Usually the computer will control an actuator by the use of low-side drivers. These drivers will complete the path to ground through an FET transistor to control the output device. Hie computer may monitor the voltage on this circuit to determine if the actuator operates when commanded. Monitoring of the system can be done either by measuring voltage on the circuit or by measuring the current draw of the circuit.
FIGURE. Computers using low-side drivers may be able to monitor the circuit for proper operation. When the relay coil is not energized, the sense circuit should see a high (12 V) volt. When the relay is turned on, the voltage should go low (0 V).
Many newer vehicles are now using high-side drivers, which control the output device by varying the positive (12-volt) side. High-side drivers consist of a Metal Oxide Field Effect Transistor (MOSFET) that is controlled by a bipolar transistor. The bipolar transistor is controlled by the microprocessor. The advantage of the high-side driver is that it can provide quick-response self-diagnostics for shorts, opens, and thermal conditions. It also reduces vehicle wiring.
High-side driver diagnostic capabilities include the ability to determine a short circuit or open circuit condition. Hie high-side driver will take the place of a fuse in the event of a short circuit condition. When it senses a high-current condition, it will turn off the power flow and then store a DTC in memory. The driver will automatically reset once the short circuit condition is removed. In addition, the high-side driver monitors its temperature. Hie driver reports the junction temperature to the microprocessor. If a slow acting resistive short occurs in the circuit, the temperature will begin to climb. Once the temperature reaches 300°F (150°C) the driver will turn off and set a DTC.
The high-side driver is also capable of detecting an open circuit, even if the system is turned off. This is done by reading a feedback voltage to the microprocessor when the driver is off. A 5-volt, 50 мА current is fed through the circuit, which also has a resistor wired in parallel. Low voltage (less than 2.25 volts) will indicate a normal circuit. If the voltage is high (above 2.25 volts), a high resistance or open circuit is detected. If the open circuit is detected, a DTC is set.