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Mallory . Module Testing

The Mallory Unilite and E-Spark Ignition Modules are highly accurate and reliable ignition triggers and amplifiers. The self-contained module consists of an integrated photo coupler, a signal processor, and a power switch. Each time the special shutter wheel rotor blocks the infrared light from reaching the photo coupler, the electronic circuitry energizes (charges) the ignition coil. When the shutter wheel allows the infrared light to reach the photo coupler, the electronic circuitry allows the stored energy in the ignition coil to discharge to the spark plugs. The Mallory photo optic modules do not require any adjustments or maintenance, unless you live in an extremely dusty/silty location that may justify the need to periodically wipe the optic lens.

What’s the Difference?

The #605 Unilite and #6100 E-Spark are “basically” the same module. However, the presence of the newer #6100M tends to confuse many people. Both look similar (in fact, the ‘new’ #605 is nearly identical to the #6100M and Accel #2005 module). Prior to the release of the E-Spark module, decision makers at Mallory wanted to improve the design of the Unilite module. The driving factors for this decision included improving the manufacturing processes (more automation versus hand assembly), and improving module durability. I have no rational explanation (I was unable to get Mallory executives to answer) why they created a separate part number, instead of simply stating “Look, a redesigned Unilite!”

The decision to offer two, virtually identical but differently priced ignition modules has proven problematic and confusing for the Mallory customer. Initially, why would someone pay nearly twice as much for an older design module that was less durable? Later, when the ‘old’ design #605 internal components were discontinued, customers were simply paying a premium for purchasing a “Unilite” (by name) versus the “E-Spark”—or for that matter, purchasing the Accel 2205 Eliminator. They’re all the same!

Suddenly, Mallory was offering identical modules and ignition conversion kits, with the newer design E-Spark module priced lower than the older design. Who does this? The newer design module was not included in complete distributors. Meaning, it was only available as a replacement module or conversion kit (from breaker point ignition).

The Mallory (Prestolite Performance) executives’ cure for this faux pas—offer different warranty terms. The modules were now identical, both product names and part numbers were now using the same internals. Purchase the E-Spark and receive a 12-month warranty. Purchase the more expensive Unilite module and receive a 24-month warranty.

The newer design modules are more durable. Photo-optic operation is the same across all the modules, but the new design modules utilize an improved circuit board. The new circuitry is slightly more tolerable of dirty power signals and minor power spikes. Power spikes, poor vehicle grounding, sticking alternator diodes/voltage regulators, battery chargers, and all the other common causes of Unilite module failures can still damage the newer design modules. This is true for most electronic components on our vehicles. We do suggest avoiding the older-style #605 module. 

Mallory 605 Unilite photo optic module
Mallory 6100M E-spark photo optic module
Accel 2005 photo optic module

Bad Reputation?

Many customers who have called for help with testing procedures or replacement suggestions for the Mallory Unilite or E-Spark modules have mentioned various web forum or social media posts that claim the Unilite/E-Spark is an unreliable piece of garbage. Claims of the modules being too sensitive, constantly fail, or other derogatory comments are common. We can tell you that many of the people posting those comments should probably not be working on their vehicles without skilled supervision. Many of these individuals are blaming the problem on the effect (a blown or damaged module) instead of what actually caused the module to fail.

Consider this analogy, often offered to our customers when discussing ignition module reliability and the various causes of module failure. Four brand new tires have been purchased and installed on a vehicle. The driver knowingly drives through a pile of sharp steel, flattening and effectively destroying the new tires. Should blame for the destroyed tires fall upon the tires as a junk product, or the tire manufacturer, or the store that sold them? The answer provided 100% of the time is, “of course not!” Often, the comments include that the driver was obviously an idiot!

Using the above comedic example, we can ask ourselves why people find it so easy to blame the ignition module itself, and not the cause? The easy answer is a lack of understanding of the product. It is easy to blame a component that someone knows little or nothing about. The ignition modules cannot really kill themselves, so we must try to diagnose what actually caused the failure.

