Because an ECM is an electrical device, the only way it can activate or change any system on an automobile is through a solenoid or relay. Solenoids are used to create movement; and relays are for switching on and off a device that requires a large amount of current or voltage to operate. As these parts are used, heat is generated each time the pull-in coil is energized. The longer duration or the faster cycling of the coil, the less time that is available for heat dissipation of the winding. Environment is also a factor in the life expectancy of this type of device. Heat and corrosion will break down the insulation on the windings and eventually become shorted, even though the device may still be operating.

Solenoid and relay circuit damage accounts for at least 65 percent of all ECM related problems and failures. Fortunately for the technician, solenoids and relays are easy to test for shorting or damage. A power supply and ohmmeter are all that is required for testing components.

Apply power to pull-in windings and check for movement.
Usually a clicking sound will be heard.
Check resistance of the pull-in windings. Most units will be 25-55 ohms. Units reading less than 25 ohms may cause damage to the ECM.
Look for signs of heat damage. Watch for melted cases, discolored windings or terminals, and burnt contact points.
All units will eventually fail, so when in doubt - throw it out! You'll save hours of trouble and countless parts house trips.

The ECM will activate the solenoid and relay drivers regardless of the condition of the device connected to it. If a shorted wire or component is connected to a circuit of this type, the excessive current draw can burn out the circuit. Some circuits are activated as soon as the ignition key is in the on position, so an ECM could be  damaged  even  before  the engine is cranked.   In an effort to save space inside the ECM,  most

manufacturers use an integrated circuit called a quad driver. Quad drivers can operate as many as four devices at the same time. If a quad driver is burned out, the ECM could lose four different functions. Some computer systems do not have trouble codes for this kind of operating error.