Circuit Description

WARNING: This page is about a different car, the 2003 Buick Regal and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.

The following DTC's are for the cooling fan relay control circuits controlled by the powertrain control module (PCM).

P0480 - low speed cooling fan relay control circuit
P0481 - high speed cooling fan relay control circuit

Battery positive voltage is supplied to the cool fan 1 relay from the cool fan 1 fuse. The PCM controls the low speed cooling fan relay by grounding the low speed cooling fan relay control circuit.

Battery positive voltage is supplied to the cool fan 2 relay and the cool fan 3 relay from the cool fan 2 fuse. The PCM controls the high speed cooling fan relays by grounding the high speed cooling fan relay control circuit.

The PCM grounds the cooling fan relay control circuits through an internal solid state device called a driver. The primary function of the driver is to supply the ground for the component being controlled. Each driver has a fault line that is monitored by the PCM. When the PCM is commanding a component on, the voltage potential of the control circuit should be low, near 0 volts. When the PCM is commanding the control circuit to a component off, the voltage potential of the circuit should be high, near battery voltage. If the fault detection circuit senses a voltage other than what is expected, the DTC will set.

The PCM will monitor the control circuit for the following:

A short to ground
A short to voltage
An open circuit
An open relay coil
An internally shorted or excessively low resistance relay coil

When the PCM detects any of the above conditions, the DTC will set and the effected driver will be disabled.