There are two types of Mallory HEI Distributors: 75 series and 85 series units. The 75 series HEI uses a Module with a Rev-limiter - Module Part #699. The 85 series HEI uses the Module Part #607.
NOTE: Pick-up coils for the HEI distributors are NOT available as replacement parts.• Remove both the distributor cap and rotor, then turn the ignition ON but DO NOT START car.
• Now connect your Red voltmeter wire to the NEGATIVE terminal (-) on the coil, by attaching the Red lead wire clip from voltmeter to the (-) terminal.
• Next attach your Black voltmeter wire to an engine ground on a good ground source. We suggest a manifold, or header bolt. Most anywhere on the engine is good.
• Now the voltage should read about 11 to 12 volts at this point. Remember to turn the key on for this reading! 12 volts must be present on your voltmeter.
• Next, place a credit card down into the distributor, blocking the 2 photo optic eyes on top the module facing each other on the top. With eyes blocked, the volts should drop way down below 2 volts or less. This drop is a must! If the voltage drops down to anything under 2 volts, then the module is good and ready to use. If the voltage does NOT drop, the module will need to be replaced.
• This damage could have been caused by many things, such as: power surge from an alternator, high resistance spark plugs and plug wires, improper ground wires, if someone has given you a jump, if you have given someone a jump, or battery chargers, which can also damage ignition modules. If voltage always stays below 2 volts and never increases up to 11-12 volts, then the module has been spiked by high voltage and needs to be replaced.
NOTE: The lack of a ballast resistor will cause this type of spike. If the voltage only drops down to 3-4 volts, this will produce a weak spark: too weak to run engine and the module will need to be replaced.When you select a coil for any High power CD ignition box like the Mallory Hyfire CD boxes, the Mallory Hyfire CD boxes MUST always use a coil with LESS than 1 Ohm of primary resistance.