Circuit/System Description

Ignition voltage is supplied to the coil side of the fuel heater relay whenever the ignition is ON. The engine control module (ECM) provides a ground to control the fuel heater relay. When the ECM is commanding the fuel heater ON, the voltage of the control circuit should be low, near 0 volts. When the ECM is commanding the fuel heater OFF, the voltage of the control circuit should be high, near battery voltage.