Here's a breakdown of the potential causes:
* Failed Blower Motor Resistor: This is the most common culprit. The resistor pack is a small, often plastic, component located somewhere in the HVAC system, usually near the blower motor itself or behind the glovebox. Over time, the resistors can overheat and fail, causing the problem you're experiencing.
* Wiring Issues: A break or short in the wiring leading to the resistor pack, or between the resistor pack and the blower motor's lower speed circuits, can also cause this issue. This is less common than a failed resistor, but possible.
* Blower Motor Issues (Less Likely): While less probable, a problem with the blower motor itself *could* contribute, though it's more likely to manifest as a complete failure rather than just a speed limitation. A partially seized motor might struggle at lower speeds but work at higher speeds.
* Control Module (Rare): In some very rare cases, a problem with the HVAC control module could be the root cause. This is less likely, as a faulty module would likely cause more widespread problems than just one speed setting.
Troubleshooting Steps:
1. Check the Fuse: Before anything else, check the fuse related to the blower motor. A blown fuse would prevent it from working at all, so this is a quick preliminary check.
2. Locate the Blower Motor Resistor: Consult your vehicle's repair manual (or online resources specific to your 1998 Chevy Express) to locate the resistor pack. It's usually easy to access and replace.
3. Inspect the Resistor Pack: Once located, visually inspect the resistor pack for any obvious signs of damage, such as burning, melting, or loose connections.
4. Test the Resistor Pack (Advanced): If you have a multimeter, you can test the resistance of each resistor within the pack to confirm it's faulty. However, this requires some electrical knowledge and the correct resistance values for your specific vehicle.
5. Check Wiring: Inspect the wiring leading to and from the resistor pack for any breaks, chafing, or corrosion.
6. Replace the Resistor Pack: If you've confirmed the resistor pack is the problem, replace it with a new one. This is a relatively inexpensive and straightforward repair.
If you're not comfortable working on your vehicle's electrical system, it's best to take it to a qualified mechanic. Incorrectly diagnosing or repairing electrical components can lead to further damage.
