Diagnostic Overview - Radiator Cooling Fan Control

When diagnosing a Cooling System it is best to start with analyzing the symptoms the system is exhibiting along with the DTCs present as a starting point for what part of the system and circuits to start testing.  Examples: Does the fan not run in low speed or high speed? Are both speeds not working? Does the fan run continuously in low speed or high speed? What DTCs are present? Keeping in mind that not all failure modes will produce a DTC. For example, in most cases the PCM does not directly monitor the relay outputs for faults. Listed below are some of the possible symptoms and failure modes. 

• A faulty relay coil will cause a DTC since it is part of the control circuitry. Burnt contacts preventing Battery voltage from passing through to the output circuit when energized will not cause a DTC. In either case the cooling fan will not work for the speed controlled by the faulty relay.
• An open or short to ground in one of the relay control circuits will cause a DTC to set. However, the symptoms will be different. An open control circuit will cause the fan to be inoperative for the speed that the relay controls. A short to ground in a control circuit will cause the fan to run continuously in low speed or high speed depending on which relay control circuit is shorted.
• An open relay output circuit (including the resistor for low speed) will cause the Radiator Cooling Fan to be inoperative in low speed or high speed with no DTCs present. However, in this system the voltage supply for the Radiator Cooling Fan Motor is shared with the relay coil feed circuit. Therefore, a short to ground in the relay output circuit, or excessive current draw from the Radiator Cooling Fan Motor can overload the fuse and cause it to open. This will cause a relay control circuit fault due to an open voltage supply for the relay coil.
• A faulty Radiator Cooling Fan Motor would typically not run in either speed if failed electrically. High mechanical resistance can cause the fan to drag and run slowly. In many cases this will cause an excessive current draw which can cause the fuse to open.