Here's a breakdown of why this happens and what to check:
* Failed Blower Motor Resistor: The resistor is a common point of failure. Heat and age degrade its internal components, causing it to fail. When it fails completely, only the highest speed (which isn't controlled by the resistor) functions.
* Wiring Issues: A broken or loose wire connected to the resistor or the blower motor itself could also interrupt the lower speeds. Check for damaged or corroded wires, especially near the resistor and the blower motor.
* Blower Motor Issues (Less Likely): While less common, a failing blower motor could also cause this problem, although usually it would manifest as a complete fan failure rather than just speed limitation. However, a partially failing motor *might* limit speed options.
* Fuse or Circuit Breaker (Least Likely): While unlikely to cause *only* high speed to work, a partially blown fuse or tripped circuit breaker *could* disrupt some fan speeds. Check your fuses and circuit breakers related to the blower motor.
Troubleshooting Steps:
1. Check the fuses and relays: Locate the fuses and relays associated with the heater blower motor in your owner's manual or online. Inspect them for any signs of damage.
2. Locate the Blower Motor Resistor: This is typically located behind the glove box, under the dash, or near the blower motor itself (consult a repair manual for your specific van model).
3. Inspect the Resistor: Visually examine the resistor for any signs of burning, melting, or physical damage. Often, a failed resistor will look visibly burned or melted.
4. Check Wiring: Carefully inspect the wiring connected to the resistor and the blower motor for any breaks, corrosion, or loose connections.
5. Test the Resistor (Advanced): If you have a multimeter, you can test the resistor's resistance at different points to confirm whether it's faulty. However, this requires some electrical knowledge.
6. Replace the Blower Motor Resistor: If the resistor is found to be faulty, replacing it is usually the solution. This is a relatively inexpensive and easy repair for someone comfortable working under the dash. You can purchase a replacement from an auto parts store.
If you're not comfortable working on your vehicle's electrical system, it's best to take it to a qualified mechanic. Attempting repairs without proper knowledge can lead to further damage.
