The distributor contains the electronic ignition module, and the magnetic pick-up assembly which contains a permanent magnet, a pole piece with internal teeth, and a pick-up coil (not to be confused with the ignition coil).
The HEI distributor is equipped to aid in spark timing changes which is necessary to maintain emissions, economy and performance. This is achieved by the Electronic Spark Timing (EST) control system. On these vehicles, timing changes are electronically regulated through the computer control module.
In the HEI system, as in other electronic ignition systems, the breaker points have been replaced with an electronic switch, a transistor, which is located within the ignition module. This switching transistor performs the same function the points did in a conventional ignition system; it simply turns coil primary current on and off at the correct time. Essentially, the electronic and conventional ignition systems operate on the same principal.
The module which houses the switching transistor is controlled (turned on and off) by a magnetically generated impulse induced in the pick-up coil. When the teeth of the rotating timer align with the teeth of the pole piece, the induced voltage in the pick-up coil signals the electronic module to open the coil primary circuit. The primary current then decreases and a high voltage is induced in the ignition coil secondary windings which is then directed through the rotor and high voltage leads (spark plug wires) to fire the spark plugs.
In essence then, the pick-up coil module system simply replaces the conventional breaker points and condenser. The condenser found within the distributor is for radio suppression purposes only and has nothing to do with the ignition process. The module automatically controls the dwell period, increasing it with increasing engine speed. The HEI system features a longer spark duration which is instrumental in firing lean and Exhaust Gas Recirculation (EGR) diluted fuel/air mixtures. Since dwell is automatically controlled, it cannot be adjusted. The module itself is non-adustable and non-repairable and must be replaced if found defective.
All spark timing changes in the HEI (EST) distributors are done electronically by the Electronic Control Module (ECM), which monitors information from the various engine sensors, computes the desired spark timing and signals the distributor to change the timing accordingly. With this distributor, no vacuum or centrifugal advances are used.
A Hall Effect Switch is used on some of the distributors in the EST system. It is mounted above the pick-up coil in the distributor and takes the place of the reference terminal on the distributor module. The Hall Effect Switch provides a voltage signal to the ECM to tell it which cylinder will fire next.
Some of the engines covered by this manual are equipped with Electronic Spark Control (ESC). A knock sensor is mounted in the engine block. It is connected to the ESC module which is mounted to the cowl in the engine compartment. In response to engine knock, the sensor sends a signal to the ESC module. The module will then signal the ECM which will retard the spark timing in the distributor.
NOTE: Late model vehicles, both 4 cylinder and 6 cylinder engines, have Distributorless Ignition Systems (DIS). Because the reluctor wheel is an integral part of the crankshaft, and the crankshaft sensor is mounted in a fixed position, timing adjustment is not possible.
Basic ignition timing is critical to the proper operation of the Electronic Spark Control (ESC) system. Always follow the Vehicle Emission Control Information (VECI) label procedures when adjusting ignition timing.
Some engines will incorporate a magnetic timing probe hole for use with special electronic timing equipment. Consult the manufacturer's instructions for the use of this electronic timing equipment.
Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.
Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.
The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5°BTDC, the spark plug must fire 5°before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.
As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC.
If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.
Ignition timing for this engine should be accomplished using the averaging method in which the timing of each cylinder can be brought into closer agreement with the base timing specification.
The averaging method involves the use of a double notched crankshaft pulley. When timing the engine, the coil wire, instead of the Number 1 plug wire, should be used to trigger the timing light. The notch for the No. 1 cylinder is scribed across all three edges of the double sheave pulley. Another notch located 180°away from the No. 1 cylinder notch is scribed only across the center section of the pulley to make it distinguishable from the No. 1 cylinder notch.
Since the trigger signal for the timing light is picked up at the coil wire, each spark firing results in a flash from the timing light. A slight jiggling of the timing notch may be apparent since each cylinder firing is being displayed. Optimum timing of all cylinders is accomplished by centering the total apparent notch width about the correct timing specification.
There are three basic types of timing light available. The first is a simple neon bulb with two wire connections (one for the spark plug and one for the plug wire, connecting the light in series). This type of light is quite dim, and must be held closely to the marks to be seen, but it is quite inexpensive. The second type of light operates from the car's battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter. This type of light is more expensive, but the xenon bulb provides a nice bright flash which can even be seen in sunlight. The third type replaces the battery source with 110 volt house current. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.
Because your car has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps around the Number 1 spark plug wire (in this case, the coil wire), eliminating the adapter. It is not susceptible to crossfiring or false triggering, which may occur with a conventional light due to the greater voltages produced by HEI.
On 1.8L, 2.0L OHC and turbocharged chevy engines, a sequential port fuel injection system (SFI) is used for more precise fuel control. With SFI, metered fuel is timed and injected sequentially through injectors into individual cylinder ports. Each cylinder receives one injection per working cycle (every two revolutions), just prior to the opening of the intake valve. The main difference between the two types of fuel injection systems is the manner in which fuel is injected. In the multiport system, all injectors work simultaneously, injecting half the fuel charge each engine revolution. The control units are different for SFI and MFI systems, but most other components are similar. In addition, the SFI system incorporates a new Computer Controlled Coil Ignition system that uses an electronic coil module that replaces the conventional distributor and coil used on most engines. An electronic spark control (ESC) is used to adjust the spark timing.
The Electronic Spark Control (ESC) operates in conjunction with the Electronic Spark Timing (EST) system and modifies (retards) the spark advance when detonation occurs. The retard mode is held for approximately 20 seconds after which the spark control will again revert to the Electronic Spark Timing (EST) system. There are three basic components of the Electronic Spark Control (ESC) system.
The Electronic Spark Control (ESC) sensor detects the presence (or absence) and intensity of the detonation by the vibration characteristics of the engine. The output is an electrical signal that goes to the controller. A sensor failure would allow no spark retard.
The distributor is an HEI/EST unit with an electronic module, modified so it can respond to the ESC controller signal. This command is delayed when detonation is occurring, thus providing the level of spark retard required. The amount of spark retard is a function of the degree of detonation.
The Electronic Spark Control (ESC) controller processes the sensor signal into a command signal to the distributor, to adjust the spark timing. The process is continuous, so that the presence of detonation is monitored and controlled. The controller is a hard wired signal processor and amplifier which operates from 6-16 volts. Controller failure would be no ignition, no retard or full retard. The controller has no memory storage.
Should a Code 43 be set in the ECM memory, it would indicate that the ESC system retard signal has been sensed by the ECM for too long a period of time. When voltage at terminal L of the ECM is low, spark timing is retarded. Normal voltage in the non-retarded mode is approximately 7.5 volts or more.