There are many Mallory customers who have successfully used the same Unilite module for 20+ years and other customers who have destroyed three modules on a Saturday afternoon! The gentleman who experienced three same-day module failures had recently purchased a rebuilt alternator from a typical auto parts chain store. After discussion, and with considerable push-back by the costumer he decided to go back to the parts store and swap out that new rebuilt alternator for another (He actually picked one up from a different store). It was initially suggested to take the alternator to a qualified shop or electric component rebuilder. Instead, the second rebuilt alternator presented the same problem—a module killer. Even though both rebuilt alternators were manufacturer-tested and warranted components, the store “bench tested” them on their equipment in front of the customer, showing they “worked fine”. However, the parts store test equipment only tests for charging and output status. It does not test for the cause of the problem that is damaging the ignition modules. Both alternators were killing modules, and both passed bench tests at this parts store.

Of course, because this occurred on a weekend, qualified shops to test the alternator were unavailable. While trying my best to assist this customer I endured considerable screaming, cussing, and personal insults. I was finally able to convince this gentleman to take the alternator(s) to a local electric rebuilder on Monday for testing.

The Monday testing found that both alternators were rebuilt using cheap (for the cost savings) diodes. The diode is the internal voltage regulator on single-wire alternators. The diodes in each alternator were observed on an oscilloscope to be “sticking,” causing a dump of considerable energy to ground (the vehicle chassis/engine). A fast oscilloscope or other digital equipment is required to perform this type of test. As the technician at this local auto electric rebuilder tested these units the customer was able to see that a 140+ amp spike was being sent to ground every time he turned off the ignition key. This explained why the engine would start once with the new module, but never again. Replacing the diode in one of these so-called “new rebuilt” alternators solved his module-killing problem. (He took the other unit back to the parts store, and surely gave them some of the hell he gave me over the weekend!)

NOTE: It is possible to have intermittent problems with the Mallory ignition modules. A damaged, but not completely dead module, can present intermittent ignition failure. Driving along, and the engine suddenly stops. Inspection of every possible component of cause finds nothing obviously wrong. In a sort of frustration or last resort, an attempt is made to start the engine. Surprisingly, the engine roars back to life. This may occur daily, once a week, or less often. Testing of the module will likely fail. This is almost a benefit, in that in some instances we’re not indefinitely stranded with a non-running vehicle. In a way, the intermittent failure is providing a warning. Hopefully, the person operating the vehicle recognizes the sign and acts upon it. Test and replace the module if found to be defective!

Step-by-Step Module Testing

Testing the photo-optic Mallory (and Accel) ignition modules is fairly easy and straightforward. The procedure consists of measuring voltage at various points, and then performing a “voltage drop” test.

The CD or inductive amplifier box is an intermediary controller between the distributor and ignition coil. The amplifier box picks up a triggering signal from the distributor and then tells the coil what to do. The ignition amplifier unit manages nearly all aspects of ignition coil function. Because the amplifier also provides low-power supply energy to the distributor, and the distributor replies with a low-power (un-amplified) triggering signal, this energy signal from the amplifier unit interferes with the proper testing procedure of the Mallory ignition module.

Bypassing the ignition amplifier unit for testing is usually simple. It is not necessary to remove it from the vehicle. Disconnection from distributor and ignition coil connections is all that is required. Some amplifier brands provide a specific harness or connector to allow bypassing the unit. Without some type of manufacturer-provided bypass, unwire the unit from the distributor and coil! After removing the amplifier, a ballast resistor (or internally-resisted ignition coil) is required for testing. The ballast resistor is required to prevent damaging a good module (if an attempt is made to start the engine). Once the ignition amplifier is unwired from the system, follow the distributor instructions for proper installation—using ONLY the distributor, coil, and resistor. See graphic below for a basic example:

Tools Required:
    • Voltmeter or Digital Multimeter (DMM)
    • Credit card (or piece of thin cardboard)
fluke 88V 07 200p 1
Unilite wiring diagram

Module Testing Steps

Please Follow Each Step, Reviewing All Notes For Each

Take the digital multimeter (DMM) and measure battery voltage. Ensure the DMM is set to DC Voltage Current for all of the steps described in this test procedure. Connect the DMM across the battery terminals. Connect the red lead to the positive (+) terminal, and black lead to the negative (-) terminal. Read the meter.

Typical battery voltage will measure between 12 and 14 volts on a fully charged 12-volt battery.

Mallory Unilite distributor rotors are usually secured tightly to the rotor shaft, but this is only a press fit. To remove the rotor, carefully grip as much of the rotor as possible and firmly pull straight up using steady pressure.


