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.
Engine Controls
Computer commands are carried out by solenoids, motors, and other control devices. Solenoids are used for many functions: vacuum control, air bleeds, carburetor fuel flow, etc. Motors are used for idle speed control and mixture adjustment.
Computers control some functions directly, while others are accomplished by more involved means. For example, some systems are capable of retarding ignition advance when detonation ("knocking") is sensed. On one system (Ford 6 Cyl. MCU), the computer activates a solenoid which cuts off vacuum to the distributor advance diaphragm. On another (General Motors CCC), the computer sends an electrical signal directly to the ignition module, which results in a delayed spark. The method used is different, but the end result is the same.
Some computer systems (Chrysler 6 Cyl. EFC and Ford 4 Cyl. MCU) control mixture by vacuum. A vacuum regulator is controlled by the computer, and provides a vacuum signal to a mixture control diaphragm in the carburetor. In another system (General Motors CCC), the computer controls a mixture solenoid in the carburetor with electrical signals, and no vacuum is needed. Servicing procedures will vary between these two systems, but the computer controls the mixture in both cases.
Use the Manual With the Right Hardware
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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.
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