Camshaft Position Sensor: Description and Operation

Distributor Exploded View:

Crank Angle And #1 TDC Sensor:

The crankshaft angle sensor and the No.1 cylinder TDC sensor are composed of a disc and unit assembly. The disc is a metal disc which has six light-transmission slits located 60° apart around its circumference. There is also an additional series of slits (1° per slit at 2° increments, with a 10° blank space) located on the outer circumference of the disc. The inner slits are for the crankshaft angle sensor (individual cylinder timing referance marks). The outer series of slits is to establish No.1 TDC.
The disc is affixed to the distributor shaft. When the shaft rotates, the slits in the disc are optically read by the unit assembly. The unit assembly, in order to be able to detect the two types of slits, incorporates two luminous diodes and two photo diodes. There is a very slight clearance between the luminous diodes and the photo diodes, and the disc rotates within this space.

Crank Angle And #1 TDC Sensor System:

The light emitted from the luminous diodes reaches the photo diodes each time the disc turns. When the photo diodes receive the light transmitted, they become conductive in the direction opposite to that of ordinary diodes, with the result that current flows as shown by the broken-line arrow in the crank angle and #1 TDC sensor system image, and approximately 5v is applied to the comparator. When this happens, 5v is transmitted from terminal 1 to the ECU. Because the light to the photo diodes is interrupted when the slit passes, the current shown by the broken line stops flowing, and terminal 1 becomes O volts. In this way, then, the pulses from the unit assembly are sent to the ECU as signals.

No.1 Cylinder TDC Signal:
Top dead center is detected by the signal obtained from the outer series of slits (1° slits) by reading the 10° blank spot. The ECU, based upon this signal, determines which of the six pulses from the crank angle sensor is the signal for the No.6 cylinder (referance cylinder), thus synchronizing the ECU with the rotation of the disc in the distributor. This allows the ECU to determine (with the use of its preset firing order) which pulse is the No.1 cylinder. The outer circumference slits are also used by the ECU to assist in a more accurate ignition timing at engine speeds below 1200rpm.

Crankshaft Angle Signal:
The six slits located at the inner circumference of the disc serve to detect the position of the crankshaft (and therefore the pistons) relative to top dead center. The ECU, based on this signal, determines the fuel injection timing, and also calculates the amount of intake-air, the timing of the ignition signal, etc. for each revolution of the engine.