Volkswagen Front Wheel Drive 1974-1989 Repair Guide

Description & Operation


See Figure 1

Electronic ignition systems offer many advantages over the conventional breaker point ignition system. By eliminating the points, maintenance requirements are greatly reduced. An electronic ignition system is capable of producing much higher voltage which in turn aids starting, reduces spark plug fouling and provides better emission control.

The Hall generator produces a voltage pulse which is sent to the control unit, which in turn switches the primary ignition circuit on and off. Located in the distributor, it consists of a trigger wheel that revolves with the distributor shaft and a stationary unit called the Hall sender. The Hall sender consists of a semiconductor layer positioned on a magnetically conducting element and a permanent magnet, both of which are separated by an air gap. When the trigger wheel shutter enters the air gap, it blocks the magnetic field and the Hall sender is shut off. When this occurs, the control unit will complete the primary circuit and you have the dwell period. When the shutter leaves the air gap, the magnetic field flows again. The Hall sender generates a voltage pulse to the control unit which will then interrupt the primary ignition circuit and ignition will occur.

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Fig. Fig. 1: Diagram of a common electronic ignition system used on Volkswagens

In 1985, an electronic ignition system with a knock sensor was introduced on some engines. By using a knock sensor input, the ignition timing can be advanced while using a higher compression ratio. The engine can now operate closer to, but not in, the knock threshold where it will operate more efficiently with a higher power output.

Because temperature changes and engine load variations caused by different electrical accessories can affect idle speed, most systems use some type of idle stabilizer. Earlier systems use an electronic idle stabilization system. This system consists of a small control unit between the Hall generator and the Hall control unit. The frequency of the voltage signal sent from the Hall generator gives the idle stabilizer information on engine speed. When the idle stabilizer senses that the engine speed has dropped below a certain rpm, it will advance the timing, causing the idle speed to increase. The use of this electronic idle stabilizer system (which advanced timing to increase idle speed) was eliminated in 1985. On later models, idle stabilization is controlled by idle boost valves and/or idle air stabilizers which bypass additional air around the throttle plate at idle in order to increase engine speed.