- Preliminary Information

1988

The DEC system has a diagnostic system incorporated in the ECM to indicate a failed circuit. When a failed circuit is indicated, the Service Engine Soon Light on the instrument panel will be illuminated. To diagnosis this system a tachometer, test light, ohmmeter, digital voltmeter with 10 megohms impedance, vacuum gauge and jumper wires are needed. With ignition switch in the On position, engine off, connect a jumper wire between ALDL diagnostic terminal B and ground terminal A. This will cause the DEC system to enter the diagnostic mode. The Service Engine Soon Light will indicate the system is in the diagnostic mode by indication a code 12. A code 12 consist of one flash followed by a short pause, then two flashes in in quick succession. This code will be displayed three times. If no other trouble codes are stored, code 12 will continue to be displayed until the jumper wire is removed from the ALDL. Begin testing with the Diesel Diagnostic Circuit Check.