Honestly, finger pain is possible until finding the proper grip. Equally important, excess pull pressure can result in one’s hand making contact with other painful components on the vehicle. The hood, firewall, accessory items, etc., are all potential hazards. I’ve done it. It hurts. Be careful!

It takes practice and experience, so don’t expect to escape unscathed the first time.

Turn the ignition key to the ON position. Connect the DMM to measure battery supply voltage at the coil.

  • First, check voltage at the PRIMARY (or positive) [+] side of the ignition coil. Connect the black lead of the DMM to a good ground (engine block preferred). Connect the red lead to the primary (+) side of the ignition coil. The meter should read equal to or very close to battery voltage.
  • Next, check voltage at the SECONDARY or negative (-) side of the ignition coil. Leave the black lead of the DMM connected to engine ground. Move the red lead of the DMM to the secondary (-) side of the ignition coil. The meter should again read equal or close to battery voltage.

— If testing for battery voltage passes these checks, continue to the next step. If battery voltage is not present or equal/close to battery voltage on both sides of the coil, one of two problems exist:

  • Ensure a power supply problem does not exist. Take a jumper wire from the positive (+) terminal on the battery to the primary (+) side of the ignition coil. Retest using the DMM to verify battery voltage at both the primary (+) and secondary (-) sides of the coil.
    • Does battery voltage now exist on BOTH sides of the coil? If yes, additional diagnosis is required to determine why power does not reach the coil through normal vehicle wiring. Remove this power jumper wire.
  • If battery voltage is low on ONLY the SECONDARY (-) side of the coil, the module is damaged and is artificially energizing the coil. This does not occur on a properly functioning ignition module. Replacement of the module is necessary.
    • The module should only charge the coil when the photo-optic is blocked. Low voltage on the secondary side of the coil with an unblocked optic is one example of a damaged Unilite / E-Spark / Eliminator ignition module.

With the DMM still connected to the secondary side of the coil from Step #3 (black DMM lead to engine ground, red DMM lead to the secondary side of the coil) this step requires momentarily blocking the photo optic L.E.D. on the module with a credit card or thin piece of cardboard.

While observing the screen on the DMM, place the credit card between the towers on the module to block the optic. The voltage should immediately drop below 2.0 volts. It may drop for only an instant, or it may hold below two volts until unblocking the module. Either is acceptable. However, IT MUST DROP BELOW 2.0 VOLTS!

If voltage does not drop, stops above 2.0 volts—even if it drops to 3.0 volts—the module is damaged and requires replacement.

Now, unblock the optic while reading the DMM. The reading on the DMM MUST return to battery voltage. If the voltage does not immediately return to battery voltage, remains at the “optic blocked” voltage, or does not fully recover, the module must be replaced.

In simpler terms:

Voltage must drop below 2.0 volts when the optic is blocked. Voltage must return to battery voltage when unblocked.

Based upon the test procedure above a few conclusions materialize:

