* Blower Motor Resistor: This is a common culprit. The resistor controls the fan speeds. If it fails, you'll typically have no fan at all, or only the highest speed might work (if it's a partially failed resistor). This is usually located somewhere near the blower motor itself, often behind the glove box or under the dash. It's a relatively inexpensive part to replace.
* Blower Motor: The motor itself could be faulty. This is less common than a bad resistor, but possible. Testing the motor directly (with a multimeter) is needed to diagnose this.
* Wiring/Connections: Check all wiring and connections related to the blower motor, resistor, and switch. Look for loose connections, broken wires, or corrosion. Pay particular attention to the area around the resistor and the blower motor itself.
* Blower Motor Switch: While less likely, the switch itself could be malfunctioning. This is harder to test without a multimeter and a wiring diagram.
* Relay: Some systems use a relay to control the blower motor. A faulty relay could prevent power from reaching the motor. These are usually easy to check visually for damage and can be tested with a multimeter. Locate your vehicle's relay diagram.
Troubleshooting Steps:
1. Locate the Blower Motor Resistor: Consult a repair manual or online forum specific to your 1990 GMC Suburban 2500 to find its exact location.
2. Inspect the Resistor: Visually inspect it for any signs of burning or damage.
3. Check Wiring: Carefully examine all wires and connectors leading to and from the blower motor, resistor, and switch. Look for loose connections, broken wires, or corrosion. Clean any corroded connections.
4. Test the Blower Motor (Advanced): This requires a multimeter. Disconnect the motor and test its resistance. You'll need a wiring diagram to know which terminals to test. A bad motor will show an open circuit or extremely low resistance.
5. Test the Blower Motor Switch (Advanced): This also requires a multimeter and a wiring diagram. You'll need to test the continuity and voltage at different switch positions.
6. Check the Relay (if applicable): Find the blower motor relay and visually inspect it. If necessary, test it using a multimeter according to its specifications.
Important Note: If you're not comfortable working with car electrical systems, it's best to take your Suburban to a qualified mechanic for diagnosis and repair. Working with car electricity can be dangerous if you don't know what you're doing. A repair manual specific to your year and model will be invaluable in this process.
