Ignition System: Operation

The Hall generator (sending unit or pick-up coil) is mounted inside the distributor on a switch plate. A trigger wheel (segmented shutter) attached to the distributor shaft under the rotor, passes in and out of the air gap of the Hall generator.

At speeds greater than 940 RPM (all speeds on Saab models), the Hall generator signals the Hall (ignition) control unit to make and break the current flow in the primary circuit of the ignition coil.

There is 1 trigger wheel shutter or tooth for each cylinder of the engine. Shutter width determines dwell, which isn't adjustable.

Fig 1: Schematic of Bosch Hall Effect Electronic Ignition System Control unit is connected directly to distributor.

As the Hall control unit breaks the primary circuit through the coil, secondary voltage is released through the high tension wiring, distributor cap and rotor to the spark plugs. See Fig 1.

If engine speed drops below 940 RPM, the idle stabilizer takes over the duty of producing the signal to the ignition control unit (instead of the Hall generator).

The idle stabilizer is mounted on the same bracket as the Hall control unit and is wired between the distributor and the Hall control unit. It senses engine speed earlier, causing ignition timing to advance. On Audi 4000 models, Hall control unit and idle stabilizer is mounted in glove box.

Advancing ignition timing causes idle speed to increase, and the Hall generator to resume its normal operation.