High Speed Operation

WARNING: This page is about a different car, the 2010 GMC Sierra, 2010 GMC Cab & Chassis Sierra, 2010 Chevrolet Silverado, and 2010 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The ECM applies a ground to the fan 1, fan 2, and fan 3 relay control circuits simultaneously, which activates all five relays. This allows battery voltage to be applied directly to the right cooling fan from the Fan High relay, and battery voltage to be applied directly to the left fan from the Fan Low relay. The left fan is supplied a direct path to ground through the energized fan control relay. This arrangement provides each fan with direct battery voltage and its own ground path. The result is a parallel circuit with both fans running at full speed. When the ECM is commanding a fan relay ON, the voltage of the control circuit should be low, near 0 volts. When the ECM is commanding a fan relay OFF, the voltage of the control circuit should be high, near battery voltage.