GM/Detroit 6.5L – Engine Management – TIMING CONTROL – Component Operation


The optical sensor is located in the fuel injection pump (figure 5-36) and includes the fuel temperature sensor. The optical sensor counts the pulses emitted by a disc in the injection pump. The disc is a silver-colored, film-like ring notched with two sets of notches.
• The outer diameters consists of 512 notches that provide the PCM with pump speed information.
• The inner diameter uses eight notches, one for each cylinder, to send pump cam reference

The optical sensor transmits pulses from these notches to the PCM as a voltage signal known as the high resolution signal (small notches) or pump cam signal (large notches). The PCM relies on the optical sensor to identify pump position. The high resolution signal is one of the most important PCM inputs for determining fuel control and timing.

Optical Sensor Circuit Operation
The PCM sends a 5 volt reference signal to the optical sensor on CKT 474 (figure 5-37). The sensor returns two signals to the PCM: a high resolution signal on CKT983 and a cam signal on CKT 982. Both are regulated-current signals read by the PCM. The high resolution signal pulses 512 times per one revolution of the injection pump. The cam signal pulses 8 times per one revolution of the pump.
When the signal on either circuit to the PCM is out of calibration, the PCM sets a Diagnostic Trouble Code (DTC).

DTC 17 — “High Resolution Circuit Fault”
DTC 17 identifies a missing high resolution signal. It sets when the PCM receives 8 cam pulses on CKT 982 without receiving a corresponding high resolution signal on CKT 983.

DTC 18 — “Cam Reference Pulse Error”
DTC 18 identifies a missing cam pulse. It sets when the PCM detects 8 missing cam pulses on CKT 982 for every crankshaft position pulse received on CKT 643 (see crankshaft position sensor/DTC 19). If DTC 17 and 18 are both stored, it could indicate a problem with CKTs 474 or 987.


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