  • If battery voltage is not initially present, test the battery and terminal connections. The battery may be low—charge it.
  • If battery voltage is not present at the ignition coil, either an electrical problem exists before the coil (corroded battery cables, electrical wiring, ignition switch, etc.), or the module is charging the coil. If the module is charging the coil without blocking the optic, the module has been exposed to voltage spikes or installation errors. Replace the module!
  • If voltage does not drop when the optic is blocked, the module is “open” and this was likely caused by a power surge, excessively high resistance in the spark plugs or spark plug leads, or improper vehicle grounding. Replace the module!
  • If the voltage remains below 3.0 volts it means that the module has been spiked by high voltage or amperage, lack of a ballast resistor (if not using an amplifier box), or the wiring was incorrect. Replace the module!
  • If the voltage only drops to 3-4 volts the engine will likely experience a noticeably weak spark. Although this is also caused by poor grounds, or a power spike, it could also point to excess resistance in the electrical system before the coil (multiple resistors: ballast and resistance wire, internally resisted coil and ballast, etc.) Make sure excess resistance does not exist, then replace the module!
  • A faulty charging system (stuck or shorted regulator/alternator), or high amp single wire alternators with a cheap diode. This can cause power spikes (voltage dumps) to ground which can reach and damage the module.
  • Ineffective or inadequate vehicle grounds. The module brown wire must be connected to the engine block. Proper vehicle grounding consists of: Battery to Engine; Battery to Body; Engine to Frame; Engine to Body; Body to Frame.
  • Using battery booster-charger to start the engine. Most of these booster-charger units provide extremely dirty electrical signals. These dirty signals cause power surges or spikes. Proper battery charging procedure requires disconnection of the battery from the vehicle’s electrical system, charging the battery, and then reconnecting the battery. Now, with a fully-charged battery the vehicle can be started normally.
  • Using non-suppression spark plug wire leads. Suppression core type spark plug wires, not solid core wires (stainless steel or copper), are required on all electronic ignition systems. Spiral core is best, but carbon or any suppression core plug wire will also work. Solid core wires can cause a back-feed of energy into the ignition system, spiking and damaging the ignition module.
  • High amp stereo equipment that is not properly grounded or when the vehicle grounds have not been updated (larger diameter) to handle the added electrical and capacity. The energy surges created by high power media systems can play havoc with a vehicle’s electrical system. Proper sized cabling, using relays, and adequate battery supply is imperative!
  • Direct shorts in the vehicle’s electrical system. Loose wiring connections, or other failing electrical components.
  • Welding on the vehicle or working on the vehicle’s electrical system while the distributor harness is connected. Unplug the 3-wire distributor harness when performing any type of work on the vehicle.
  • Faulty starting system (excess starter drag, not enough voltage/amperage getting to the starter, worn brushes/armature). The power surge when cranking can damage the ignition module.
  • (Not that many people are using these anymore!) CB radio spiking on mic click into electrical system (typically only on power modified CB radios).
  1. Verify battery supply voltage
  2. Remove Distributor Cap & Rotor
  3. Turn Ignition Key ON
  4. Is there Battery Voltage (+/- 14v) on the PRIMARY side of the coil? YES = GOOD!
  5. Is there Battery Voltage (+/- 14v) on the SECONDARY side of the coil? YES = GOOD! None, or Considerably Less Than the Primary Side of Coil = BAD!
  6. Voltage Drop Test – Voltmeter still on Negative side of coil. Voltage Drops BELOW 2.0v = GOOD! NO Drop = BAD!
  7. Voltage RECOVERS to BATTERY VOLTAGE after unblocking the optic? YES = GOOD; NO = BAD

Any one BAD in the procedures above, equals no need to test further — replace the module.

A Few Tips and Lesser Known Facts

The Mallory Unilite®, E-Spark®, and Accel Eliminator modules require a drop in voltage on the trigger side (green wire) of the module. Although the power supply side (red wire) can handle 14.0+ volts, a ballast or low voltage controller signal is required. Furthermore, although the ballast resistor is wired inline on the primary (power supply +) side of the ignition coil, it is actually dropping the voltage to the secondary (trigger side -) of the ignition coil. Subsequently, this means that ONE of these components are required as part of the installation:

  • The correct ballast resistor. For example, the Mallory part #700 variable ballast resistor is the best, but another ballast of compatible specs is also acceptable (usually around 0.85 Ohms).
  • An OEM resistance wire in the factory wiring harness of the correct length so that the proper voltage regulation is achieved.
  • An internally resisted ignition coil (Warning: Internally resisted ignition coils cannot be used with external ignition amplifier units). These coils eliminate the need for an unsightly external ballast resistor.
  • An ignition amplifier, such as the those offered by Pertronix, Mallory, Crane, Accel, MSD, or other comparable units. Aftermarket ignition amplifiers provide a low voltage signal to the ignition module, eliminating the need for any other type of voltage reduction.

Optional Module Protections

Adding other components to the ignition system can provide limited “minor protection” as an attempt to prevent ignition module damage.

Installing these components provides no guarantee of preventing a voltage spike or other event from damaging or destroying the module.

Options include installing the Mallory Circuit Guard, part #29371. This simple component plugs inline on the 3-wire distributor harness and claims to provide two features.

