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 MANUALSCHEVROLET1990HI-CUBE 7.4 N, 400/M40REPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCETHEORY AND OPERATIONENGINE CONTROLS - THEORY & OPERATIONCOMPUTERIZED ENGINE CONTROLS (DIESEL)MEMORIES
1990 Chevrolet Hi-Cube 7.4 N, 400/M40
Computerized Engine Controls (Diesel): Memories
1990 Chevrolet Hi-Cube 7.4 N, 400/M40SECTION Memories
The 3 types of memories used in ECMs are: Read Only Memory (ROM), Random Access Memory (RAM) and Programmable Read Only Memory (PROM).
- Read Only Memory (ROM) - ROM is programmed information that can only be read by ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.
- Random Access Memory (RAM) - RAM is the scratch pad for the CPU. Data input, diagnostic codes and results of calculations are constantly updated and temporarily stored in RAM. If battery voltage is removed from ECM, all information stored in RAM is lost.
- Programmable Read Only Memory (PROM) - PROM is factory programmed engine calibration data which "tailors" ECM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. The PROM can be removed from ECM. If battery voltage is removed, PROM information will be retained.
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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.