulations moved to the forefront.  Despite little use in production cars, aftermarket CD ignitions were popular upgrades in 1960’s and 1970’s. Today they remain popular for the muscle car crowd, in particular MSD branded systems.

With new emission regulations beginning in the late 1960’s, manufacturers needed to come up with a better ignition system to help reduce emissions.  While past electronic ignitions were more focussed on high-performance, the focus shifted to meeting emissions requirement.  The big advantage of an electronic ignition was that it was more accurate, stayed in tune longer and reduced maintenance.  This also helped ensure an engine burned cleaner for a longer period.  First to develop a mainstream electronic ignition was Chrysler.  Introduced in May 1971 on 340 manual powered cars, it became a regular option on some 1972 Chrysler products.  For 1973, all Chrysler products had the new Electronic Ignition System (EIS) as standard equipment.

Wiring Schematic for the early version of Chrysler EIS.

The Chrysler EIS was similar to the Delco-Remy transistorized ignition.  It eliminated the breaker-points in the distributor, and used a magnetic based pickup in place of breaker points and rubbing block.  The ignition was controlled by an external device which contained the transistors.  Chrysler called this the electronic control unit or ECU rather than an amplifier.  From the outside, the distributor didn’t appear to be significantly different but inside was a different story.  Below the rotor and attached to the distributor shaft was a reluctor, replacing the traditional rubbing block.  The reluctor had 6 or 8 spaced ridges, depending on the number of cylinders. Mounted stationary in the distributor was the pick-up coil assembly.

Other than internal changes, the EIS distributor wasn’t significantly different.

Like the Delco-Remy distributor, as the reluctor rotates, one of the ridges will pass the pickup coil and it will produce small positive voltage.  When one of the spaces passes the pickup coil, a negative voltage is produced. The end result is a small AC voltage.  When the positive voltage begins, this will signal electronic control unit and the fast switching transistors will open the primary circuit, which then induces the secondary circuit and the spark plug will fire.

Chrysler EIS distributor in detail. This shows detail on how the AC voltage signal is produced to signal to control box.

This ignition was a big improvement over the old breaker points, allowing for more precise ignition timing, less chance of misfire, improved starting and high RPM performance while reducing maintenance requirements.  However, this system still restricted the power to the primary circuit.  Chrysler continued to use a ballast resistor.  It was different than the previous resistor, in that it had dual roles.  It reduced the voltage at to the primary circuit with the same 0.5 Ohm resistor as the Chrysler breaker point ignition, while a second side of the resistor was a 5 Ohm resistor that protected some of the electronics in the ECU.  Since the resistor reduced the power to the ignition coil with the same 0.5 Ohm of resistance as the breaker-point ignition, it meant that this ignition had the same overall amount of energy to the previous breaker points system.   However, this saved production costs as the ignition did not require a special coil. The Chrysler EIS was revised over the years, with different control boxes variations, and a change back to a single ballast, but the system still had the same fundamentals.

The dual ballast resistor and the control unit.

Back at GM, we know that the original Delco-Remy transistor ignition dated back to the early 1960’s and was last used on 1971 Chevrolet high performance engines.  On the other hand, Pontiac who’d been a pioneer of electronic ignition hadn’t offered any electronic systems since the late 1960s when the CD ignition left the option list.  Delco-Remy designed a modern ignition to replace the transistor ignition and Pontiac ended up being the division to use it.  This new ignition was called the Unitized (or Unit) Ignition and it incorporated several design features to help address issues from the original transistor ignition.

The extra complexity of the Delco-Remy transistor ignition was one area that engineers addressed.   The engineers decided the best solution was to combine all ignition components into one “unitized” component, hence the name Unitized Ignition.  This system consisted of a specially designed distributor which contained magnetic pick-up coil unit, a mechanical advance mechanism, an ignition module and the coil.  It sounds quite similar to the HEI distributors that GM would release later in the decade, but there was one major difference.  The Unitized ignition wires were a one piece specialized part.  Meaning you couldn’t just replace one wire, you had to buy a proprietary set.  It also used a specialized distributor cap and ignition coil, all of which were significantly more expensive and not found in the aftermarket.

