Current is supplied to the ECU driver circuits (#10 and #20 in example) through the fuel injectors. Current flows either directly from the ignition switch or from the EFI Main Relay. When the ECU driver circuit turns on, current flows to ground through the injector solenoid coil. The magnetic field created causes the injector to open against spring tension. When the ECU driver circuit turns off, the spring closes the injector valve.
There are two common types of driver circuits currently in use on Toyota EFI engines; both of these driver circuits work on the voltage control principle. One uses an external solenoid resistor and a low resistance injector, the other using a high resistance injector without the solenoid resistor. In both cases, the high circuit resistance is required to limit current flow through the injector winding. Without this control of the current flow through the injector, the solenoid coil would overheat, causing injector failure.
A third type of driver circuit was used by Toyota on overseas models using the 4A-GE engine with D type EFI. Referred to as a current controlled driver circuit, it has never been used by Toyota on vehicles sold in the U.S.A. but is widely used by other auto manufacturers. This type of driver circuit uses a low resistance injector and limits current flow by controlling the gain of the driver transistor. The advantage to the current controlled driver circuit is the short time period from when the driver transistor goes on to when the injector actually opens. This is a function of the speed with which current flow reaches its peak.
In terms of injection opening time, the external resistor voltage controlled circuit is somewhat faster than the voltage controlled high resistance injector circuit. The trend, however, seems to be moving toward use of this latter type of circuit due to its lower cost and reliability. The ECU can compensate for slower opening time by increasing injector pulse width accordingly.
Caution: Never apply battery voltage directly across a low resistance injector. This will cause injector damage from solenoid coil overheating. Use the proper SST inspection wire will ensure proper series resistance.
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