HALL EFFECT TESTING

A Hall effect system is used on any Saab with a distributor after 1981. The hall effect unit is analogous to the old points, except they are electronic and much less prone to burning out. No maintenance required. The crank sensor is still used on DI systems in order for the DI control units (Trionic and Motronic included), to determine which pair of pistons is up. Ionization voltage through the plugs is then used by DI systems to decide which plug of those two choices to fire. Pin numbers for the crank sensor differ from the ones I give here for the distributor mounted sensors, but operation is the same for the actual hall effect.

The sensor works by supplying a constant current to a semiconductor. Exposing the semiconductor to a magnetic field causes a voltage to be produced across the semiconductor. This is a property of the semiconductor. Blocking the magnetic field turns the produced voltage on and off. So now you have an electronic switch that doesn't arc and burn itself up like points.

On a Saab, either distributor hall effect units or crank mounted crank sensors, the magnetic field is blocked by use of a shutter wheel. The crank sensor has half as many shutters because it turns twice as fast as the distributor. It has cut out 'windows' separating shutters that allow the semiconductor to be exposed to the magnetic field, one shutter per cylinder on distributor mounted sensors. When the shutter blocks the field from the semiconductor, the voltage falls to 0 Vs. The length of the shutter determines the dwell, or time for coil saturation. This is the equivalent of points being closed. When the opening between the shutters lines up with the gap between the magnet and the semiconductor, current is produced. This is the equivalent of points opening and fires the coil.

The control unit reads the signals from the sensor. When it sees the sensors voltage at 0V, it energizes the coil. When it sees voltage from the sensor, it interrupts the primary circuit to the coil, causing the collapse of the magnetic field there and the induction of the high secondary voltage in the coil.

There are two circuits between the hall effect and the control unit (ignition amp or module), one a power supply across the semiconductor and the other the switching circuit. Pin 5 of the control unit connects to the + of the hall effect sensor and supplies at least 7.5V across the conductor, coming back through pin 3 of the control unit via the - terminal of the sensor. The 0 terminal of the sensor is connected to pin 6 of the control unit. The control unit sends out voltage from pin 6 that comes back via the - terminal. When the shutters interrupt the field and no voltage is produced from the semiconductor, the control unit reads its maximum voltage on pin 6 and energizes the coil (about 8V, switching voltage). When the gap of the shutter well exposes the semiconductor to the magnetic field, the semiconductor produces a voltage in the opposite direction of the switching voltage and pin 6 of the control unit receives less voltage (electron flow is from neg. to pos. remember), and breaks the primary circuit in the coil.

An analog rpm signal can be picked up from pin 1 (neg.) of the coil as the primary windings pulse (RPM signal). The control unit produces a digital form of this signal out put at its pin 7. (TD signal) 86 and 87s used a pulse amp that took this digital signal and amplified it for use by other components. (TD-O signal) Among these are the upshift light, and tach. A quick test for a missing hall effect voltage is starting the car and watching for the tach bounce. If the tach bounces, the signal is there. This eliminates at least the hall effect sensor, but not necessarily the ignition amp. However, it usually is good if it puts out the TD signal.

Diagnosing the system proceeds as follows.

Insure a properly operating charging system and good battery, temps above 32F.

Unplug connector at distributor and check voltage at its pins + and - (Outer pins). Should be at least 10V. If not, check wiring (continuity test) from distributor back to control unit. If wiring good, proceed as follows.

        Unbolt the ignition amp. On 83 or earlier, supply ground wire from unit body to car.

       Remove the connector from the unit. Check across pins 4 (Green/white) and pin 2 (black, ground). With key 'on' voltage should be 10V or higher. If so, replace control unit. If not, trace feed to pin 4 from the ignition switch via the white 29 pin firewall connector pin 28. Or check ground on pin 2 at the left wheel housing member.

If 10V was present across + and -, proceed as follows.

     Remove all the wires from the coil. Check primary resistance of coil across the terminals. It should be between .5 and .9 ohms. Check the secondary across each terminal and the center coil wire. It should be 7-8 Kohms. If not, replace the coil.

If so proceed as follows.

Reconnect all the wires to the coil. Reconnect control unit. Unplug connector to distributor. Check across terminals 1 and 15 (pos.) of coil with voltmeter. With key 'on' voltage should rise to greater than 5V and then fall to 0V in a couple of sec. Check pin 15 of the coil for battery voltage when the key is on, this runs from the switch through the same white connector on the same pin 29 as before. Check continuity from pin 1 of the coil to pin 1 of the control unit (with control unit disconnected). If so, the control unit is functioning properly.

If not, replace the control unit.

Now check the signal coming out of the hall sensor. reconnect the control unit and the connector for the distributor. Pull back the boot on the control unit connector and check across its pins 6 (brown) and 3 (copper). With the key 'on', turn engine by hand, the readings on the voltmeter should change from between less than .4V to 3V or higher as the shutter wheels block the magnetic field from the semiconductor. Check between terminals 5 (green) and 3 (copper). with key 'on', you should see 7.5V. If so, the car should start. If any of these readings are bad, then check the distributor, making sure the rotor turns with the engine. If the distributor is turning, then the hall effect is bad and can be replaced.