Unitized Ignition’s components were all contained within the distributor assembly. Note the propriety wire set, which included the distributor terminals

Another enhancement with this ignition was that it provided 12 volts of power to the primary circuit – there was no resistor used.  With no resistor, the Unitized Ignition energy was further increased.  The higher primary voltage results in a higher secondary voltage. The previous Delco-Remy transistor ignition used a resistor on the primary side of the ignition, meaning it reduced primary voltage and the overall ignition energy.  However, the resistor used on the Delco-Remy transistor ignition was less restrictive resistor than the breaker-points ignition.

The other major advancement was the reduction in size of the amplifier, which GM renamed the ignition module.  The module was made significantly smaller allowing it to be mounted on the distributor housing.  The Unitized Ignition eliminated the complex wiring harness with multiple connections of the transistor ignition system to only one 12 volt connection to the distributor.  Largely, the Unitized Ignition was really just further refinement and evolution of the original magnetic pulse transistor ignition, bringing it one step closer to the HEI system.

The unitized ignition was first made available on 1972 Pontiacs, but it was not overly common.  It was initially a mandatory option for the 455 HO engine, but Pontiac made a change partway through early in the model year making it a regular production option, listed at $77.  It remained on the option list for 1973 and 1974 until it was phased out and replaced by the HEI system.

Ford’s Solid State Ignition was very similar to Chrysler’s

Ford was the next manufacturer to introduce a mainstream electronic ignition.  First used on late 1973 Lincoln engines, Ford introduced what they called Solid State Ignition.  All 1974 California cars received this ignition, along with 49 state cars powered by 400 and 460 engines.  In 1975 the ignition became standard on all Ford cars.  Like Chrysler, this ignition’s primary purpose was to help reduce emissions.

Ford Solid State Distributor in close detail and it is very similar to Chrysler’s. This diagram shows how the AC voltage is produced.

The Ford ignition was very similar in design to the Chrysler ignition.  It too used a distributor with a magnetic pick and an armature (Ford’s name for a reluctor) and even the internal components looked very similar.  The ignition contained a large externally mounted control box, which Ford called a module.  This module contained the transistors that were triggered by the AC voltage of the distributor pickup.  Like Chrysler, Ford kept a resistor in place, albeit a resistor wire, and the resistance value was the same as the breaker point ignition.  This means the overall ignition energy was not increased over the old breaker point ignition.  Initially this allowed for reusing the same coil, but a running change for 1975 used a new ignition coil with lower primary resistance which utilized a special connector.

At first glance these might appear the same, but each Ford revised the wiring and module for each year from 1974-76.

The Ford ignition system essentially functioned the same as the Chrysler EIS. It too improved starting performance, emissions and high RPM performance while reducing maintenance.   However, Ford being Ford, resulted in its engineers changing the ignition on an annual basis from 1974-76.  Over that three year span Ford redesigned ignition module each year.  And although the systems appeared identical all of the wiring was also revised each year.  This meant that each year from 1974 to 1976, Ford used a specific module and wiring harness, none of which interchanged with other model years. This ignition eventually became more refined and consistent, evolving into the Dura Spark ignition.

As time went on, it became apparent that while electronic ignition wasn’t overly popular as an option, but it did become a necessity for producing cleaner burning engines. In the final installment of this series, we will look at the final mainstream electronic ignitions from Ford, GM and Chrysler from the mid to late 1970s. Ford would develop its Dura Spark I and II systems, GM would develop its HEI system and Chrysler EIS was further changed with its Lean Burn system becoming the first to incorporate some basic computer controls.

A special thanks to Daniel Stern for supplying some of the research material on vintage ignition systems.

Further Reading:

Automotive History: Electronic Ignition – Losing the Points, Part 2

Automotive History: Electronic Ignition – Losing the Points, Part 3