  1. Electrical signal filtering that cleans up a dirty power signal caused by poor electrical connections, or electrically-charged dust in the alternator. Compare this to the seemingly ancient rabbit ear antenna sitting on top of an old television set. The rabbit ear antenna never provided a perfectly clean picture, but it took away some of the snow. The Circuit Guard removes some of the peaks in a dirty electrical signal, similar to the old rabbit ear antenna.
  2. A compact capacitor within the Circuit Guard is designed to absorb “small” power spikes, preventing them from reaching the module. It does absorb some small spikes, but is not a guarantee to protect the module. A decent sized spike will still get through! This is not a replacement for laziness in finding the cause of module failures; although, this component is inexpensive insurance to help prevent module damage.

More on Module Reliability and Comparisons

For reference, the Mallory Unilite®, E-Spark®, or Accel Eliminator modules are no more or less sensitive than a GM HEI, Ford Duraspark, or other brand ignition modules. Most people don’t correlate the failures of one type of component to between brands, or have knowledge of failure causes. Parts stores and product manufactures often simply hold out their hands for customers to drop more $$ on a new module with no explanation.

The main issue with these photo-optic modules (as presented to this author by Mallory engineers) is due to component size, and a patent issue that Delco/General Motors holds and has not been released for general licensing or use. Mallory modules require an external ground wire connection that leaves them susceptible to attack through the ground circuit. This sounds silly, to the point of ridiculous, because all vehicle electrical components require grounding. However, the size of most electrical components allow for internal circuitry that mitigates possible electrical problems linked to voltage spikes or other issues. For Mallory, this presented a problem related to size. To avoid a legal issue with General Motors, regarding patent infringement, the module would require a 2-3 times increase in size to provide similar protections that the common GM HEI module possesses. Such an increase in size would prevent the photo-optic module from fitting inside the distributor. Mallory wants to keep these distributors self-contained.

This is also why an amplifier unit, ballast or other resistor is required. These units are still the most accurate ignition triggers available and offer other benefits! If the vehicle’s electrical system is in proper working order, maintained, and the vehicle owner or mechanic are properly trained, module failures should be few.

In Closing

As mentioned near the beginning of this article, sometimes the ignition module will experience an intermittent problem that can be difficult to diagnose. For example, intermittent stalling may occur. Driving along and suddenly the engine stops. The most common time for this occurrence is soon after the engine has reached normal operating temperature. The first response is to try and restart the engine. The next entails getting out of the vehicle, opening the hood and investigating for the most common causes: (air/fuel/spark). Does the engine receive fuel? Has a battery or ignition cable come loose? Does ignition spark exist? Others may simply stand over and peer into the engine bay, waiting for something. If nothing looks obvious, then what?

That perplexed look turns to frustration, and out of that frustration another attempt to restart the vehicle. It Starts!? More confusion! Did it fix itself? A more reasonable explanation addresses one of the most troubling automotive issues—the intermittent problem. Often difficult to diagnose, but relatively easy to fix.

In this example, an experienced tech will hopefully consider heat or reaching normal operating temperature as a possible cause. In the very least it could be related to the cause of the problem.

Using an analogy, consider our bodies, and especially for those of us getting older or being less active. Our bodies don’t always warn us in advance of doing something that we’ll pay for later. If we go out on the weekend and are horsing around with our kids, or doing out of the ordinary yard work or home projects, helping a friend move, or playing sports, we may feel tired but still feel pretty good from working a few muscles that do not get used as often anymore. Sore muscles or a mild headache may kick in later that evening, reminding us of the day’s activities. The next day we may feel perfect, almost renewed! Then the second or third day comes along …

A few days later we may feel like absolute garbage. We can barely move, various muscles in our necks, backs, legs and shoulders are aching. Sleeping may be uncomfortable that second or third night.

Why am I describing this painful retrospect? I’m using this example to describe what has happened to the ignition module!

Of course the module was not out climbing mountains, shooting hoops, drinking too much beer or running bases, but at some point the module was affected by something that damaged it. The vehicle stalling experience is the module’s way of telling us that it has been hurt and needs to be checked and/or replaced. This can be a benefit with the Mallory module, by not leaving us stranded somewhere, offering a little advance warning. When the module got up to temperature, something in the circuitry shut off the flow of power through the module and it was now unable to do its job until it cooled slightly. This may not occur again for days or weeks, but if the stalling becomes regular it is definitely a tell-tale to take action. Most of the time, performing the module test procedure above will provide conclusive evidence that it needs to be replaced, but in rare instances the module will pass the test above and still be at least partially defective. Replacing the module would be the last resort.

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