The ignition system is an integral part of any engine's performance, just a much as the fuel delivery is, whether it is carbureted or fuel injection.

We will start out with the basic operation of the early HEI (High Energy Ignition) as it debut in mid 1974 and work toward the ESC (electronic spark control) as it appeared in 1981. But first the basics.


Courtesy of GM

The standard parts for a HEI distributor that replaced the breaker plate and points in the previous distributor:

The HEI system controls the primary current in the ignition to time the spark via a magnetic signal of the pick up coil. When the teeth are aligned, the magnetic field increases, which induces a electric current through the pick up coil windings. AS the timer core and pick up coil move away from each other, the magnetic field collapses and the voltage signal is diminished.  The magnetic field and pick up coil generate an AC (alternating current) in the pick up coil leads and the windings.

Courtesy of GM

When the pick up coil voltage falls below a certain threshold, the ignition module opens the primary current flow within the module. The ground path is eliminated from the coil primary windings, collapsing the magnetic field. The voltage that is induced in the coil (secondary) circuit is the result of the mutual induced current from the magnetic field collapsing.

The diagram above  is typical of the early HEI using 4 pin ignition module and vacuum advance, non EST 1974-1980.

BASIC HEI OPERATION- Electronic Spark Control

 The ECM uses other electronic devices to help control the spark timing under various conditions (such as under load, deceleration, etc). This includes Electronic Spark Timing and later with the conjunction with Electronic Spark Control (ESC) to control detonation.  The basic principle to control the primary (coil) current by a transistorized ignition module is also used with Distributorless Ignition System (DIS).

There are two modes to the EST timing. Bypass, which is basically base timing ( no computer controlled), when the engine is being cranked, or running below 400 rpm. The second mode, is EST, which the computer has control over the timing (during normal operation).  Base timing is not fixed, there is some advance engineered into the module circuit. 

In bypass mode (shown above), the pick up coil is the trigger signal for the ignition coil.  When the engine is cranking below a predetermined threshold (400-600 rpm), the computer has no control over the timing. Only when it reaches the threshold speed, does the computer take over timing. 

The set timing connector is used to take the timing control away from the computer. This is used only when setting the "base" or mechanical timing of the engine. Depending on the year/model. this may mean shorting the "A" and "B" terminals on the ALDL (diagnostic terminal).

In EST mode (shown above), when the engine reaches the threshold speed, the ecm applies a 5 volt signal to the ignition bypass circuit.  This signal switches the bypass "off" and turns on the EST circuit for triggering the ignition coil. Spark timing is now controlled by the ECM based on information programmed in the "chip" and input signals from the MAP, MAF, engine coolant temperature and engine rpm speed. 


HEI 7-pin module types. (A) HEI large cap with integral coil (B) HEI small cap-remote coil V6 and V8 engines (C) HEI small cap-remote coil 4 cylinder engines.


 Some engine use a hall effect switch, which used used in place of the reference terminal on the HEI module. The Hall effect switch is added to the primary ignition circuit to permit use of the Electronic Spark Timing. On other engines and applications, the Hall effect switch is used to trigger the fuel injection and used in camshaft timing reference for the position of the number one cylinder (4.3 CPI and CSFI engines, 5.0 and 5.7L Gen I Vortec V8 engines).

The Hall effect switch is a electronic circuit which emits a voltage signal controlled by the presence or absence of a magnetic field. 

The switch is mounted in the distributor above the normal pick up coil/module assembly. It contains a small magnet that has a specified gap between it and the electronic switch.  One or four steel vanes (depending on application) are attached to the distributor shaft. As the shaft rotates, it alternately blocks and unblocks the electronic circuit and the output signal is used as the reference signal to the ECM.


Ignition voltage is supplied on ignition module terminal "B" at 12VDC+. This voltage is passed on through the transistor (in direction of arrow) through a resistor. The collector is connected to ground.  The base of the transistor is connected to the transducer, which is the conversion of magnetic energy to electrical. When a magnetic force is induced (not blocked by the shield) the transistor is in its "on" state and voltage passes through the transistor. This will put the distributor reference at less than 1 volt. The resistor in the power supply line at the ignition module terminal "B" will keep the voltage biased at this turn on voltage.  When the magnetic force is blocked with the vane between the magnet and the transducer, there is no voltage induced into the switching transistor, thus it will not conduct any voltage through the transistor and therefore is switched "off". The reference voltage is not being "grounded" close to 0 volts, and therefore it is at near battery voltage.

With the ignition key on, engine not running, a voltage will be present at the output of the switch, whether less than 1 volt or close to 12 volts and that will depend on the position of the vane in the distributor. 

When the engine is running, the output voltage measured will be approximately 6 volts. With the reference line to the ECM from the Hall effect is open, the ECM has no distributor pulses and also it will not deliver no injector pulses.

Electromagnetic Interference 

Electromagnetic Interference or EMI for short, is a phenomenon that affects sensitive electronic equipment. This includes HEI, EST and the ECM. When high current carrying conductors such a spark plugs are routed too close to the ignition wiring for the EST controls, coil trigger harness. This will induce false signals on the reference line to the ECM. This in turn commands the ECM to send false and erroneous information to the EST system.  This will adversely  affect the timing control and injector pulses. This can the engine to miss or stall in dangerous situations.

So it is important to keep as much distance from the spark plug wires to the EST wiring, plug in connectors and wiring harness. It is important the clips and straps are used to keep the harness wiring from being loose and potentially coming in close contact with high current carrying conductors. The same can apply to the O2 sensor wiring as well, as it will cause problems in fuel delivery (too rich or too lean, stumbling and stalling).


                    Page 2: Troubleshooting basics for the HEI-EST system (coil in cap)

                    Page 3:  Troubleshooting basics for the HEI-EST system (remote coil)