Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Main Relay (Computer/Fuel System): Description and Operation
1991 Hyundai Sonata V6-3.0LSECTION Description and Operation
Control Relay Location:
Control Relay System:
The operation of the Fuel Pump is controlled by the ECU through the Control Relay. While cranking, current (supplied by the ignition switch "start" position) flows through coil L2 to close one of the two switches (S1). As a result, the fuel pump is supplied power. Additionally current (supplied by the ignition switch "ignition" position) flows through coil L3 to close the other switch (S2). As a result, power is supplied to the ECU, Air Flow Sensor (AFS), igniter, injectors and battery power is available to the L1 coil of the S1 switch. After engine start so long as the engine remains running (power no longer supplied by the ignition switch "start" position) the ECU provides a ground to the L1 coil, thus allowing the battery supplied current to close switch S1. As a result the fuel pump continues to be supplied power, but is now under the full control of the ECU. In the event of engine stall or other abnormal conditions, the ECU interrupts the ground, opening the L1 coil and interrupting the Fuel Pump power circuit.
NOTE: Failure of the control relay may interrupt power supply to the fuel pump, injectors and/or the ECU, resulting in start failure.
RENDER: 1.0x
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.