Troubleshooting the GM HEI Distributor

by Don Bowman
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GM HEI Distributor Basics

The GM HEI distributor uses a hall effect for the triggering and sensing the No.1 cylinder location and the rpm of the engine. It uses an ignition module within the distributor to regulate the timing curve as well. It will automatically advance the timing on acceleration a specific amount and retard the spark when decelerating and starting.

It uses a standard rotor, under which is located a centrifugal set of weights and springs that act as the final mechanical advance. The GM HEI distributor's springs can be replaced with three different weights of springs that, in effect, allow the advance at lower rpm. The ignition coil is housed in the distributor cap, which makes the GM HEI distributor a very efficient, self-contained unit. It can be used with many ignition-enhancing-after-market capacitor discharge units such as MSD or Jacobs (multiple spark distribution).

This system creates a multitude of sparks rather than one spark, and it lasts over many degrees of stroke. Some of the early units also had a vacuum advance that mechanically retarded the spark for start and deceleration and increased the spark approximately 10 degrees on start-up. The combination of the vacuum and the mechanical weights did all of the controlling of the spark in the early units. The module in these units primarily acted as a switch.

Testing the GM HEI Distributor

A no-spark condition is checked by checking the distributor for power at the connector on the side of the cap. If there is power, disconnect the electrical connector and remove the cap. Check the rotor and the cap for excessive wear. Check the coil tower for excessive wear. Remove the top plastic cap on the distributor cap. Use an ohmmeter and check the coil positive terminal to the metal case of the coil. The reading should be infinity. Check the coil tower and the negative terminal. The reading should be 900 ohms. Check the positive terminal to the negative terminal. The reading should be around 700 ohms. If any of these tests show drastically different readings, the coil is bad. If the coil is good, the cap and rotor are not cracked or worn significantly and there is no spark at any wire, replace the ignition module.

Engine Runs But Has no Power

Hook up an advance timing light by hooking the carbon connection over the No. 1 cylinder wire and hook the positive and negative clips to the battery. If there is a vacuum advance, pull the hose off the vacuum source and plug the leak. Start the engine, pull the trigger and shine the light on the right side of the harmonic balancer. Turn the knob on the timing light until the straight 0-degree line on the harmonic balancer is lined up with the 0-degree mark on the timing chain cover. Read the degrees of advance by the mark on the timing light advance knob. For instance, if it is 8 degrees before top dead center, check the label under the hood for the timing specifications. Adjust the distributor, if necessary, by loosening the hold-down nut on the distributor base and turning the distributor and rechecking until it meets specifications. Turn the distributor counter-clockwise to advance and the clockwise to retard the spark.

Once set, reconnect the vacuum hose and see how much the distributor advances. If the advance rises about 10 degrees, the vacuum advance mechanism is operational; if not, replace it. Raise the rpm to 2,500 and recheck the timing. It should be around 32 degrees plus or minus one degree. If the timing does not rise, the centrifugal advance mechanism isn't working. If it rises but is less than 32 degrees, adjust the distributor to achieve this number.

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