Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSCHEVROLET1990FORWARD CONTROL 7.4 N, AUTOMATIC, 475/M40REPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODES - DIESELENTERING OR EXITING DIAGNOSTIC MODE
1990 Chevrolet Forward Control 7.4 N, Automatic, 475/M40
Entering Or Exiting Diagnostic Mode
1990 Chevrolet Forward Control 7.4 N, Automatic, 475/M40SECTION Entering Or Exiting Diagnostic Mode
- With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). See Fig 1. The Diesel Electronic Control (DEC) system will enter the diagnostic mode.
- In this mode, the ECM will display Code 12, by flashing the SERVICE ENGINE SOON light. Code 12 consists of one (1) flash, followed by a short pause, then 2 flashes in quick succession.
- Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until the diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.