How to Troubleshoot a Mallory Unilite Distributor With No Spark

by Don Bowman

When a Mallory Unilite distributor fails to produce a spark to the plugs, the vehicle will not run. The Mallory Unilite distributor uses an infrared light lens and a sensor between which a wheel with tangs, called shutters, passes. The infrared light makes a completed circuit when the sensor picks it up. As the distributor rotates, shutter tangs pass in front of the lens blocking the infrared signal, the circuit is opened and the coil is charged. When the shutter passes, and the signal is restored, the coil is fired discharging its voltage through the plug wires.

Locate the coil, which will always be close to the distributor, and see if it is a 12- or 6-volt coil. All vehicles need a coil, which acts like a capacitor to build enough electrical energy to fire the spark plugs. The distributor simply directs the spark to the proper cylinder. Many coils are 6-volt and need a ballast resister installed in the power line to the coil. If a 6-volt coil is used without the resister it will eventually overheat. If it is a 6-volt coil, check the coil side of the resister for voltage with the key on. Place the black ground voltmeter lead on a good ground on the engine and probe the coil side of the resister. It should be the target voltage of 6 volts. If not replace the resister.

Check the positive side of the coil for the proper voltage, be it 12 or 6 volts. Repair the wiring if the voltage is not correct. Move the red probe to the negative terminal of the coil. The voltage should be the same. If not replace the coil.

Remove the distributor cap and pull off the rotor. Inspect the carbon secondary button in the center of the cap for severe wear or irregularities. Replace the cap if the button is missing or worn.

Place the red lead on the negative terminal on the coil. Place a credit card between the infrared light and the sensor. The voltage must drop below 3 volts. This is imperative. If the voltage does not drop below 3 volts replace the Unilite module. If the voltage does not rise back to 12 volts at the coil negative terminal once the card is removed, replace the module.

Tip

  • check The Mallory Unilite is sensitive to voltage spikes. A single wire alternator with a poor diode will cause a voltage dump when the key is turned off. This can send a 30 or higher volt spike through the system. As a matter of fact, a bad diode trio in a conventional alternator can do the same damage. Short circuits, high resistance in plug wires, too large a gap in the plugs or sparking the battery are all possible causes of a failed module. Remember, Unilite modules have no parts to wear and will last forever under normal circumstances. If one fails, do not install another until the source of the problem is found and repaired.

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About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).