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.
The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC. This only holds true, however, when the engine is at idle speed.Īs the engine speed increases, the pistons go faster. If the setting for the ignition timing is 5☋TDC, the spark plug must fire 5°before each piston reaches TDC. The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC).
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. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.īecause 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. Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. Consult the manufacturer's instructions for the use of this electronic timing equipment. Some engines will incorporate a magnetic timing probe hole for use with special electronic timing equipment. Always follow the Vehicle Emission Control Information (VECI) label procedures when adjusting ignition timing. 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.īasic ignition timing is critical to the proper operation of the Electronic Spark Control (ESC) system. NOTE: Late model vehicles, both 4 cylinder and 6 cylinder engines, have Distributorless Ignition Systems (DIS). Electronic Spark Control General Information The module will then signal the ECM which will retard the spark timing in the distributor. In response to engine knock, the sensor sends a signal to the ESC module. It is connected to the ESC module which is mounted to the cowl in the engine compartment. A knock sensor is mounted in the engine block.
Some of the engines covered by this manual are equipped with Electronic Spark Control (ESC). The Hall Effect Switch provides a voltage signal to the ECM to tell it which cylinder will fire next.
It is mounted above the pick-up coil in the distributor and takes the place of the reference terminal on the distributor module. With this distributor, no vacuum or centrifugal advances are used.Ī Hall Effect Switch is used on some of the distributors in the EST system. The module itself is non-adustable and non-repairable and must be replaced if found defective.Īll 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. Since dwell is automatically controlled, it cannot be adjusted. The HEI system features a longer spark duration which is instrumental in firing lean and Exhaust Gas Recirculation (EGR) diluted fuel/air mixtures. The module automatically controls the dwell period, increasing it with increasing engine speed. The condenser found within the distributor is for radio suppression purposes only and has nothing to do with the ignition process. In essence then, the pick-up coil module system simply replaces the conventional breaker points and condenser. 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. 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 module which houses the switching transistor is controlled (turned on and off) by a magnetically generated impulse induced in the pick-up coil. Essentially, the electronic and conventional ignition systems operate on the same principal. 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. 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.