Why the Highest Setting Only Fails (Even After Replacement):
* Voltage/Signal Issue: The highest setting often requires a different or higher voltage/signal than the lower settings. A problem in the wiring harness (broken wire, poor connection, corrosion) could interrupt this specific signal without affecting lower settings.
* Resistor Pack Problem (Most Likely): Many older vehicles use a resistor pack to control blower speed. The highest setting frequently uses the *least* resistance (or no resistor at all for full power). A faulty resistor in the pack – specifically the one related to the highest speed – could explain why only that setting fails. This is especially likely since the problem persists after replacing the controller.
* Blower Motor Relay: The relay switches power to the blower motor. A failing relay might not be able to handle the high current demand of the highest setting.
How to Repair/Troubleshoot:
1. Check the Blower Motor and Fuse (Easy First Step): Ensure the blower motor itself is working. You might hear a faint hum if you turn on the fan at a lower setting. Also, check the fuse related to the blower motor; it may be blown.
2. Locate the Blower Motor Resistor Pack: This is usually found somewhere near the blower motor itself (often under the dashboard, sometimes accessible from the engine compartment). It's a small, rectangular component with multiple wires connected.
3. Inspect the Resistor Pack: Visually inspect the resistor pack for any signs of burning, melting, or loose connections. The resistor associated with the highest speed may be visibly damaged.
4. Check Wiring at the Resistor Pack: Carefully examine all the wires connected to the resistor pack for breaks, corrosion, or loose connections. Pay close attention to the wires related to the highest setting. Use a multimeter to test continuity (should be very low resistance on a good connection) on each wire.
5. Check Wiring at the Controller: Though you've replaced the controller, double check all wiring connections going to *it*. A loose or corroded connection could still cause the problem.
6. Test Blower Motor Relay (If Applicable): Locate the blower motor relay (often in a fuse box under the hood or dashboard). With the relay removed, you can usually use a multimeter to test its continuity and operation.
Tools You'll Need:
* Multimeter (to test voltage, continuity, and resistance)
* Wiring diagram for your 1994 G-20 van (very helpful; you can find them online through repair manuals or forums)
* Screwdrivers (various sizes)
* Test light (optional, but can help visually check for power at certain points)
Important Notes:
* Safety First: Disconnect the negative battery terminal before working on any electrical components in your vehicle.
* Wiring Diagram: A wiring diagram is invaluable. It shows you where the resistor pack and other relevant components are located and how the wiring is connected.
* Professional Help: If you're not comfortable working with car electronics, it's best to take your van to a qualified mechanic.
The most likely fix is a faulty blower motor resistor pack or a wiring issue associated with that resistor or the high-speed setting. Focusing on those areas will likely solve your problem.
