Here's why:
* High Speed Driving: At high speeds, your truck's radiator is cooled sufficiently by the airflow rushing through the radiator. The cooling fan isn't needed to supplement this airflow.
* Stopped at Red Lights: When stopped, the airflow drastically reduces. The cooling fan is *essential* to pull air through the radiator and dissipate heat from the coolant. If the low-speed fan isn't working (or is working intermittently), the engine overheats, causing the coolant temperature to rise and resulting in hot air from the vents. The high-speed fan might kick on eventually, but by then significant overheating might have already occurred.
Possible Causes of a Failing Low-Speed Cooling Fan:
* Fan Motor Failure: The motor itself might be burnt out, seized, or have faulty internal wiring.
* Fan Relay Failure: The relay, an electrical switch, might be failing to send power to the fan motor.
* Wiring Issues: Worn, damaged, or corroded wires going to the fan motor or relay can prevent the fan from operating.
* Cooling Fan Control Module (CFCM) Failure (if equipped): Some vehicles use a dedicated module to control the cooling fans. This module could be malfunctioning.
* Low Coolant Level: Although less likely to be the sole cause of this specific issue, low coolant makes it harder for the system to cool and could exacerbate the problem, causing overheating at idle.
What to do:
1. Check Coolant Level: First, ensure your coolant level is correct. A low level needs to be addressed regardless of fan function.
2. Listen for the Fan: When the engine is running at idle and is starting to overheat (gauge shows higher temperature), listen carefully for the fan to engage. It should start running at low speed. If you hear a clicking sound, that could suggest relay trouble.
3. Inspect the Fan and Wiring: Visually inspect the fan motor and its wiring for any obvious damage.
4. Test the Fan Relay: You can test the fan relay with a multimeter or by temporarily swapping it with a known good relay (same amperage rating) – be cautious when working with electrical components.
5. Consult a Repair Manual: A repair manual specific to your 2001 Dodge Ram 2500 will have diagrams and troubleshooting steps for the cooling system.
6. Professional Mechanic: If you are uncomfortable performing these checks or repairs yourself, take your truck to a qualified mechanic for diagnosis and repair.
Ignoring this issue could lead to serious engine damage due to overheating. Address it promptly.
