Fuel Delivery and Air Induction: Description and Operation

Fig. 1 Electronic Control Module (ECM):

Fig. 2 Engine calibration unit (PROM):

Fig. 3 ALDL terminal identification. 1984-85:

Fig. 4 ALDL terminal identification. 1988:

The DEC system is used on 1984-85 California light duty emissions and all 1988 V8-379 engines. The ECM, Fig. 1, is located in the passenger compartment and is the control center of the DEC system. The ECM monitors input information from various sensors and controls the EGR and TCC systems. On 1988 models, the ECM also controls the Cold Advance/Glow Plug system. A removable calibration unit called a PROM (programable Read Out Memory), Fig. 2, tailors ECM operation to the the vehicle application by providing the memory data used to perform calculations. In addition the ECM provides diagnostic logic which detects faults in the systems that the ECM monitors or controls. On 1988 models, the ECM will illuminate a Service Engine Soon lamp on the instrument panel when a malfunctions in the sensor or control circuits is encountered. On 1984-85 models, there is no Service Engine Soon lamp on the instrument panel, therefore the Diesel Diagnostic Check Tool, which is equipped with a Check Engine light, must be used. On 1984 models, the ECM will not store flash codes. On 1985 and 1988 models, the ECM will store a code or codes which will identify the problem area. These codes can be read through the ALDL. The ECM will not store the code once the problem has been corrected. Therefore, intermittent malfunctions will only be signaled when the problem is present. The ECM must be maintained at a temperature below 185°F, otherwise the ECM may become inoperative.
The ALDL, Figs. 3 and 4, is a twelve terminal connector that allows test equipment to diagnosis malfunctions in the DEC system. The ALDL is located under the instrument panel on all 1988 models and 1984-85 C and K Series. On 1984-85 G and P Series, the ALDL is located under the driver's seat. The connector terminals are used to diagnosis the system.