General Information - Activating Mechanisms


A number of sensors in your vehicle provide information to the on-board computer or PCM (Powertrain Control Module). The PCM is also connected to a variety of "output actuators" - activating mechanisms - which respond to the conditions that the sensors monitor. These include the fuel injectors, ignition coils, and various relays and solenoids. Relays regulate the flow of electricity to various components; solenoids usually operate valves or other mechanisms that regulate air, coolant, or vacuum flow.

Below are a number of examples of the different types of activating mechanisms inside your vehicle.

Note: this is a general description. Consult a repair manual written specifically for your car, truck, van or SUV for exact descriptions. Your vehicle may have some, all or none of these mechanisms.

The Exhaust Gas Recirculation (EGR) valve. When your engine is "under load," in other words when you are passing, going up a hill, or hauling a trailer, the temperature in the combustion chambers of your engine's pistons go up beyond 2500 degrees F (1380 degrees C). At this temperature, excessive amounts of oxides of nitrogen (NOx) are produced. NOx is a major ingredient in smog: when combined with hydrocarbons, other organic compounds in the atmosphere, and sunlight, it forms ozone, nitrogen dioxide, nitrogen nitrate and other potentially poisonous substances. The PCM regulates the EGR valve to allow exhaust gases to recalculate back to the intake manifold where they dilute the incoming fuel/air mixture. This lowers the combustion chamber temperature, thus reducing the amount of NOx produced during high-load conditions.

Fuel injectors. Fuel injectors spray a fine mist of fuel into the intake ports. There it is mixed with incoming air. The fuel injectors are under control of the PCM.

Idle Air Control (IAC) Valve. The IAC controls the amount of air allowed to bypass the throttle plate when the throttle plate is at its idle (or nearly closed) position. The PCM controls the IAC valve. When your engine is under more-than-normal load (for example running the air conditioning compressor during low speed turning or braking, or during high power-steering pressure), the engine can run rough and perhaps stall. To prevent this from happening, the PCM opens the IAC, which increases the idle speed enough to overcome the extra load imposed on the engine.

Ignition coils. The PCM controls the ignition coil(s). The coil(s) supply the electricity to "fire" the spark plugs. How your engine is set up and the number of cylinders determines the number of ignition coils. Ignition coils are sometimes together in "packs," in other words, two or more ignition coils make up a pack that services the spark plugs.

Warning: The ignition system in trucks, cars, vans and SUVs generates extremely high voltages, up to 40,000 volts. Use extreme care when servicing ignition system components like spark plugs, and ignition coils and packs.