The fuel pump won’t be activated again until the PCM sees an RPM signal. Whenever the key is turned on, the PCM grounds the fuel pump circuit for a few seconds, activating the fuel pump and pressurizing the system. This closed switch allows voltage to flow on the fuel pump activation circuit, activating the pump. When the PCM fuel pump driver activates the relay coil, the coil acts as an electromagnet that pulls the electrical contacts closed which completes the fuel pump circuit. The ground circuit is activated by the PCM driver when appropriate. It’s fed a switched ignition feed (12 volts with key on)and a ground. The control side (or coil side) is the low amp side. The “control” side which is basically a coil and the “switch” side which is a set of electrical contacts. The higher amperages are kept under the hood, away from sensitive areas The relay contains basically two sides. Higher amperage current creates more heat but also in the event of a failure, can cause a PCM failure. For obvious reasons it’s better to not have higher amperage current anywhere near the PCM. Just as the name implies, a “relay” permits the transmittal of higher amperage current to the fuel pump without that current having to travel through the PCM (Powertrain Control Module). The fuel pump is activated by a PCM-controlled relay.
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