GM HEI Distribution Wiring Diagram

The high-energy ignition (HEI) ignition system has gone through tons of improvements over the last and present centuries, ditching the old standard cap, rotor, and point system it once used. The new HEI design features a vacuum advance mechanism, the ignition coil, and a permanent magnet assemble that features a pole lined with teeth and a pick-up coil. You could find a few models that feature an external coil, however, most coils have been combined with the cap.

GM HEI Distribution Wiring Diagram

GM HEI Distribution Wiring Diagram 01

GM HEI Distribution Wiring Diagram 02


GM HEI Distribution Wiring Diagram 03

How Does an HEI Distributor Work

The High Energy Ignition (HEI) features a star-shaped wheel on the center shaft which passes in front of the magnetic pickup. Whenever the point of the star passes close to the pickup, it produces a pulse that is used to trigger the coil. The coil is located in the distributor cap and the control unit for the magnetic pickup is located in the distributor.

The distributor needs one wire to supply the module with power. The module will send out a voltage to the magnetic pick which will be interrupted every time the star wheel causes a pulse in the voltage. This pulse will be returned to the module where it will ground the coil and cause a spark plug to fire.

Being a point-type ignition, the magnetic pickup of the HEI distributor is not subject to irregularities in firing at high engine speeds.

Signs of a Bad HEI Distributor

Engine Fail to Start

For the HEI distributor to function, the positive hot wire from the battery that feeds the distributor needs to deliver voltage. If not, the starter will crank the engine but there will be no fire coming from the spark plugs.

To check for battery voltage, turn the ignition key on and ground the lead of a test light against a metal source, and place the test light probe on the “BAT” side of the distributor to determine the voltage to the distributor. If the test light fails to illuminate, then the battery voltage is absent and you have to check the battery charge

Plug Wire Engine Miss

A faulty HEI plug wire that has a corroded connection or too much resistance will lead to a constant engine miss that resembles a dead cylinder.

Reduced Fuel Economy

The HEI system is designed to control the spark timing changes for the engine which, in turn, regulates emission, fuel economy, and engine performance. A reduction in fuel economy could indicate that the electronic spark timing (EST) in the HEI control system has failed. This system works together with the vehicle’s computer and when it fails, a trouble code light will come up on the dashboard to alert you.

Weak Spark

Weak spars will not produce the sufficient voltage needed to ignite the combustion gases. This can be due to an HEI distributor with a worn pick-up coil or corroded magnetic poles. You will notice this as a sporadic miss when the engine pulls under a heavy load or the vehicle has to climb a steep mountain pass or go uphill. To test for a weak spark, pull the plug from the engine and ground it against a metal source while the engine runs. If you notice a yellow or intermittent spark, this indicates weak HEI coil output.

Hard Starting

If you notice that the engine requires repeated cranking to start, it could be due to worn or carbon-corroded cap electrodes that have chipped or increased in gap size.

Smog Check Failure

Insufficient coil voltage or spark in the HEI distributor will allow excessive hydrocarbons and carbon monoxide emissions. This could be due to the incomplete burning of raw gas in the combustion chamber. A weak ignition spark could result in catalytic converter failure by soaking palladium pellets with fuel which destroys the chemical process within the converter.

Troubleshooting the GM HEI Distributor

Testing the GM HEI Distributor

A no-spark condition can be checked by checking the distributor for power at the connector on the side of the cap. If you find power, disconnect the electrical connector and remove the cap.

  • Check excessive wear at the rotor and the cap.
  • Check the coil tower for excessive wear
  • Remove the top plastic cap on the distributor cap. Using an ohmmeter, check the coil positive terminal to the metal case of the coil. The reading should indicate infinity.
  • Check the negative terminal and the coil tower. The reading should be 900 ohms
  • The positive terminal to the negative terminal should be reading around 700 ohms

If any of these tests show readings that drastically deviate, then the coil is bad. However, if the coil is good and the cap and rotor are in perfect working condition but there is still no spark at any wire, then the ignition module should be replaced.